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Vapor Lock?

Piper J3

Piper J3

Well Known Member
Vapor Lock?

A couple of items caught my attention today. Weather in northern Ohio was 68F thanks to the massive El Nino.

I flew an uneventful 30 minute flight, landed, and shut off engine. Airplane sat for about ten minutes. I restarted the engine and within about 30 seconds it began to shake and then quit. Restart went well and all indicators were in green. I did a complete run-up and engine performed well. Takeoff for my second flight was fine. About five minutes into the flight I got a high fuel flow warning with fuel flow around 12 GPM. High readings remained for a couple of minutes. I slowly reduced power to about 50% and GPM went back to 4.5ish. I gradually returned to cruise power and fuel flow remained fine for the duration of the flight.

I think the fuel system got a vapor lock when sitting with a hot engine. Under-cowl temperatures most likely vaporized fuel in the lines and perhaps even the carb float bowls. I buy this explanation and will remember symptoms for future consideration.

My question is why the elevated fuel flow readings? Is this vapor lock bubbles returning to the fuel tank? Any ideas appreciated
 
I've had quite a bit of trouble with this (http://www.vansairforce.com/community/showthread.php?t=130177). Hot start immediately after shutdown works great. Hot start after sitting 10-15 mins was real hard with a lot of vibration for about 20 seconds then it smooths out and runs great. I insulated the lines from the fuel spider to the carbs and that helped a lot but didn't totally fix my problem.

I've never noticed a high fuel flow, though.
 
Can you download the Dynon data to see what the fuel pressure was during the incidents? Others have had low pressure, high flow, and sputtering engine, all due to bad engine driven fuel pump. Vapor lock is unlikely at 68 degrees F. Another thing to check is carburetor float weight and dirt in the carburetors.
 
Good Morning J3,

You are getting to be one of this forum's best customers! Two questions for you.

1. Since gou bought this airplane as a completed project, do you know if the original fuel pump was replaced by the "improved" version?

2. Does your airplane have an electric fuel pump switch installed, and do you use it?

And a suggestion: in the scenario you described, let the electric pump run a couple of minutes before attempting startup, in order to flush cool fuel from the tank through the system.

Yes, this is sure crazy weather for this time of year. Up here in NY spring-like conditions prevail. Cant fly though, until my carbs come back from Lockwood.:mad:

Best regards,
 
Negative to both questions...

I need to research how to identify old/new mechanical fuel pump. I have been reading about adding a switch for electric pump and don't follow the reason for this. Seems like you would always want the boost pump running. Good suggestion on running the boost for a period of time prior to hot restart. I'll try to remember that during the hot days of summer.

Yes, I have been using this wonderful forum to get knowledge about the RV-12 I purchased flying from the original builder. I appreciate you guys taking me under your "wing" and showing patience / understanding for someone asking questions everyday.

I now have about twenty hours on the 12 and can't even describe how nice it flies. Control harmony, speed envelope, docile handling, ergonomics, safety, operating cost, easy maintenance, visibility extraordinaire, etc...

My wife worked for Northwest Airlines for 17 years and has flown all over the world. I would fly her in my Cub and she'd hang halfway out the door. I flew her in the 12 for the first time last week and she couldn't believe it. We have triplet 7-year-old granddaughters who I have yet to take flying with me. I see lots of adventures ahead for all of us.
 
Piper J3 said:
... don't follow the reason for this. Seems like you would always want the boost pump running.
Click to expand...

I installed a switch so I could turn on the Master in the hanger and turn the clattering fuel pump noise off without having to pull fuse. It's always on in flight ...

Piper J3 said:
... Good suggestion on running the boost for a period of time prior to hot restart. I'll try to remember that during the hot days of summer.
Click to expand...

Unfortunately, that doesn't circulate fuel through the lines from the fuel spider to the carbs. I believe that's where the vapor lock is occurring.

Piper J3 said:
Yes, I have been using this wonderful forum to get knowledge about the RV-12 I purchased flying from the original builder.
Click to expand...

Yeah, this forum is a great resource for experienced and inexperienced alike ...
 
