Fuel Pressure low - new pump

Put the new style fuel pump on about 15 hrs ago. No issues till now
On take-off today (~ 60 deg OAT), got a couple LOW FUEL PX warnings.
Brought the airplane back around to land, and noticed that the px was a bit higher.
Advanced the throttle back to WOT, and the px went down again. Not the point of alarm, but lower than at cruise pwr.
Fuel px in cruise is about 3+ psi
elec alone - ~ 1psi

Checked the gascolator just now - all clear

TT ~ 260 hrs
Mostly run on autogas

Sounds like a restriction?
Anybody else experience similar?
Thx much

M

Fuel press elec only should be above 2.2 psi - mine runs around 2.5 - 2.7. So that's a lead.

Any performance problems?

This might be a press indication problem and not a restriction problem. What fuel press sender do you have?

Bob Bogash
N737G

Mike,

Don't know how new your auto fuel is, but the 15th of this month they start raising the vapor pressure for colder weather. The info below is from a local fuel supply.

Reid Vapor Pressure.
Sept 15. 10% max
Oct. 11.5%
Nov 13.5%
Dec thru Mar 15%
Apr 13.5%
Mar thru Sep 9%

Fuel pressure high new pump

I almost issued a new thread with the title above, then I saw this thread.
Yes, I have the new pump and after downloading the Skyview data for the first 13 hours of flight, I have a fuel pressure that is in the upper range of what Rotax lists as acceptable (per VAN's support line). What I read is mostly in the 6 psi range. At power setting above 5,000 rpm I am in the high 5 but as soon as I decrease power below 5K I am in the 6 range, well into the yellow. Yesterday I removed the fuel pump fuse and it went from yellow to green so I am going to install a fuel pump switch to be on the safe side.
To be clear, here are the set-up ranges that VANs recommended for the new pumps fuel pressure (this is what they have on their S-LSA)
range 1, red low: 0 - 0.7
range 2, yellow low: 0.7 - 2.2
range 3, green: 2.2 - 5.8
range 4, yellow high: 5.8 - 7.2
range 5, red high: 7.2 - 8

Seems there is quite a bit of variation in fuel pressures with the new pump. We've heard of several instances like this one. We are developing a teflon hose package for this engine, but have slowed down due to the data we are receiving. We know what Rotax says, but even with the new pump, there are fuel pressure from 1.5 to 6.8.
We are not sure whether this with just changing the pump, or the complete pump/hose package.
Tom

New pump pressure issues

Some of the pumps have had different fuel pressure, but have mostly been high and not low. The standard pressure seems to around 4.5 - 5.5 psi, but some have certainly gone higher. That is why Rotax raised the max pressure from 6.2 up to 7.2. The low pressures haven't usually been the pump. When testing the pumps when first on the market Kodiak found there was a huge difference between gauges and pressure senders even from the same company. This has been my experience too. The other problem has been wiring issues like poor grounding and the like. This new pump needs a recirculation while the older pumps could get away without it even though it really should have had one. I have had many a discussion with builders and a couple of MFG's not installing one. Adding this and being highly suspicious of sending unit readings have remedied many problems. Any time I have a fuel pressure issue I double check it with a mechanical gauge.

Here is an example to illustrate the discrepancies. I prefer carb sync gauges over the electronic ones because you can diagnose with gauges. When I started to test the gauges from the same company they could be .5-1.0" vacuum off from each other. So now I buy 8 or so and test them all side by side and pair them up according to the most equal readings. I know Lockwood kind of does the same thing.

I wish gauges and senders were created equal like they are supposed to be.

Fuel pressure low, esp on takeoff

Still having a problem with fuel pressure as noted in the first thread.
Data collected:
Definitely more pronounced during takeoff
Seems to be related to deck angle + WOT, i.e. WOT and a few degrees nose up -->Lower pressure indicated.

After a couple minutes, the pressure comes back up
The pressure comes back with a lower throttle setting (during climb, not cruise)
Not dependent on full tank or lower gas qty
Not dependent on 100LL or auto gas
Not a gascolator issue
Pressure recovers in cruise, any throttle setting.

All of that rambling to lead to a (sort of) conclusion:
Is it possible that it's an airflow problem with the new pump drain?
My thought is that an (air) pressure delta at the pump vent (in the lower aft cowl) is causing the pump to indicate a lower pressure.

