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Fuel Pump Issue

Mine

My plane has always needed the boost pump on in the climb if climbing to more than 8-9000 ft. If I turn off the boost pump before leveling off in cruise the fuel pressure will drop below 10 psi, triggering my alarm. Never had the engine stumble, just turn on the boost and fuel psi comes up to 20+, wait until level in cruise, turn off boost, psi stays at 20+. Mike Seager gave me the sop of keeping the boost pump on in the climb, I'm guessing for this reason.

My theory is the reduced cooling in climb plus the higher pump head (pitch up attitude) results in incipient vapor lock. I put a max reading temp strip on the mechanical fuel pump and I think it registered high at >190 IIRC. I may put a shroud and blast tube on it at annual, but I might not. Boost pump or slower climb rate resolves it. Seems to be worse at higher ambient temps.
 
I had low PSI issues a while back using the original Tempest fuel pump. After doing all sorts of troubleshooting, I got fed up and swapped out to a new Lycoming pump... zero problems since. I don't know if the original pump had an issue or if using genuine Lyco was the solution. Regardless, low psi issues went away and life is good.

I've had a blast tube directed at the mechanical pump from day one, but no shroud. The fuel flow sensor is in the standard spot in the tunnel.

Cheers,
Jay
 
My plane has always needed the boost pump on in the climb if climbing to more than 8-9000 ft. If I turn off the boost pump before leveling off in cruise the fuel pressure will drop below 10 psi, triggering my alarm. Never had the engine stumble, just turn on the boost and fuel psi comes up to 20+, wait until level in cruise, turn off boost, psi stays at 20+. Mike Seager gave me the sop of keeping the boost pump on in the climb, I'm guessing for this reason.

My theory is the reduced cooling in climb plus the higher pump head (pitch up attitude) results in incipient vapor lock. I put a max reading temp strip on the mechanical fuel pump and I think it registered high at >190 IIRC. I may put a shroud and blast tube on it at annual, but I might not. Boost pump or slower climb rate resolves it. Seems to be worse at higher ambient temps.

This is very interesting and good information and that Mike Seager recommends to keep the boost pump on in the climb. it is good to know that 10psi is enough to feed the engine.
 
same for me

Hi Ivan
I have the same issue. I keep the fuel pump until 2 or 3 min level. Never had engine trouble even if the pressure was down to 12psi. Always going back to 30 with the boost pump.
What i notice. It's more when it's hot.
The other thing is that i removed my left heat muff ( i placed aluminum shield 1/3 the circonference of the muffler ) and replace it with a fresh air that go from the front directly to the left heat ( cold) box. Inside my tunel, i have a Y and both front and back heat come from the right heat muff. I use the fresh air to cool under the panel

Since that day, i solve my hot tunel issue ( probably because fresh air wash the hot air that come from the right heat box when in the close position.

Second, i have less problem with hot start and my low fuel pressure alarm. With the heat generated by the heat muff, i just can imagine the heat that is discharge under the cowling when the heat box are closed and it's not so far from the fuel pump.

My Rv-7 had the same issue. So i don't think too much about it anymore.
 
Interesting

I find it peculiar that some need the boost pump on and some don't.
It would be interesting to find out the differences in installations between those who do and those who don't need a boost pump for climb.

I usually turn the boost pump off just out of the traffic pattern after I change tanks to mogas.
Pressure usually drops 2 or 3 psi but recovers in about 5 seconds.

Blast tube and fuel pump shroud installed since new.
Engine compartment temp sensor installed on firewall just above the exhaust
pipes usually indicating around 150F.
Constant flow return line fuel system.
 
Drop in pressure in level flight

Ours will randomly drop in pressure in cruise. It will maintain 22 to 23 psi and every so often( once every 10 hours) will drop slowly to as low as 12 psi...at first it scared me and I would immediately switch on electric pump and it would rise immediately to 27 psi...then as it happened again I decided to watch it before switching elect pump on and it slowly drops to 12 psi and then starts climbing slowly back to 22 again. This only has happened in level flight because like Bryan stated Mike teaches to leave pump on in climb. We did install blast cooling shield because of Bob L experience. I am somewhat confident it is not wiring or sensor because of its responsiveness to switching on electric pump. Aerosport recommended new mechanical pump which we will replace at annual. It would be interesting to disect the mechanical pump when replacing but it may be hard to know the reason since it is so random.
We also have two fuel filters, one in each wing root and it will happen on either tank.
I think in our case the engine does start to slightly loose power ( going by sound and feel only) at 12 psi and fuel flow will drop about 1 gph during this event.
This is good discussion and would like feedback to solve for all of us.
And Bryan....it is NOT because we have a 430 instead of a 650:)
 
