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RV-12 fuel anomaly

J

JRo

Well Known Member
My RV-12 has a switch for the fuel pump. I use the electric fuel pump for start, take-off, climb, descent & landing. Returning from last X-C with electric pump off in cruise, I noticed low fuel pressure (wiggling between 1.5 psi and 2.5 psi, bottom of green set at 2.2 psi). Engine running normal. Switched on electric fuel pump, fuel press instantly returned to green, fuel flow rose into the red ( 11 gph), smelled fuel. Then within 7-8 seconds, fuel smell rapidly dissipated, fuel flow returned to normal (5-6 gph), fuel pressure remained steady in the green. Since I had plenty of altitude, returned home using airport-to-airport routing, switched pump off again. Fuel pressure dropped again, flirting continuously with the lower yellow arc. Engine ran normally.

Spent the day today opening up, inspecting, tightening. Found no leaks. Could not duplicate with electric pump on, or even in a ground run. Test flew in the pattern; same scenario over again: Low fuel pressure with pump off. Momentary spike in fuel flow and smell of fuel momentarily when pump switched on, then normal.

Anyone seen this one?

Thanks,

Jim R.
 
Sounds like your mechanical fuel pump diaphragm is leaking. Is there any evidence of fuel at the weep hole (old design) or run off tube (new design).
 
Is this one of the older AC or Pierberg pumps? If so looks like its time for a new pump. Do you have a check valve inline? You may need to check it.
 
from the thread starter:

No. No sign of leaking at all. This is a brand new RV-12 and engine. 81 hours on the tach. Engine came from Vans last summer, first flight was 9/13.

I suspect the mechanical fuel pump as well. But wouldn't that be almost out of the question for such a new rig?

Please tell me the RV-12 isn't a problem child. It took me almost 2 years to build. I just wanna fly now....
 
I agree. Roger is certainly an expert. I wouldn't trash the ROTAX based on one part failure. These little engines seem to be reliable little beasts that will bring you home.
 
I agree with RFSchaller. Others have experienced the same symptoms, high fuel flow and low pressure caused by a bad engine driven fuel pump. Pumps can fail at any time, even if relatively new.
Joe Gores
 
Jim, Got your phone message but too tied up to return call. Advice above is good. I had to replace my pump early on too. It was the old style. Does your pump have the overflow tube?
 
Yes, my pump is apparently the "new style" as there is an aluminum vent or drain tube leading down & back from the engine-driven pump. How does one go about submitting a warranty claim on a Rotax part? Where do I have to go to get a warranty part/labor. Fuel tank, rear window, landing gear attachment, faulty ignition wiring, bad radio antennae connection, crimped instead of soldered CHT & oil temp pick-ups, insane warm-up times, now a bad fuel pump? This RV-12 is beginning to seem like a bad choice. I don't wanna work on planes; I wanna fly them!

Jim R.
 
Jim there is an easy fix for your problem even though you might not like it.:( Simply leave the electric fuel pump on all the time like the plane was designed. 550 hours on mine with electric pump running continuously with no problems. Later after you just fly some and enjoy the plane you can go back and trouble shoot the problem when you have more patience, sounds like you maybe suffering from builder burn out.:rolleyes:
Get a good night sleep and take a break from all the fixing for a while. Good luck.:)
 
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Jetguy said:
Jim there is an easy fix for your problem even though you might not like it.:( Simply leave the electric fuel pump on all the time like the plane was designed. 550 hours on mine with electric pump running continuously with no problems. Later after you just fly some and enjoy the plane you can go back and trouble shoot the problem when you have more thpatients, sounds like you maybe suffering from builder burn out.:rolleyes:
Get a good night sleep and take a break from all the fixing for a while. Good luck.:)
Click to expand...

I'm with you Jetguy.....the RV-12 (as designed) was to be flown with electric fuel pump "on". For you guys that for some reason think it is better to switch it off.....I have question? What kind of pressure drop is caused by the pump when not running? Could this be a problem. Jim, did you bypass the pump with a one-way check valve to eliminate said (presumed) pressure/restriction drop?