Piper J3 said:
Negative to both questions...

I need to research how to identify old/new mechanical fuel pump.
Click to expand...

IF your pump has no drain tube on the bottom, it is the old style pump. If it has 3 hose connections, it is the new style pump.
 
Pic of newer fuel pump, it's in the upper right hand corner of the pic.;)

dddb1774977a57fc745a792d5158d166_zps70c65351.jpg
 
J3,

I think I mentioned this in a post way back when you first purcahased your airplane, maybe it bears repeating. You may have already done these things, if so forgive my reminder.

The fact that your airplane is a very early serial number, and was flown only a few hours, gives me concern that the previous owner may not have been checking and complying with (when appropriate) Service Bulletins, Notifications, and the like from both Van's and Rotax. Your fuel pump is just one example of an upgrade that probably should have been done. There are many nore. If you have not done so, I strongly suggest you go to Van's website and flyrotax.com and check the lists of all the various bulletins for the RV-12 and the 912ULS. Then physically check the airplane for compliance.

Like I said, if you have already done this forgive my meddling.

John
 
Thanks for concern. I actually began this process a couple of weeks ago. I printed the SB lists from both Van's and Rotax and I'm beginning to work my way through each item making a comprehensive list of when/how completed. Are there any SB's on the prop? Am I missing anything else? Daunting task...
 
Piper J3 said:
Thanks for concern. I actually began this process a couple of weeks ago. I printed the SB lists from both Van's and Rotax and I'm beginning to work my way through each item making a comprehensive list of when/how completed. Are there any SB's on the prop? Am I missing anything else? Daunting task...
Click to expand...

Indeed, there is at least one SB on the prop. Go to the Sensenich site and follow the menus down to service bulletins. Specifically check SB11-04-15, relating to some early hubs that slipped pitch over time. I had one of those. You need to check the numbers on the hub to see if it needs to be returned to Sensenich for replacement.

There was also a change to increase prop bolt torque as I recall. I dont remember if that instruction came from Sensenich or Van's. Check current documentation for present day correct torque.

Yes, this research is a bit of a job, but if you keep good records once it is done, it is done, and your airplane and documentation will be up to current standards.
 
BigJohn said:
There was also a change to increase prop bolt torque as I recall. I dont remember if that instruction came from Sensenich or Van's. Check current documentation for present day correct torque.
Click to expand...

Really? I hadn't seen anything for this. My hub and manual say 20 ft/lbs. Is this correct?
 
f1rocket said:
Really? I hadn't seen anything for this. My hub and manual say 20 ft/lbs. Is this correct?
Click to expand...

Current manual calls for 18-20 ft/lbs. Maybe it did not change, I may be thinking of something else. Its been five years and my brain is much older than that! 🤔
 
I checked today and I indeed have the new style fuel pump. Question - how is the fuel pressure regulated? Is pressure regulator contained inside the mechanical fuel pump? If so, pressure is factory set? Thanks again...
 
BigJohn said:
Current manual calls for 18-20 ft/lbs. Maybe it did not change, I may be thinking of something else. Its been five years and my brain is much older than that! 🤔
Click to expand...
Probably thinking about the Rotax engine mount cap screw torque. There's a `sticky' thread about that issue.
 
Piper J3 said:
Vapor Lock?

........ About five minutes into the flight I got a high fuel flow warning with fuel flow around 12 GPM. High readings remained for a couple of minutes. I slowly reduced power to about 50% and GPM went back to 4.5ish. I gradually returned to cruise power and fuel flow remained fine for the duration of the flight"............
Click to expand...

J3, now that we have pretty much eliminated the possibility of an old style fuel pump as a cause, lets explore another possibility. High fuel flow indications at altitude have been noted several times, if both fuel pumps are running. My particular airplane demonstates this behavior nearly every time I climb above 5000' or so. That's the main reason I installed the electric fuel pump switch. A few years back I worked with Dynon to try and find a cause and fix, without success. There are several threads on this in the forum around 2012. Ultimately, the only way I was able to prevent this behavior was to shut off the electric pump at altitude.