Drop in pressure in level flight

Ours will randomly drop in pressure in cruise. It will maintain 22 to 23 psi and every so often( once every 10 hours) will drop slowly to as low as 12 psi...at first it scared me and I would immediately switch on electric pump and it would rise immediately to 27 psi...then as it happened again I decided to watch it before switching elect pump on and it slowly drops to 12 psi and then starts climbing slowly back to 22 again. This only has happened in level flight because like Bryan stated Mike teaches to leave pump on in climb. We did install blast cooling shield because of Bob L experience. I am somewhat confident it is not wiring or sensor because of its responsiveness to switching on electric pump. Aerosport recommended new mechanical pump which we will replace at annual. It would be interesting to disect the mechanical pump when replacing but it may be hard to know the reason since it is so random.
We also have two fuel filters, one in each wing root and it will happen on either tank.
I think in our case the engine does start to slightly loose power ( going by sound and feel only) at 12 psi and fuel flow will drop about 1 gph during this event.
This is good discussion and would like feedback to solve for all of us.
And Bryan....it is NOT because we have a 430 instead of a 650:)
 
I have a temp labels on my alt, mech pump and magnetos. Max temps reached on alt surface facing #1 exh pipe= 300F with no issues so far. Mech fp max= 225F with no shroud. Magneto max= 200F. No blast tubes. Min fuel pressure alarm set at 15 psi. Has alarmed on a couple of hot climbs. I have ff transducer in tunnel which does restrict flow. Turning on boost pump and leveling off cured it. Never ran rough. I did order the upgraded SS fuel pres transducer from GRT to replace the automotive quality VDO sender. Have not installed yet. I also have cowl outlet temp switch that has an adj opening temp. It is set at 230F and has only alarmed once at 16,500' cruise due to thin air, near gross, near aft limit with 6 degree pitch angle. Normally 3 deg at 12,500' cruise.
 
This is not unusual.

Hot fuel, sucking uphill in the climb and lowering outside pressure. Hot pumps too.

Bonanzas are the same!

Leave the pump on and until 5-10after TOC.

Installing the pump cooling shroud helps.

You are not abnormal if that helps.
 
I've already shared this with Ivan.

When I replaced my pump over a year ago, I installed the shroud at the same time. This means I'll never know the true root cause of my issue.

I can state that I can now take off with the electric pump off and the mechanical pump has no problem maintaining proper fuel pressure on the climb during the summer temps.

I suspect that's due to the shroud, but I don't have the data to prove that.

Bob
 
Hot fuel, sucking uphill in the climb and lowering outside pressure. Hot pumps too.

I think this statement from David sums it up best.

After all the research, anxious moments and cancellation of a trip followed by the installation of a new fuel pressure sender, a new Fuel Pump and a cooling shroud I come to find out that decreased fuel pressure in the climb is not at all abnormal.

The one nagging question that remains is how can I fly for over 500 hours and never have this happen. Why now???
 
Does this make sense?

I think this statement from David sums it up best.

After all the research, anxious moments and cancellation of a trip followed by the installation of a new fuel pressure sender, a new Fuel Pump and a cooling shroud I come to find out that decreased fuel pressure in the climb is not at all abnormal.

The one nagging question that remains is how can I fly for over 500 hours and never have this happen. Why now???

David seems to have nailed the variables, but the summing needs numbers. "Why now?" you ask. 500 hrs and off shore fuel. No data, but since this has not occurred in 500 hrs, you ask why now - really appropriate question.

So lets see,

1. angle of climb - no change - Check
2. Temperatures, fuel, ambient, etc. - well, if it was just coming out of Bahamas, then the tank temps with ambient, could have been a major factor, but you have repeated this, right? - probably not the root cause.
3. System suction pressure loss from tank to mechanical pump - well, we don't know this one. Let's explore

3.+ since the system has not changed, it is not a configuration,
But - the pressure drop may have increased - how? - fuel filter.
or a minor tube facture, or a loosening fitting.

You did not say if you have checked the suction screens and the fuel filter. You could have gotten a bad batch of fuel, or just shaken loose a connection that is sucking a minute about of air. These factors might be your best place to start getting some data of why now.