Anybody have similar experience with monitoring fuel px during T/O?
Thx for the comments.
Mike B

Mike Buettgenbach said:

Still having a problem with fuel pressure as noted in the first thread.
Data collected:
Definitely more pronounced during takeoff
Seems to be related to deck angle + WOT, i.e. WOT and a few degrees nose up -->Lower pressure indicated.

After a couple minutes, the pressure comes back up
The pressure comes back with a lower throttle setting (during climb, not cruise)
Not dependent on full tank or lower gas qty
Not dependent on 100LL or auto gas
Not a gascolator issue
Pressure recovers in cruise, any throttle setting.

All of that rambling to lead to a (sort of) conclusion:
Is it possible that it's an airflow problem with the new pump drain?
My thought is that an (air) pressure delta at the pump vent (in the lower aft cowl) is causing the pump to indicate a lower pressure.

Anybody have similar experience with monitoring fuel px during T/O?
Thx for the comments.
Mike B

Click to expand...

Mike.....we did have one customer who was experiencing inconsistent pressures. When he swapped to our retrofit fuel hose kit those problems appear to have stopped. Now, it is quite possible that disassembling everything and putting it back together may have fixed some other "issue" that existed. But, I wanted to throw that out there in case you are getting close to the Rotax 5 year hose replacement, and/or you are interested in swapping over to conductive teflon fuel hoses.

Have a great day
Steve

Some other RV-12s had low fuel pressure on takeoff. It turned out to be a bad engine driven fuel pump. I am not saying that is the problem in this case, but it is worth checking.
Joe Gores

Mike- this seems to be a problem with 'some' RV12's both with and without the new version fuel pump. I say "some", because it doesnt happen with all of them.
(if it did it would be easy). We've heard of guys reporting low fuel pressure indications, and had NO noticeable performance loss. We had several that changed only the fuel pumps, and the pressure was worse. Several that changed pumps and hose assemblies and were fine. So---pin pointing the root cause has been puzzleing. Also---It didnt seem to matter what part of the country (or countries) the planes were in.

My suggestion to all of them was to get a definate fuel pressure reading from a KNOWN GOOD direct reading guage, and compare it the EFIS readings they were getting. That--you would think--would eliminate the possibly of a bad pressure sender, or a mismatched calibration between the sender and the EFIS ( Skyview in this case). Several found that the senders were off alittle, and found the actual pressures where Rotax wanted them.
Honestly, all of this puzzled us for a while. We sold a couple of fuel hose packages and some guys were reporting excellent performance, and we had one that showed a fuel pressure reduction. He had previously had fuel pump issues, but that was cleared up. When using the boost pump, pressures were fine. He changed the fuel pump under waranty, and things thave been good.

I questioned why the boost pumps were switched on in all phases of flight, not just as a boost for takeoff, or landing. I surmised that the added 1-2 psi was needed to allow the engine pump to keep up. You guys ARE turning alot of rpm, and possibly the pump wasnt able to keep up. The boost pump didnt care about rpm. The perplexing problem is that this pressure issue is erratic in nature. It doesnt happen in all RV12's, nor with either OE or aftermarket fuel accessories. And same in the same area may or may not have issues.
It almost been on a case by case basis. Its almost to the point where you have a KNOWN boost pump pressure of say 4.0 psi, and bypass the engine pump. Havent tried that yet---but I'm sure we can find a tester to do it.

Tom

Mike Buettgenbach said:

All of that rambling to lead to a (sort of) conclusion:
Is it possible that it's an airflow problem with the new pump drain?
My thought is that an (air) pressure delta at the pump vent (in the lower aft cowl) is causing the pump to indicate a lower pressure.

Click to expand...

I guess it is possible.
It would be easy to check by disconnecting the line at the fuel pump and making one test flight.

TS Flightlines said:

I questioned why the boost pumps were switched on in all phases of flight, not just as a boost for takeoff, or landing. I surmised that the added 1-2 psi was needed to allow the engine pump to keep up. You guys ARE turning alot of rpm, and possibly the pump wasnt able to keep up. The boost pump didnt care about rpm. The perplexing problem is that this pressure issue is erratic in nature. It doesnt happen in all RV12's, nor with either OE or aftermarket fuel accessories. And same in the same area may or may not have issues.
It almost been on a case by case basis. Its almost to the point where you have a KNOWN boost pump pressure of say 4.0 psi, and bypass the engine pump. Havent tried that yet---but I'm sure we can find a tester to do it.