If this happens again, pull the pump breaker and see if the fuel flow indication goes back to normal immediately. Keep in mind, this is an indicator reading issue, and not a safety issue, unless the alarm scares you to death! ��
 
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Fuel flow has been behaving for several flights now, so I'm inclined to think it may be an anomaly. I'm keeping close eye on this.

My concern now is high fuel pressure. I have seen readings as high as 12psi on the D-180 and 6ish for duration of cruise flight. Seems related to throttling from cruise to idle rpm. I have been reading that many others have seen similar. I'm very concerned about the high reading. Perhaps faulty sensor or faulty pressure regulator. I need to install a temporary mechanical pressure gauge and verify readings. From what I have read the mechanical pump isn't capable of developing this high of a pressure.

Forgot to mention that engine starts and runs beautifully.
 
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Heat soak of the carbs is a real problem with under cowl temperatures. The cowl cant just release the heat right away. Before starting your engine when its hot try letting the electric pump run for a bit, this will circulate fuel to the carb bowls and the return will pump out hot fuel (air bubbles most likely) from your fuel pump. (the return line has a small restrictor so it will take a little while)
If you have a re-occuring problem you may what to consider using some 100 LL fuel, or mix some in your auto fuel. The 100 LL is less likely to vaporize like auto fuel.
 
There is no fuel pressure regulator. The fuel pressure is dependent on several factors including fuel return line orifice, fuel consumption, fuel line size, obstructions to fuel flow and etc. The most likely cause of a high fuel pressure indication is the notoriously inaccurate sending unit.
 
Piper J3 said:
I checked today and I indeed have the new style fuel pump. Question - how is the fuel pressure regulated? Is pressure regulator contained inside the mechanical fuel pump? If so, pressure is factory set? Thanks again...
Click to expand...

The mechanical pump has a spring on the plunger shaft. This is what regulates the fuel pressure. You should note that the new Corona pump has higher pressures than the old one marked with A/C. You cant adjust the fuel pressure. If you find excessive pressure be sure that the return line restictor, found inside the banjo fitting on the top of the engine, is not contaminated with debris. If this gets plugged the pressure can go too high and cause some fuel flooding.
 
Eric,

I disagree with your statement that he pump spring determines fuel pressure. The diaphragm pump is a positive displacement pump. In systems with positive displacement pumps the discharge pressure is determined by the system characteristic (resistance to flow) as long as the pressure is less than the stalling point of the pump where it can't move. Unlike centrifugal pumps whose speed varies as discharge pressure changes.

Rich
 
High fuel flow

High fuel flow indications can be caused by air/vapour passing through the transducer. Used to see that on the Shadin in the Twin Comanche when I occasionally forgot to switch tanks and let one run dry. While the engine was windmilling (can be for quite a while with fuel injection) there would be fuel flow spikes. Had my first vapour lock experience with the 12 recently on a near 100 degree day. I only use electric pump for takeoff to safe height and for landing. It happened top of climb at 3500 after heat soaking in the sun for a couple of hours. Couple of stumbles and high fuel flow warning - around 29 litres (6-7 gals). Turned on pump and all good. Now using pump more extensively in hot weather, to top of climb and into cruise until temps settle. Great thing about aviation is that you never stop learning!

Jack
 
In general, I agree with Eric - -

The spring determines the max possible pressure if static. Other factors vary the measured pressure at the output of the pump. Usage, restrictions, etc. This type pump delivers about the same output pressure regardless of fuel usage. If less flow is needed, the diaphragm just moves less distance.
 
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Sorry, John, I don't agree. I'm happy to be shown the error of my ways, but everything I've been taught about positive displacement pumps says they deliver the same volume every stroke so the flow curve of pressure vs. flow for a fixed speed is a vertical line up to the point where the pump stalls. All the spring does is return the diaphragm to the initial position where it begins the stroke.
 
Take one apart - -

The spring actually pushes on the 'dry' side of the diaphragm which creates the pressure. There is only a full stroke with an open outlet. The flow varies only depending upon usage.
 