Good Luck. Fly Safe
 
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Good points made by David!
Like all of you I am curious to find out as to why Ivan's fuel pressure problem
Showed up at 500 hours?
A cavitating mechanical fuel pump or vapor lock resulting in fuel pressure loss is often the result of cumulative factors.
While a fuel delivery system might tolerate a number of less than ideal components "gascolator" or installations where fuel is allowed to heat up, adding just one more factor "unknown at this time" can be enough to push pumping ability over the edge.
BillL is probably on the right track, pointing out a developing small leak somewhere along the fuel delivery system. Additionally a mechanical fuel pump that may have deteriorated just a bit over time and no longer develops 100% of its rated pumping ability could also be considered.
While I always keep my boost pump on for landing and take off, I can confidently take off on the mechanical fuel pump only showing 20 plus psi fuel pressure all the way up to 14000 feet on a hot day over 100F using 91 octane gasoline.
My guess is that if you "need" your boost pump on for your climb to keep fuel pressure up you have a fuel system that is not far from vapor locking and cannot tolerate any other negative factors such as small system leaks, very hot temperatures, substandard gasoline, or even a slightly deteriorating fuel pump.
Just food for thought.
 
Good thoughts

I would be curious to know how many of you having issues, have the Tempest pump installed vs Lycoming pump.
 
Thanks for the compliments but I just learned this stuff from George Braly and from the unleaded Avgas project.

What could have changed? Slight clogging of the breather lines in your tanks. A bit more butane in the avgas blend you have?
 
Another data point

While I was building my 10 Scott S told me about the breather vent line possibly getting fuel in it when the tanks are full while climbing. I tried my best to install the line as close to the top of the tanks as I could. My fuel pressure will drop way down to 5 psi if I top the tanks completely and stay on a long climb. Of course I'll turn on the pump but I've seen 5 psi before I do. The engine has never stumbled during this phase. If I don't fill the tanks all the way it will not happen. Sometimes it doesn't happen with full tanks, just when I am doing long climbs with topped off tanks?? Just another reference.
 
Interesting, i had no idea others were having this issue. After 400 hours, i am experience a similar issue. I have tried to tighten lines up and cleaned the obvious fuel filter in the tunnel as well as the wing tank screen.

Another question: are there any other fuel screens in the fuel flow path? IE, i am wonder if the mechanical fuel pump has a screen or filter? Also, does the Silverhawk throttle body have one?

Stock plans. Just like Ivan, i find it interesting that this fuel PSI issue has come up all of a sudden. Literally, one flight was fine. The next one the problems have all started.

Thanks!
 
Interesting, i had no idea others were having this issue. After 400 hours, i am experience a similar issue. I have tried to tighten lines up and cleaned the obvious fuel filter in the tunnel as well as the wing tank screen.

Another question: are there any other fuel screens in the fuel flow path? IE, i am wonder if the mechanical fuel pump has a screen or filter? Also, does the Silverhawk throttle body have one?

Stock plans. Just like Ivan, i find it interesting that this fuel PSI issue has come up all of a sudden. Literally, one flight was fine. The next one the problems have all started.

Thanks!

Yes, there is one in the Silverhawk fuel control.
 
Ivan,

You've had the shroud on now for a month. How has it been performing since the change? The same or was there an improvement?

Hopefully, the RV-10, Diane, and you are all basking in the Florida sun and not all the winter weather in Guelph.

bob
 
Yesterday we were all basking is the cold windy drizzle, but that gave way to sunny and cool today. :p
 
Pressure fluctuations

Just want to go on record that my RV-8 built in 2002 with an 400 hour since new IO-360 also sees these fuel pressure fluctuations. I always top my tanks when I fill up and it seems to be directly related to full fuel tanks in my case. Does not seem to be an issue after they burn down a bit. I hate it and wish it was not there but it is. Turning on fuel pump brings the pressure right back. Engine has never stumbled but I turn on the fuel pump as soon as the alarm goes off. Have seen it less than 7 pounds. Ed
 
Fuel pressure drop Update!

Ivan,

You've had the shroud on now for a month. How has it been performing since the change? The same or was there an improvement?

Hopefully, the RV-10, Dianne, and you are all basking in the Florida sun and not all the winter weather in Guelph.

bob

I am sorry to have taken this long to get back here with my findings. The reason is two fold, first of all I don't tend to visit the VAF forums too often and the other is that I wanted to gather more data before I post my findings.

In a nutshell it has been an arduous journey to figure out what was wrong. To make a long story short I have come to the conclusion that nothing is really wrong with the system and that the problem is caused by heat.

The drop in fuel pressure occurs only in the climb and only after the boost pump has been turned off. The mechanical engine driven fuel pump is mounted on the back of the engine in a very hot environment where it is not only heated by heat transfer from a hot engine but also from hot air routed in the same area from the cabin heat valves when they are closed off to the cabin. In fact one of them is pointed right at the Mechanical fuel pump.

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Fuel pump and cooling shroud.