Tom

Click to expand...

Has nothing to do with the ability of the engine driven pump to keep up.
It has to do with making the airplane simple to operate (no switch to forget to turn on), and reducing the likelihood anyone will ever experience vapor lock engine failure.
The Rotax is designed to use auto fuel and most people are because of the reduced cost. It is much more sensitive to vapor lock issues (that is why cars have the pumps in the tank). If you push fuel instead of sucking it, vapor lock likelihood is greatly reduced.

I agree that fuel would rather be pushed than pulled. The question is why do we have such a disparity in fuel pressures? Ok--- many builders have NOT checked the actual pressure with a mechanical guage for verification.
I havent heard of any vapor lock issues with the 12, just the variances in fuel pressure.

Eventually, the root cause will show itself.
Tom

TS Flightlines said:

I havent heard of any vapor lock issues with the 12

Click to expand...

Maybe because most of them are flying with a full time fuel pump pushing cool fuel to the engine pump?

Vapor lock usually (not definitive) isn't an issue for engines with recirculating lines. Yes it could happen, but hasn't been an in flight issue. RV12's and many other aircraft have not had the problem. These new Corona pumps must have a re-circulation line or you will have issues with the carbs. This has been one of the hardest points to get across for users of the old AC and Pierberg fuel pumps. This new pump is different. Boost pumps usually add anywhere from .5-1.0 psi and shouldn't be any more than that if set up properly. Pump pressures do vary, but pressure sending units vary tremendously and are usually the cause of different pressures from plane to plane. When Rotax first tested all the new Corona fuel pumps because of high pressure reports they spent a tidy sum making a very accurate test stand. What they found more than any thing was senders where more inaccurate than the pumps and pressures were all over the map. Also owners did not have re-circulation lines and had old carbs that had never been maintained so the pressure leaked past and flooded them.
Any vapor lock I usually hear about is after the engine is shut down and then 30-45 minutes later the pressure is low on start up, but goes back to normal after a few minutes with the introduction of cooler fuel. Having your fuel lines in fire sleeve will help with the problem, but isn't a cure.

If you have a fuel pressure you question I would highly recommend you put a mechanical gauge in line and check it against the aircraft gauges.

Just for a little reference. The new 912is engine uses around 20 liters at approx. 45 psi an hr., but circulates 114 liters hr. So re-circulation lines are important.

p.s.
The engine driven mechanical pump can easily keep up with fuel flow on a low wing.

roger lee said:

Vapor lock usually (not definitive) isn't an issue for engines with recirculating lines. Yes it could happen, but hasn't been an in flight issue. RV12's and many other aircraft have not had the problem.

Click to expand...

When the RV-12 development program began, there was no such thing as a recirculation line/system on a Rotax 912 (at least it wasn't documented in the installation manual, nor was the engines delivered equiped for it.
When the second RV-12 prototype first flew (with another new engine) the return line idea was in the planning stages but no one at Rotax mentioned it and that new engine was delivered without provisions for one.
The extra electric pump was added near the tank to help alleviate the chance of vapor lock.
In very hot climates it may still be beneficial for that reason, and to help keep an engine running in case of engine driven pump failure... considering the track record of engine driven pumps on 912's, it can't hurt.

That is the answer-

Scott,

Was any thought given to having a fuel pump switch? I've always thought that with the electric pump running continuously an engine driven pump failure might not be discovered especially with the new drain line routing any diaphragm leakage out the back of the cowling.

For these reasons I retired the pump to what was my unused Nav Light switch.

Rich

I assume you meant to say rewired?

As already mentioned, a decision was made to not have a control switch for the pump. This aligned with making it (for the most part) a simple stick and throttle airplane when it comes to pilot workload (the avionics is a different subject), and having a system that would help counter vapor lock issues and potential fuel pump failure, without any intervention from the pilot.

Experience has shown that a failure in the engine driven pump usually produces a reduced fuel pressure along with an increased fuel flow... enough that an RV-12 pilot familiar with his airplane would likely take notice and investigate.

Record your fuel pressure history!