As Scott has pointed out.....using auto fuel (which this plane is meant to burn) brings varying problems with vapor point in gas which can lead to engine outages....hence in the -12 the electric fuel pump pushes fuel to the engine driven pump, eliminating this potential problem. Shutting the pump off IMO is like playing Russian roulette.
 
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When you have more patients, you will have more income. Then you can buy more gas... then you can fly more. Now, are you a psychiatrist, or just regular doctor?
Seriously though... have the fuel pump checked. Newness doesn't mean anything in the mechanical world. Stuff breaks. Stuff needs adjusting. Flying comes after all that... so that you can live another day. To have more fun and more patients.
 
My non-Rotax 2 cents and experience with my single Bing carburetor engine:

I just completed my first condition inspection. In addition to the fuel pump advice I would like to add that I discovered my carb bowl gasket was pinched. If the gasket is pinched or twisted, that may be the source of your pressure drop and fuel vapor leak...just a guess.

Regards,
 
Thank you John; good advice. Builder burnout no doubt. And I, too, consider the plane flyable. But flying at altitude with the electric pump off is the only way to detect a problem with the mechanical pump. I originally installed a switch for that reason, as well as for the fact that the only way not to listen to the poor little electric pump struggle whenever the master switch is on is to pull the fuse, at which point other critical items are lost (such as the electronics cooling fans). I also read on this blog that fuel flow readings are more consistent and accurate with the electric pump off. And wouldn't I be sacrificing redundancy as well?

Also sounds as though enough folks have has issues here that perhaps Rotax should be addressing a pattern of failures...?

thanks again,

Jim R.
 
There is another way to determine if the engine driven fuel pump is working. Before engine start, note the fuel pressure with electric pump running. Then start the engine and watch for a fuel pressure increase.
Joe Gores
 
Me too!

I have the same symptoms except for the gas smell. I posted on an other thread the following, without success:

My fuel pressure runs around 5 psi, at the top of the green and close to the yellow. I installed a switch to cut the electric fuel pump, mostly for not being bothered when I need the master on while in the hangar. A couple of time I cut the electric fuel pump in flight and was scared to see the fuel pressure drop to between 1 and 2 when operating with only the mechanical pump. I did not see any change in RPM (all this checked with savyyAnalysis) but it scared me enough to leave the electric pump on in flight. Note that with the engine off the pressure from the electric pump reads 3.7 psi, consistent with the mechanical pump of 1 to 2 for the total around 5. My question: is it normal to get such a low pressure from the mechanical pump alone? I don't remember seeing this behavior in the Sportcruiser in which we systematically stop the electric pump outside the take-off or landing phases.
__________________

Clearly things are more complicated than just a mechanical pump failure. Indeed with the electric pump providing 3.7 psi, a higher fuel pressure than 1 or 2 by the mechanical pump would put the fuel pressure in the red or beyond.
I now fly confidently with the electric fuel pump on... although I would like to have an explanation about what's going on!
 
1 psi is equal to about 3 feet of head pressure in a gravity fed fuel system. The fuel pressure transducer used in the RV-12 is not accurate. If the pressure goes up when the electric pump is turned on and then goes up some more after engine start, then I know that both pumps are working. But I have little faith in the actual fuel pressure numbers.
If the pressure goes down for no apparent reason and the fuel flow goes up, that is a symptom of a leaking diaphragm in the engine pump. Not only can fuel leak out, but air can leak in.
Joe Gores
 
from thread starter:

Rotax says the minimum fuel pressure is 2.2 psi with engine running. Mine is 1.5 psi. I consider that unacceptable. As for the advice of ascertaining that the mechanical fuel pump is operating after start; that's fine. But running with the electric pump off is the only way to detect a malfunction of the engine-driven pump in flight. I flew this plane 90 hours with the electric pump off in cruise and a very satisfactory fuel pressure. This occurrence saw a rapid pressure drop to 1.5 psi. So I can't be convinced that nothing happened.

Being simple, I don't understand the remark at all about psychiatry. As for being tested, let me assure all that in 18,000 hours I've been tested plenty; more than you'll know. If anyone would like to return to the mechanical problem at hand, I'm most interested in fuel system advice.