A couple of changes I have done since this issue began, first of all I lowered the PSI setting at which the Master warning would come on from 20psi to 17psi. According to the Lycoming manual std. pressure for this pump is between 18-28psi. My VP-200 has the ability to turn on the Boost pump automatically and I lowered that setting from 18 to 15psi. Since this change the boost pump has not been turned on automatically which indicates that the fuel pressure has not dropped below 15psi.

Another change I made was to add a cooling shroud and blast tube to the mechanical pump. There is no doubt that this pump is running hot and heating up the fuel as it goes through it.

And lastly, I am now keeping the boost pump on much longer after take off than I did before. In the past I would turn it off as soon as I cleared the pattern but now at least through 3000'.

The changes in setup I made combined with the addition of this cooling shroud has not eliminated the problem all together but it is much reduced. This past weekend on an 88 degree F day here in central Florida I did have the Master warning come on a couple of times in a climb to 8500'. What is interesting is that these drops in fuel pressure were momentary in nature. A quick check I noticed that pressure at 16psi but it quickly built back up to normal pressure of 23-24psi and the Master warning went out. This was an exception because, I believe, the temperature was so high. On must departures now since these changes the fuel pressure stays put right where it should be.

During these events or at any other time in the past has the engine ever stumbled or starved for fuel. EGT's have never fluctuated which to me means that the mechanical fuel pump can deliver enough fuel at pressures as low as 15psi. or lower. I read on this forum someone stating that he has seen 10psi without any problem, some even lower than that.

I hope this helps and that it answers some of the questions.

Best regards,

Ivan Kristensen
 
Random observations...

Pump body temperature is a secondary interest at best. The real interest is the inlet-outlet rise in fuel temperature, or better, the tank-to-pump-outlet rise.

The pump body is not the only place fuel can pick up heat. In the case of the RV-10, we hear reports about high tunnel temperatures, and a lot of the fuel system is in there.

Everybody's pump body temperature is basically the same, i.e. equal to engine block temperature. If a particular pump install actually does run hotter, it's because there is an additional heat source. It makes a lot more sense to remove the additional heat source than to try cooling the pump.

A blast tube is operating at about the same pressure delta as a heater box, so running them in opposition isn't very effective.

Fuel flow and fuel temperature rise would have an inverse relationship. The pressure problem appears in climb, high power, i.e. at high fuel flow. High flow equals less fuel dwell time in any particular hot component, meaning less temperature rise than at low fuel flow. Heat may be a factor, but it is probably not the only factor.

We all agree that vapor bubbles cause pumping problems. It would be worthwhile to consider all means of vapor bubble formation, in particular mechanisms that work in concert with heat, for example, an elbow fitting with a sharp internal corner. Hoses are available with curved tube end fittings for a good reason.

BTW, I vaguely recall reading about a study done at Piper, in which a complete fuel system was constructed with clear tube for visual observation, and that everyone was amazed at all the locations in which cavitation was evident at high fuel flow. Anybody else recall that?
 
Random observations...

Pump body temperature is a secondary interest at best. The real interest is the inlet-outlet rise in fuel temperature, or better, the tank-to-pump-outlet rise.

The pump body is not the only place fuel can pick up heat. In the case of the RV-10, we hear reports about high tunnel temperatures, and a lot of the fuel system is in there.



We all agree that vapor bubbles cause pumping problems. It would be worthwhile to consider all means of vapor bubble formation, in particular mechanisms that work in concert with heat, for example, an elbow fitting with a sharp internal corner. Hoses are available with curved tube end fittings for a good reason.
?

It seems the major variables between planes are in no particular order:
Type of fuel valve and height above the floor, type of boost pump, filter and filter location, type of fuel lines in the tunnel, and what, if any efforts have been made to keep tunnel temps down.
Fuel vapor occurring in the fuel selector would give the mechanical pump fits. While fuel should siphon right through the selector regardless of height, I don't know that it does.
In hot places some folks remove hose from heater valves and divert to somewhere near cowl exit.
 
In my own evaluation, there are several good reasons to leave the right heater valve off and go with a single valve. Heating fuel, at the pump and in the tunnel, is one. With just the left valve installed and plumbed to the rear heat outlet, the airplane is shirtsleeve warm at altitude when surface temperatures are down to 30 F or so. Below that you might need a light jacket, no big deal in my mind. But, I sleep in the snow for the fun of it, YMMV.
 
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Boost Pump Suction Filter

Might already have been said (as I read these posts briefly) but open up and check your filter ahead of the electric boost pump. Particularly on new tanks and installations, some debris could be collected in this filter which would cause the symptoms you describe. I would recommend opening this filter anyway soon after the first few flights / engine runs. I was talking to a factory representative yesterday about this very scenario and the symptoms match having restriction in the boost pump suction filter.
 
In hot places some folks remove hose from heater valves and divert to somewhere near cowl exit.