This thread is an opportunity to mention that with Skyview we have a wonderful feature to keep track of what's going on in the engine and outside too. Just download on the USB stick the data after each flight and copy it to a computer. Even if you don't use the information right away, in case of problem you will be able to look back at a parameter like fuel pressure or fuel flow and see how it evolved in time. Since my first flight, some 50 hours ago I have kept a record of all my flights and thanks to SavvyAnalysis great (and free!) site, I can review the whole history of all flights through the 25 parameters provided by Skyview.
My fellow RV-12 builders, we are living in a world that just a few years ago would have looked like a pipe dream to guys like us, let's enjoy!

Scott,

I suspected that was the reasoning. I wanted to see if there was another issue I had missed. I see both sides of the argument, but I think I'll stick with a switch.

Thanks,

Rich

Compared to the aircraftspecialty setup this original Rotax fuel manifold does look a bit convoluted. It seems a bit over engineered. You have to wonder if this setup is contributing to the fuel pressure problems if it's not the fuel pump. I have the old vdo 360-043 0-30 PSI sender and have had these problems to. Worked great for many hours of use then out of the blue had low pressure warning. Removed the sender and cleaned it and tried something new and drilled out the small restrictor hole about 3 hundred of an inch. Now it does fluctuate some but never falls low enough to set off the low end alarm on the D-180. If I messed it up they are only about $39.00 anyway. Hey, but you make the call!

Hi John,

Just a note on the re-circulation line restrictor. In the installation manual it says it's a .35mm orifice, but that is a misprint that we are trying to correct.
It is in fact a #35 Mikuni slow or idle jet. The difference is the .35mm is just a number and hole size and the #35 Mikuni is based on flow rate. Mikuni is based on flow. If someone had REAL high fuel pressures then technically it could be lessened by the installation of a slightly larger Mikuni idle jet, but first you better make darn sure the pressure is actually high and not just a poor reading from a sender or gauge.

Jetguy said:

Compared to the aircraftspecialty setup this original Rotax fuel manifold does look a bit convoluted. It seems a bit over engineered. You have to wonder if this setup is contributing to the fuel pressure problems if it's not the fuel pump. I have the old vdo 360-043 0-30 PSI sender and have had these problems to. Worked great for many hours of use then out of the blue had low pressure warning. Removed the sender and cleaned it and tried something new and drilled out the small restrictor hole about 3 hundred of an inch. Now it does fluctuate some but never falls low enough to set off the low end alarm on the D-180. If I messed it up they are only about $39.00 anyway. Hey, but you make the call!

Click to expand...

Here is a comparison to the fuel manifold that we use. It has really nice unrestricted flow paths and also the benefit of standard aviation flares so that a field replacement hose can easily be fabricated.

The hoses that we sell have a 10 year warranty on them and are "replace on condition" versus a mandatory 5 year replacement.

So far field testing and customer feedback has been excellent. Everyone has been very happy with the setup. As a side benefit, we also shave a little weight over the standard rotax hose setup, even with our stainless steel fittings.

Please feel free to contact us with any questions.
Steve

Restrictor size

When we were working on this, we couldnt get any info on the restrictor size, so we bought the restrictor through a Rotax dealer. We measured it for ourselves and found it to me .35mm or about .015. Thats what are using, that way it didnt change what Rotax was sizing for OE applications.
Tom

Hi Tom,

The restrictor listed in the Rotax manual is a misprint. It really isn't supposed to be a .35mm it is supposed to be a #35 Mikuni idle or slow jet. Mikuni works off of flow rate and not mm size. People are trying to get this corrected. We just went over this in the Rotax update school a couple of weeks ago. People having high pressure issues and put in a .35mm may still have an issue. This has caused a few minor issues, but I'm sure some restrictor fuel flow is better than none.

Roger---We bought a 963820 which by the diagram goes in the return side of the banjo fitting, like in the Van's return assembly VA216. I measured it at .015, and thats how we came up with the orifice size of the return restrictor. Those builders that are flying this setup have not reported any low fuel pressure issues that I am aware of.
Early in the development there were 2 builders that did have a lower fuel pressure. We were able to secure a VA216 and then the 963820 to conpare the 2 restrictors. We made the change, and the issues went away.
Tom

I'm all for using a known good mechanical guage for checking the fuel pressure!