Thanks,

Jim R.
 
My inclination is to go with Joe Gore, suspecting the pressure sensor for the following reasons:
- The drop in pressure occurred twice in flight, down to 1.5 and back to normal after switching the electric pump on.
- We had a 1 hour flight recently with only the mechanical pump at a steady in the middle of the green, 3 to 4 psi
- There is something funny going on with the sensor as the pressures of each pump alone do not add-up when both work, otherwise I would be in the red high constantly
The difference I have with the thread issuer is I never smelled any gas odor but the other symptoms look the same.
 
The location of the fuel pressure sensor is roughly at the top of the fuel tank, so elevation head should be zero (the sensors do have a minimum reading >0)and any pressure indication above the sensor minimum should be due to the pump(s). I've flown plenty of aircraft without a fuel pressure gage relying on a switched boost pump if things went bad. I don't see much value in the pressure sensor or the fuel flow sensor. They seem to provide more spurious indications than warnings of imminent failure. (My flow sensor sent me on one wild goose chase.). I know many will scream "heresy!", but it's my takeaway from all the postings. I check my old style mechanical pump weep hole on every preflight, and I think it will be my first indication of a deteriorating pump diaphragm. I put a switch on my electrical pump.

I know many will not agree with my reasoning, but that's what makes America great! :D
 
For those who have got the fuel pump switch here is my take about these postings:
For the new mechanical pump which have an aluminum fuel line on the weeping hole it is not easy to find out if there has been fuel dumped in small quantities. However we need to have an easy way to check the mechanical pump on a systematic basis. Consequently I am going to do my engine start, taxiing and run-up with the electric pump switched-off and monitor the fuel pressure as well as the drop in RPMs. I then will switch the fuel pump on for take-off, checking the little bump in pressure and leave it on during flight. Just a few small changes in my checklist that will bring peace of mind in flight.
I see two benefits of the fuel pump switch under these conditions:
- potential detection of mechanical pump failure
- less fumbling with the fuel pump fuse when working in the hangar as this fuse is not intended at being used as a switch
I also believe that VANs ought to go this way for the E-LSA (S-LSA guys don't mess with their plane in the hangar :D)
 
WingedFrog said:
For those who have got the fuel pump switch here is my take about these postings:
For the new mechanical pump which have an aluminum fuel line on the weeping hole it is not easy to find out if there has been fuel dumped in small quantities. However we need to have an easy way to check the mechanical pump on a systematic basis. Consequently I am going to do my engine start, taxiing and run-up with the electric pump switched-off and monitor the fuel pressure as well as the drop in RPMs. I then will switch the fuel pump on for take-off, checking the little bump in pressure and leave it on during flight. Just a few small changes in my checklist that will bring peace of mind in flight.
I see two benefits of the fuel pump switch under these conditions:
- potential detection of mechanical pump failure
- less fumbling with the fuel pump fuse when working in the hangar as this fuse is not intended at being used as a switch
I also believe that VANs ought to go this way for the E-LSA (S-LSA guys don't mess with their plane in the hangar :D)
Click to expand...

The check list in the POH supplied to all E-LSA builders and with all S-LSA airplanes already has a check process.
After master switch on, the pilot is direct to verify fuel pressure in the green from just the electric pump.
After engine start, the pilot can note a slight jump in fuel pressure because of the engine driven pump.
 
I installed the new fuel pump with a short section of clear tubing directly attached to the pump, before transitioning to black tubing via a double-barb connector. ELSA.
 
rvbuilder2002 said:
...
After engine start, the pilot can note a slight jump in fuel pressure because of the engine driven pump.
Click to expand...

This is a test on a cold engine at low rpm. I like better the test during run-up at higher rpm and engine warmed-up some... although I admit not knowing anything about mechanical fuel pumps failure modes :confused: just a gut feeling!
 
Scott, I understand the logic behind the startup check, but a failure in flight of the mechanical pump would not be identified promptly. Switching off the electrical fuel pump makes it obvious when the mechanical pump fails.

Richp
 
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