This is one thing I have been giving serious though to. I rarely use the front heater anyway as it is just too hot. Even at -15C the rear heater seem to be enough to keep the cabin comfortable.

One thing is for sure, the temperature of the air coming out of the heater vents is VERY hot. Eliminating one of these heat sources from the space behind the engine would/should be a good thing.
 
Replaced mechanical fuel pump under warranty and so far (10 hrs) no fluctuation. As well is in cruise it will maintain 23 psi where is old one was 21 to 22 psi. I dissected the old pump before sending back but no obvious reason for the drop in pressure. Our issue only appeared in cruise but I also leave electric on till level off.
 
This has been a great thread for me to read.

I have a little over 40 hrs on my new engine install. IO-375 on RV7A.

After reading this thread I realize that in both instances that I have experienced loss of fuel pressure it was with full tanks and during a climb to relatively high altitudes.

The worst one happened at 16,000', I was still climbing to get over some weather. Fuel pressure alarm came on, (Dynon Skyview, Betty says "Fuel Pressure" in head set, love it). The pressure dropped to 12 PSI and I felt the engine losing power, boost pump cured it. I descended and turned the boost pump off to see the pressure drop again slowly. One more time on the boost pump then off again and the pressure stayed good for the rest of the descent. I was heading for nearest airport. The OAT was well below 0F up at 16,000' so I was thinking ice...

Spent the night in Bend OR. No water found from sumping tanks again.

I topped off the tanks before leaving Bend the next morning. At 10,000' the pressure dropped again. Boost pump again and descended to lower altitude and flew on to Central CA. Changed out pre filter as precaution, old one was not clogged at all.

I have tested the plane up to 17,500' and it ran fine, but the tanks were not full during the test. This has only happened two times, both times with full fuel and climbing at altitude.

Now I believe the problem my be what others have discovered. Full tanks in a climb puts fuel in the vent lines, that plus the low ambient pressure up high makes me think vapor lock as probable cause. A little extra restriction in the system plus fuel in the vent lines may be enough to cause the problem.

FYI, I went through my fuel system looking for leaks. All flares are fine with no cracks. All fittings were snugged up, nothing found loose.

I used a pre existing hole in my firewall for my fuel line bulkhead fitting that was pretty close to my cabin heater valve.

Changes made to try to help situation:

I moved my firewall penetration for fuel line further away from the cabin heat valve. In the process I eliminated a 90 degree fitting in the system, managing to bend some alum tubing for the connection from boost pump to firewall.

I added another 2" flange to my cabin heater valve and routed it scat down to the cowl exit area. The heat was coming out pretty close to the mechanical pump.

The pump has always had a shroud and blast tube.

When testing the fuel vents, I was surprised at how hard it was to blow through them with the fuel caps off. It was difficult to move air through the tubing but I could get air to the tanks. (Try it and I bet you will be surprised also!) With thousands of RVs flying with stock fuel vents and no problems, I think I should leave them alone, but, it is tempting to do something with the system to get better venting. Any ideas?

Now I know to test the system with a sustained climb and full tanks to see if I can replicate the problem. Maybe accepting the idea of needing to run boost in those conditions is the answer? I would prefer to have a system that is less prone to vapor lock as I would like to run auto fuel but this experience has me staying with 100LL for now.

I can't help but consider putting pumps between the wings and fuse to push rather than pull the fuel...

I may find a way to eliminate a few more fittings with less restrictive high dollar free flowing types.

Randall in Sedona
 
Randy---I dont know that I'd put pumps in the wing roots. We plumb filters and gascolators there, but havent done any pumps. Granted, electric pumps would rather push fuel than pull it, but I can envision all sorts of issues with pumps in the wing roots., But----I can plumb it for you.
Tom
 
Randy---I dont know that I'd put pumps in the wing roots. We plumb filters and gascolators there, but havent done any pumps. Granted, electric pumps would rather push fuel than pull it, but I can envision all sorts of issues with pumps in the wing roots., But----I can plumb it for you.

Envision a few for us Tom. I'm serious, because the standard may well move that way. We're already moved to the all-electric airplane, and 10 years ago it was easy to find a spirited defense of the vacuum pump.

Consider....electric boost pump packages (AFP, Andair, EFii, etc) have become compact. In the context of liquid delivery, keeping the engine-driven pump and mounting one boost pump in each wing root is no different than mounting one boost downstream of the selector valve. Actually, in the context of hydraulics, it is better.

Ok, now make a shift in mindset. Right now most of us "save" the boost pump by only running it for takeoff and landing. Right here in this thread we have pilots who talk of a need to run the boost pump in climb as if it is a Bad Thing. We might be right to question that mindset, as the same pumps run 100% of the time in an automotive application.