You do not have to take one apart. Brian Carpenter has already done it for us and made an EAA "Hints for Homebuilders" movie, "Rotax 912 Fuel Pump Overview"
The spring is located on the dry side. But it does not press against the diaphragm, it pushes the rod back into the gearbox. The rod pushes the diaphragm which pushes the fuel. The spring pushes the rod which pulls the diaphragm which sucks more fuel into the pump.
The pump is positive displacement. It can not stall (unless the engine stalls), but something could break.
 
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A difference between the pump described on Wikipedia and the Rotax pump is that the automotive pump shaft is pulled by a cam, whereas the Rotax pump shaft is pushed by a cam. The automotive pump spring pushes the fuel whereas the Rotax pump spring sucks fuel.
At 1:40 into the movie, Brian Carpenter pushes the pump shaft. When he stops pushing the shaft in, the spring pushes the shaft back out.
 
If the external return spring on the plunger shaft drives the suction stroke and there is no relief valve, there has to be another spring under the diaphragm to limit the compression stroke. Otherwise the pump would hydro lock when the fuel demand is satisfied (no fuel flow).
 
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Think I have it now - -

After watching the football game last night, I was able to think about this use of a pump. I have changed my mind on this pump. It is not like the old automotive pumps I used to know. This pump moves the 'plunger' the same amount on every stroke or cycle. The elastic diaphragm controls the flow and pressure. Sorry for carrying this out, but fun to solve things ( even if they are not critical ).
 
Mike,

Extra fuel goes down the return line to the fuel tank. Positive displacement pump curves I'm familiar with show leakage past seals when they reach shutoff head. I agree that something has to give if the discharge flow goes to zero!

Rich

P.S. I like the engineering geek discussions too!:D
 
Fuel pump pressure is controlled by the diaphragm spring. This should not be confused with the external plunger return spring that drives the suction stroke.

The fuel pump plunger on the diaphragm is single action, its only used during the suction stroke, it is NOT used to push the diaphragm during the pressure stroke, the diaphragm spring alone drives the pressure stroke. The pump will only release fuel when the output pressure is less than the diaphragm spring pressure.
 
I agree!;) we are over thinking a simple design. Pump pressure basically changes with engine speed. Higher RPM means cam turning faster means diaphragm moving faster.
 
Agree Joe - -

Thanks for putting the links to these videos. The elastic diaphragm controls the pressure and also the volume of fuel going thru. Fairly simple once you see what they are doing. The material the diaphragm is made of is critical. It has to be soft enough to limit pressure. If less fuel output is being required, the diaphragm just stretches as needed.
 
I concur with your description of the fuel system. BTW I'm the OP. I like your test for a blocked restrictor in the fuel return line. I will check this before the next flight. Appreciate the insight...
 
I got a number of emails from Vans forum members for this post and ask for some pics and insight.

Here is a nice set of pics someone sent me in their email.

http://ctflier.com/index.php?/topic/3431-the-inside-of-the-rotax-fuel-pump/

The return line isn't just for vapor. It will allow fuel or vapor through. It is there to allow cooler fuel to circulate from the tanks through the fuel lines during the engine run to help prevent vapor lock from the engine heating the fuel lines under the cowl. After the engine stops and sits the fuel hose can and does get hot enough to produce some vapor as some see with a low fuel pressure after sitting a while and starting the engine. This should go away as cooler fuel circulates through the system. The restrictor should be a #35 Mikuni idle jet and not a .035" hole. There was a misprint in the manual for many years. It is a very small hole.
If this hose was open and without any restriction the fuel would take the path of least resistance and you wouldn't have any fuel pressure. This
re-circulation line does not control your fuel pressure. What does help control the fuel pressure is the diaphragm tension or tightness. The first pumps had some high fuel pressures. This was caused by the MFG making the diaphragms too tight. This was fixed quite a while back.

