HELP - Rotax 912 100hp low fuel pressure at cruise rpm ...
We just picked up a new Pipistrel Virus SW from Moriarty NM, 6 hours on the hobbs. 3 hours into the trip to Canada we noticed low fuel pressure - 0.05-0.08 bar showing on the Dynon @ 5200 rpm, 0.1 at 4800 rpm. We diverted to Pampa TX to look into it - on descent into Pampa (idle) we saw the pressure back up to about 0.3 bar (right in the middle of the "ok" range" 0.15-0.40).
I think the possible causes are: a) bad pressure sender b) restricted fuel filter (?) c) bad fuel pump.
We got a fuel pressure gauge from an auto parts place and the plan tomorrow morning is to tee it in to the line beside the sender and verify the pressure reading both at idle and during run-up. That should narrow it down to (a) or (b/c). Then we can insert it before/after the fuel filter to check for any restricted flow there.
As far as I know this installation does not have a boost pump - it's gravity fed from the wings to a gascolator low on the firewall then to the engine-driven pump. This is a factory-new plane.
Any suggestions, advice or help (Rotax Mechanics in the area?) is greatly appreciated.
Check the actual connections to the sender. I've seen the wires cable tied too tight such that the wire to the connector breaks underneath the insulation. Along with this check for a bad ground. Next likely is the sender itself.
My PiperSport had a similar problem. It turned out to be a little water in the line to the sensor.
Thanks for the help. We have not resolved the issue 100% but we did satisfy ourselves that it was a measurement issue, not an actual low fuel pressure. We did a couple of full power climb tests over the airfield in Pampa TX where we stopped to check it out and although the fuel pressure showed extremely low the engine exhibited absolutely no signs of running lean at all. There was no fuel leakage from the drain hole in the fuel pump and no other visible signs of a problem.
When we observed the fuel pressure go from normal - then very low, then back to normal within 10 minutes at cruise with zero changes in engine parameters we concluded that it had to be a measurement issue.
Water in the sensor line is on the "to-do" list to check along with all sensor wiring back to the EFIS and ground before replacing the sensor itself. I guess removing the sensor and letting the line bleed a bit with the tank head pressure should clear out any thing that ought not to be there. We did not observe any unusual readings in the second half of the trip - so maybe any contaminants in the sensor line has bled itself out?
P.S. total flight time from Moriarty NM (just east of Albuquerque) to Kitchener, Ontario Canada was 12.6 hours - with fuel burn averaging around 4.3gph - and indicated airpseed of 130-135kts.
Two different aircraft have been brought to me recently with the problem you describe. In each case the problem was with the VDO fuel pressure sending unit. Once replaced normal fuel pressure was presented.
Ed Snyder LSRMA
Originally Posted by njones
replacing the sender - thread selant at all?
I have one of the 2-bar automotive type fuel pressure senders in my RV-8. I noticed the fuel pressure on the EFIS was '---" which signals either bad connections or a bad sender.
The sender is screwed into a manifold block. My understanding is that this is supposed to provide the contact to ground.
But I also am concerned about thread channel leakage. Should I use thread sealant (e.g. permatex #2) on the final threads of the sender? Or should it be installed without any sealant whatsoever?
I am struggling with the same issue and I think I may have found the answer:
Originally Posted by njones
I have a low wing RV-8. The pressure starts out ok and gets lower as time goes on.
I discovered that the tank air vent nut is loose on the inside of the tank. The tank is not re-pressurizing as fuel is removed. When I fix this I'm hoping the low pressure issue goes away.
To check this, I attached a flexible tube tot he tank vent input, put my fingers on the tank vent exit which is inside the tank near the gas cap. When I blew air through the tube, I felt both air and gas.
I then put my ear on the wing right over the point where the tank vent fittings through the tank wall are located. As I blew air into the vent I could plainly hear lots of lovely bubbles being made.
Also, if your vents are clogged, you'll get the same result.
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