As Tom mentioned...the restrictor that we use in our package has had no issues with low fuel pressure and has proven to be a very reliable setup.

I was surprised that the more -12 builders that I talk to, the more I keep hearing about the weird fuel pressures and sometimes showing as low as 0. I can't help but wonder if there is something weird going on in that manifold as JetGuy alluded to.

One thing that I am happy to see is that Rotax appears to be going a bit more conventional with their new engines and their hose setups. The other benefit to this is that if someone needed to do a field replacement, any mechanic would be able to fabricate a replacement hose, rather than using a proprietary one.

We have seen lots of interest in our fuel retrofit kit. Its an easy installation at the 5 year replacement point, although lots of builders have been doing it earlier.

Steve

Hi Tom,

I'm not saying anyone has had any issues or it won't work. It is the wrong size and a misprint because Mikuni goes by flow rate that Rotax calculated and not by orifice size like Bing. You could use the wrong oil, spark plugs and even fuel pumps and the engine will run just not up to spec. A re-circulation line/restrictor even one a little too small is far better than none. Dealers make mistakes all the time and they also don't always buy from Rotax, but buy local trying to get the part easier, quicker or cheaper. These types of things is why it is important to attend Rotax update schools otherwise you would never learn about these issues and changes. The Rotax manuals do have typos.

How many here remember the idle stop screw (typo) to raise rpm it said to turn it counterclockwise and to lower rpm to turn it clockwise typo. It was there for many years before it ever got changed.
On another subject when Rotax tested all the fuel pumps that came into the US they made a very expensive test stand to check each right down to the tenth of a psi. What surprised them was when they tested a number of fuel pressure senders how far off and inaccurate many were and they were not always equal. This is one reason Rotax is getting fuel pressure squawks all over the map for readings. I advise everyone with an odd reading to put a mechanical gauge inline to verify. We trust companies that make senders to be accurate and stay that way, but after enough years of doing this I know it isn't always true. I use gauges to show me approximations and trends and many times take the actual odd reading with a grain salt until I can test it. We buy a set of vacuum gauges to set our carb sync and many people don't know that two identical gauges may be 1 or more inches of vacuum off. Lockwood found this out and buys a number of gauges and then test them and matches gauges up that are the same.

Pilot Jet

Guys, I am confused about the discussion of the return line "restrictor". I wish y'all would use the same terminology. There is mentioned a "Mikuni Idle Jet". There is a "Pilot Jet" shown on the RV-12 plans. On 46-09 it tells you to remove it from the M8X1X17 Banjo Bolt and "it will not be replaced but save it in a safe place". Then on 46-11 the plans show the M8X1X17 Banjo Bolt reinstalled specifically without the Pilot Jet and torqued to 90 in-lb.

Are these all (Line Restrictor, Mikuni Idle Jet, Pilot Jet) the same part?

And, unless I read the plans wrong, it is not used in the RV-12 fuel lines.

Hi Tony,

In the VA-216 fuel return line there is another restrictor like the one you removed. When changing the fuel lines for the TS Flightline hoses I found the fuel return line only pushed on the barbed connector without a clamp. The restrictor is in the barbed connector.

Tony_T said:

Guys, I am confused about the discussion of the return line "restrictor". I wish y'all would use the same terminology. There is mentioned a "Mikuni Idle Jet". There is a "Pilot Jet" shown on the RV-12 plans. On 46-09 it tells you to remove it from the M8X1X17 Banjo Bolt and "it will not be replaced but save it in a safe place". Then on 46-11 the plans show the M8X1X17 Banjo Bolt reinstalled specifically without the Pilot Jet and torqued to 90 in-lb.

Are these all (Line Restrictor, Mikuni Idle Jet, Pilot Jet) the same part?

And, unless I read the plans wrong, it is not used in the RV-12 fuel lines.

Click to expand...

As was mentioned, the fuel restrictor on the Van's setup is integral to the VA-216 fuel line.

In our retrofit kit, we have an external restrictor that threads right onto the flare fitting. This allows field replacement of a hose, while maintaining the integrity of the return line restrictor setup.

Hope that helps.

Feel free to email with any questions!
Have a great weekend,
Steve

Thanks, question answered. I would not have figured this out, and was concerned that the pilot jet I mentioned was supposed to be in the line.