One more thought shift please. The engine driven pump is there and runs 100% of the time anyway. Why not think of it as the backup? In this brave new world, the electric pumps are primary, and being boost packages with bypass valves, the engine driven pump will pull fuel through a failed one just as it does now.

In truth, nothing mechanical has changed. The only pioneering required is to link pump power to tank selector position, so that selecting a tank turns on its dedicated pump. Even that is not required if you're willing to manually switch pumps when you switch tanks. You know, sorta like we already turn on the boost to switch tanks?
 
Ok Dan---My 2 brain cells are thinking about this.
I see several options, and if there is inverted fuel except the 10 of course. Yep---can be done. With filters.
Question would be what pumps do we want to use? A Facet low pressure to prime the injection boost pumps, or 2 injection boost pumps in the roots, and eliminate the cabin one? Redundant backups may be a good thing here.
Tom
 
Question would be what pumps do we want to use? A Facet low pressure to prime the injection boost pumps, or 2 injection boost pumps in the roots, and eliminate the cabin one?

As noted, the same pumps we use now...Facets for carbs, AFP, Andair, or EFii boost pump packages for injected installs. Both the Facet and a standard boost pump package will allow fuel flow with a dead electric motor, so the engine pump can continue to supply fuel.

An EFii electronic fuel injection install has two electric pumps, no bypass, no engine pump.
 
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I dont see any issues with that Dan. Mounting the pump would be the only issue, but most of the root areas of the RV's are reinforced anyway. pump mounted on fuse side, filter on wing side.
I'll play with this some!
Tom
 
What is driving the two pump solution? Are we convinced it would solve that particular problem?
We just doubled the cost, which is pretty healthy, at least in FI engines. Two pumps to maintain now and we added more fittings, wire, etc....
What say you smart people?
 
What is driving the two pump solution? Are we convinced it would solve that particular problem?

I think it would solve the fuel fluctuation problem, if you think fuel pressure fluctuation is a problem. From what I read, none of the posters reported engine problems just fuel pressure fluctuations.
For the first time this year I have noticed fuel pressure fluctuations dropping as low as 10 psi, however this is on 91 octane mogas only and in climb.
Leaving the boost pump on keeps the pressure steady at 25psi.
It is fair to assume that having the fuel pushed rather than sucked uphill would eliminate the pressure fluctuations.
We just doubled the cost, which is pretty healthy, at least in FI engines. Two pumps to maintain now and we added more fittings, wire, etc....

I am certainly not afraid of an all electric fuel delivery installation but you are right about price and added complexity along with additional electrical system redundancy.
I see no advantage in having the fuel pumps located in the wing roots as opposed to somewhere along the floor of the fuselage. You cannot have the pump located any lower than the floor of the fuselage or else they will protrude out the bottom of the wing root fairing. Having the pump or pumps installed under the seat or in the tunnel will put them in a position where they will be flooded and in a hydraulically correct position to push fuel to the carburetor/fuel controller.

Having the mechanical/electrical fuel pump redundancy is not only my preferred choice but it is also the least expensive and least complicated choice.

As to the fuel pressure fluctuations, I'll get used to keeping the boost pump on
until I level off and until we run out of this crappy winter mogas.




.
 
Ernst---in some models, like the 7's, there just isnt extra space in the cabin for 2 pumps. Dan's thought about wing root installs does make sense, if only for the fact that there is sufficient space for the pump and the filter. And---its close to the wing exit, so it would have a shorter "suction" distance.
Better would be in-tank installs, but that has its own problems. LOL---flop tubes would be an entirely new creation. (Remember? Kahuna :D).

Maybe the thing to do would be to see if any of our other high altitude flyers have similar issues. If not, then find what is different about Randy's situation and fix it.
Tom
 
I'm not necessarily proposing a solution...more a thought exercise. We tend to be soooo fixed in our thinking sometimes.
 
Pumps in the wing root would push the fuel through the fittings and plumbing around the selector valve rather then sucking the fuel through same.

In my case, the fuel must travel through a 45 deg AN Fitting and (2) 90 Deg fittings before heading through more bent aluminum tubing to the pre filter and the boost pump. Then two more 45 deg fittings before getting to the mechanical pump. So, in my system the mechanical pump is sucking fuel through (2) 90s and (3) 45s, plus the usual tubing bends. I suspect this is more restriction than most systems, but it worked well in my Subaru days. (I am looking at ways to reduce the number of fittings or replace them with high flow types, maybe pull the return system and replace the valve with Van's etc)

When I built the plane, I wanted to have the Andair valve handle point to the tank in use, orienting it this way caused complications in the plumbing resulting in the arrangement described above. Mine is a 6 port valve with full return system that worked fine when the system had 35 GPH flowing back to the tank in use. Now that I have a dead end injection system perhaps the restriction from those fittings is enough to cause vapor lock, speculation.