The fuel pump does in fact have two springs. One external and one internal. The pump has a double check valve and an internal screen to help filter the fuel before it gets to the carbs. That said if you have an issue after the fuel pump outlet with a hose there is no more filter before entry into the carbs. The pump works on differential volume/pressure inside the pump. This is what operates the check valve as it is free floating with its movement. If there was never a volume or pressure change internally the fuel wouldn't move through the pump. As the pump operates off the eccentric in the gearbox and it pushes on the plunger and pushes on the diaphragm. As the diaphragm moves one way it allows fuel to enter the pump and into the pressure side of the pump. As the diaphragm moves the other direction the check valve slams shut and the fuel is forced out to the carbs. So as one spring pushes the diaphragm one way the internal spring pushes it the other. As the fuel moves from suction side of the pump to the outlet pressure side the check valve opens on one side and closes on the other. If there was no check valve then there would be no pressure.

Believe it or not this operates a lot like your heart with its valves from the atria and ventricles. When your valves (your check valves) in your heart leak because they don't seal you have a heart murmur and leakage. If you have a bad valve (prolapsed valve) then blood moves back and forth, but not through the heart. Then it's time to get your pump fixed and sometimes like our engine, replaced.
 
Good job Mike Miller. According to Sink, you are correct that there is an internal spring between the reciprocating shaft and the diaphragm. The spring must push the diaphragm to pump fuel out and the spring must also pull the diaphragm to suck fuel in. It is too bad that ctflier.com will not allow outsiders to view the fuel pump pictures.
 
take one apart

John, I did take my old pump apart. I agree that at least our original fuel pump is not a positive displacement pump, has a telescoping plunger, has two springs- the lighter one to control pressure. What we don't know if our new replacement pumps are made the same way.
 
Agree John - -

I would call this a variable displacement pump. The internal spring controls pressure. The plunger moves the same amount each cycle, but the diaphragm only moves as much as needed to supply fuel.
 
This doesn’t fit with the vapor lock discussion but I thought I’d post here…

I had high fuel pressure alarms back in October - December and traced the problem to electrical contacts on the fuel pressure sensor. I cleaned the contacts and tightened the “pinch” on the spade lugs. Problem was solved for a dozen flights and fuel pressure readings were very consistent ~ 6psi.

Airplane has been hangered for three weeks because of weather. I flew yesterday in 30F temperature and my fuel pressure alarm problem has returned. I saw readings that spiked near 13psi with a lot of fluctuation. Toward the end of the flight the pressures settled at ~ 6psi. I’m pretty sure this is a false error. I don’t see any indication of fuel overflow at the carbs and the engine is running flawlessly. I think my next step is check for intermittent ground. I may run a new ground wire to the engine crankcase and also to the fuel pressure sensor to see if that helps. I'm running 93 octane E10 exclusively. New fuel pump 893110 S/N 13.4493 was installed a year ago.

My wife isn't too thrilled when the alarm keeps going off...
 
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Hey Jim - -

Since 'new', I think all the electrical issues I have had were involved with connections. I went back over ever crimp, and used two hands to re-crimp them until I felt I was going to destroy them. I also added a very small dab of Vaseline on the connectors. It spreads on its own and stops corrosion. I almost never have such problems now. It has worked for me. I also changed some of the connections to Delco weather-pack connectors. Much easier to disconnect when needed.
 
^^^

Experience here is in agreement with John's post. All my early spikes and alarms were from poor electrical connections. If you still have the original style fuel pressure sender on your plane the spade connectors can be replaced by ring terminals. Doing this solved my fuel pressure spikes.
i-XfDz7K2-M.jpg

The sender has screws for the ring terminals. The spades are probably OK for analog gauges but the EFIS is very sensitive to spikes that you would likely never see on a gauge.
John also suggests something to mitigate corrosion. Here we are using DeOxit.
https://www.radioshack.com/products/deoxit-d5s-6-spray-contact-cleaner-and-rejuvenator?variant=5717849605#.VYAe92AdL8s
We call it the "Magic Elixir". I give all my slip-on and screw-on electrical connections under the hood a little shot of this when I have the cowls off. It is a clean spray and doesn't leave a mess or pick up dirt.
 
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