Another possible approach to the problem would be to put a fuel pressure regulator at the spider and let the excess fuel flow back to the tank in use again... It worked OK before, but not sure that would work on this system. Comments? Anybody tried that?

One instance of loss of fuel pressure happened at 16,000' in below zero OAT, the other time was at 10,500 and 17F OAT. I have already made a few changes that may have solved the problem, and after reading through this thread I have learned that the need for boost pump use in climbs and high altitude cruise is not uncommon.

I would like to have fuel system with a little more margin to vapor lock situations so that I can run auto fuel as desired.

The problem with most of this business is I am making changes based on speculation rather than measurements. As stated above, I may have already solved the problem, but with this approach, only flight testing will tell and even then I may not be able to exactly replicate the situation that lead to the problem. (Just a little self criticism here as I probably should have measured fuel pressure on the suction side before making any changes and then measured after to see the difference, maybe somebody knows how much difference in pressure drop there is between an AN fitting and bent tubing? Enough to cause fuel to boil?)

FYI, I have eliminated a 90 Deg fitting from the outlet of the boost pump by bending alum tubing. I also ducted the cabin air heat down to the cowl exit and away from the mechanical pump.
 
Randy and others---
we may be in uncharted territory here in some cases. Altitude, type of fuel, a somewhat uncommon fuel system, OAT, all of this are factors. Randy, you may infact become the test pilot for this system.
Perhaps some of our other members can elaborate on some of this. I know Paul did alot of testing---Dan for sure, so possibly we can help Randy with his issue.
Tom
 
When testing the fuel vents, I was surprised at how hard it was to blow through them with the fuel caps off. It was difficult to move air through the tubing but I could get air to the tanks.

This is very interesting, an unobstructed 1/4" vent line should make it very easy to get air into the tank????
A little fuel in the vent lines is certainly not uncommon especially with full tanks, but should not inhibit the free flow of air venting the tanks.
If in fact the tanks are under an even miniscule amount of vacuum would explain the occasional collapse in fuel pressure especially in those installations with multiple elbows and fuel line construction which impedes the flow of fuel.
In many installations the forming of vapor bubbles may only be a matter of a few degrees of fuel temperature or one more elbow in the fuel line or a tiny bit more draw needed to pull fuel uphill to the mechanical fuel pump.
Any of these 3 conditions, fuel temperature, fuel pressure and a streamlined fuel supply line can be addressed to eliminate a fuel vapor problem.

I have been burning mogas E10 for the last 300 hours without any trouble and this is the first year that I have observed a bit of fuel pressure fluctuation on climb out using mogas without the boost pump. As I previously stated, the pressure stays stable flying on avgas without the boost pump and stable with the boost pump on during climb using mogas. No boost pump required for cruise on either gasoline.

I have a constant flow return lijne plumbed out of the mechanical fuel pump, calibrated to return 6GPH @25psi.
I believe this installation provides a constant flow of cool fuel to the mechanical pump and addresses the fuel temperature issue.

I have not considered checking the fuel vent lines for possible obstruction but I have always noticed the air pressure building up during fueling when using a funnel. A forward facing fuel vent line should provide just a tiny bit of pressure
in the tank but certainly not a vacuum. Measuring tank pressure during flight might provide some more clues.

An electric boost pump in the wing roots would certainly go a long way in overcoming any of these issues and a good idea other than for the reasons I mentioned previously.
 
New fuel system

Since I posted about my fuel pressure loss at altitude etc. I thought I would post a follow up on what I have done in hope of eliminating the problem.

Well basically, I put in a whole new fuel system. I have pulled out my old feed and return lines and Andair 6 port valve and replaced them with the standard Van's fuel selector valve. (The old system was in place to facilitate the Subaru injection system, much like the EFII system with fuel returning back to the tank it came from.)

Before the changes the fuel left the tanks in the normal stock Van's manner. It then turned up 90 deg (bent alum)to get to the Andair selector valve, then a 45 Deg AN fitting, then an AN90 Deg, then coming out of the valve another AN 90 Deg. After that it droped down to the EFII pre filter and boost pump. Exiting the boost pump was another 90 Deg fitting and then two more 45 deg fittings to get to the mechanical pump.

I have eliminated (3) 90 Deg AN fittings, and (1) 45 Degree fitting, plus the tank selector valve now sits lower so the fuel does not have to be drawn as high (by about 1.5"). In addition, the routing of the fuel lines is much less resistant to flow as in fewer bends and elevation changes.

I am also trying out the Rocket style fuel vents (coils in the wing root) only I have increased the size of the coils to 3/8". The vent to atmosphere is located a few inches below the leading edge of the wing, in the wing root fairing. I went with three coils of the 3/8" tubing and used a 3/8" bulkhead fitting cut at an angle with screen glued in place like the originals under the firewall, only much larger. I suspect this area is a high pressure zone being just under the leading edge. I used an odd little adapter where the new vent line attaches at the tank, converting from 1/4" OD to 3/8" OD flares. The vent tubing inside the tank remains the same 1/4" OD tubing.

I only had parts around to install the new vents on one side so maybe I can figure out a way to measure the pressure in each tank and see how much difference the new vent makes, if any. It sure is easier to blow though. I think, in theory, the effort required to blow through the 1/4" tubing that bends all around inside the fuse, equates to less pressure reaching the tank.

So that is the new system. All new tubing in an arrangement that should allow a much freer flow, so less suction required and hopefully an improved venting system. FYI, I am leaving the old vent system in place and can easily hook it back up if I don't like what I have done for any reason. I cannot think of any easy way to measure the air pressure inside the tanks to compare the effectiveness of the vent change, any ideas? This would be a great time to measure since I have one tank vented in the stock manner and one with the coils in the wing root.

I intend to test the new system starting with topped off tanks and climbing to 17,500 and leveling off for a while to see how it does.
 
It's been over a year...

Anything new or positive conclusion to this very interesting story?

Bevan
 
I saw Bevans post to this, and thought, wow-----I forgot to play with this. (among other things, been busy, and OTHER issues.)
If Randy could chime in here at let us know the status, I think we all could learn something.
I re-read a few of the posts and was struck by the post saying the vent line was hard to blow through. So---is is -4 aluminum tubing, or hose, and IF its hose, what kind? The issue I see is that at Randy's base in Sedona, at 4300', its going to be hard to test things at 16000-17000 feet. IF it had hose as vent lines, and they were rubber hose, it is a possibility that at altitude the liner could constrict, or even collapse, creating an issue. If is is aluminum tubing, then it hard for me to imagine the tubing collapsing, and then opening back up as the plane descends. I guess crazier things have happened.
Hopefully Randy will give us an update and see if his changes have somved the problems.
Tom
 
am not sure if it will help, but thought I let you know about the problems I had
with the fluctuating fuel pressure on my YIO-360-M1B engine. Actually I
have solved the problem now. The cause for the fluctuating fuel pressure
indication was VAPOR LOCK in the fuel pressure hose. I have connected
the fuel pressure hose to the silverhawk injector which is located at
the front lower end of the engine. The fuel pressure hose I have routed
underneath cylinder 1 and cylinder 3 back to the firewall where the
pressure sensor is located. It seems that the fuel in the pressure hose
got heated up during longer flights at higher levels - thus causing
vapor locks in that hose and thus causing the erratic readings of the
fuel pressure. I have put an additional heat protection sleeve on the
fuel pressure hose now and re-routed it as far away as possible from the
hot areas near the exhaust pipes of cylinder 1 and 3. Problem solved, no
more erratic fuel pressure readings.
best regards,
Josef
 
I saw Bevans post to this, and thought, wow-----I forgot to play with this. (among other things, been busy, and OTHER issues.)
If Randy could chime in here at let us know the status, I think we all could learn something.
I re-read a few of the posts and was struck by the post saying the vent line was hard to blow through. So---is is -4 aluminum tubing, or hose, and IF its hose, what kind? The issue I see is that at Randy's base in Sedona, at 4300', its going to be hard to test things at 16000-17000 feet. IF it had hose as vent lines, and they were rubber hose, it is a possibility that at altitude the liner could constrict, or even collapse, creating an issue. If is is aluminum tubing, then it hard for me to imagine the tubing collapsing, and then opening back up as the plane descends. I guess crazier things have happened.
Hopefully Randy will give us an update and see if his changes have somved the problems.
Tom

In my 9A I've got the Tuckey-style outboard tanks and Rocket-style coiled 1/4" aluminum tubing (3 turns) at the wingtip, then vented to the bottom of the wing with the aluminum 45-degree fitting with screen pressed in it. My vent air comes into my outboard tanks and flows through them into the outboard tank fuel pickup, which is connected to the inboard vent fitting (flow-through venting). I've now got almost 5 hours of flight in Phase I at 16,500' and 17,500' without the slightest appearance of any venting issue going on. I would be more inclined to suspect vapor lock on the inlet side of the pump due to the reduced ambient pressure at this altitude, than a venting problem.
 
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