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albertaflyer

Active Member
Just wondering what would cause a fuel pressure drop. I have an o360 with the stock mechanical pump and a faucet boost pump. What would cause it to lose pressure (down to .5 psi) even on the slightest of nose up attitude (without boost pump on)? Even with full fuel it still will drop unnervingly low! If i turn the boost pump on it will climb back up to about 5 psi. The engine has never stuttered or missed but I would just like to know what may be causing this. There is a tee after the mechanical fuel pump, one goes to carb the other to a remotely mounted sending unit on firewall... Any ideas? Is it because the mechanical pump is sucking the fuel from quite a distance and through quite a few restrictions (fuel selector, elbows, fuel pump that is not on)
 
not much help here -

But only in nose-up attitude? I think I would first verify the fuel pressure with an independent pressure guage. If the pump is good, it should have steady pressure, even with a "slight nose up attitude". Are you venting fuel overboard in climbs through the pump drain hoses? I would think that if the boost pump allows pressure to run normally, then it may probably be the mechanical pump.
Did this problem start after recent maintenence? (just asking), or did it come on by itself? Also, just for troubleshooting purposes, are you using a "rubber" based fuel hose like 701, or 303? If so, might possibly have a liner restriction, but I doubt it, since the boost pump stabilizes the pressure.
Some thinkgs to think about. Hope it helps-
Tom
 
Without telling you to ignore what you're seeing (there could always be something unique about your situation), this has been extremely common among carbed engines in RV's ever since people started using electronic instruments (and maybe before - I don't know). It might have to do with the positioning of the fuel flow transducer up high on the firewall, with the Carb down low - that's a significant loss of head pressure.

Whatever the cause, many of the carbed RV's I have flown show almost no fuel pressure with the engine running at a good clip (climb, or high-throttle cruise). The engines all run fine. I often see less than 1.0 psi when the engine is doing just great. 1450 hours, and it hasn't quite yet due to lack of fuel pressure.

I'd love to see someone definitively study this and come up with a reason....but you are not in a minority here!

Paul
 
My 7A with carbed 0-360 did the same thing. Van's aslso confirmed this as being quite typical. Any upward attitude will cause a pressure drop as the fuel will need to be lifted further as a result of the change in angle. Carbed Lycs only need 0.5 psi to operate. But as Paul recommended, check all the filters and screeens and look for any restrictions. As part of my annual, I blew back, with air, from the gascolator to the tanks to make sure there was no blockage, and I also did a volume test using the electric fuel pump to pump a timed amount to calculate how much the pump was delivering.

Roberta
 
I biased my Rocky Mountain engine monitor by +1 PSI since I got tired of the flashing 0 (zero) PSI in flight.
 
Fuel pressure

Had Piper warriors do that in Alberta during real cold weather. The mechanical pump would freeze up in climbs. Turned the boost pump on and back to normal pressure. Don't think it's been that cold in southern alberta this year though to cause that though.
 
My injected IO-360 has been doing this off and on for 900 hrs. Unnerving, but engine has never missed a beat as a result.
 
Same

O-360 A1A, 350 hrs and it has often gives me a low pressure warning when in a climb without the elect fuel pump on, the engine continues to run great. My GRT engine monitor is set for a warning when it goes below 1 psi. I have also read Lycoming's statement that only 0.5 psi is needed so it is very possible I am still good but need to figure out how to change the limit to 0.5 psi.
 
I had the same problem with my O-360 in my Piper Archer. I replaced the mechanical pump 6 months ago and the problem is gone.
 
Wow... That's why I like this forum! Ask a simple question and you get a handful of answers! Thanks a bunch. I have checked everything except the mechanical pump. I didn't think it could be the pump because as long as I'm flying level or nose down, it's fine. But I will have to check it now for peace of mind. Thanks again!
 
I'll just chime in with another confirmation that this seems quite normal; I am also a new 6A driver with a carbed A1A, and I have also seen the pressure drop from its usual 5 psi down to much lower levels, sometimes without any apparent reason. As others report, the engine continues to run happily, and if it really bugs me, turning the boost pump on briefly restores the normal reading. Fortunately, I was checked out by a very experienced RV builder/flyer who warned me in advance that this would likely happen (it is apparently that common an occurrence) so it didn't create too much consternation for me the first time I saw it. If you've checked your filters, etc. I think you can relax and chalk it up to a little RV eccentricity:)
 
albertaflyer said:
Wow... That's why I like this forum! Ask a simple question and you get a handful of answers! Thanks a bunch. I have checked everything except the mechanical pump. I didn't think it could be the pump because as long as I'm flying level or nose down, it's fine. But I will have to check it now for peace of mind. Thanks again!
Click to expand...

As has been stated many times in many threads (search "fuel pressure") the fuel pressure senders we use often have problems with consistent readings at the low pressure a carbed engine needs as pressure altitude changes with climbs/descents. You don't want to ignore low readings without prudent investigation (I had a mechanical pump go bad) but once everything has checked OK the sensor will most likely be the culprit.
 
fuel pressure drop

Three minutes into the flight, my friend said, your fuel pressure just went to zero. Pucker factor went to 10! Engine run O.K., did not show any signs of trouble. Electronic checks on fuel gauge and fuel transducer, showed nothing unusuall. Figured the fuel transducer might be faulty, decided to take it off. As the fuel line was loosened, air came out, then fuel. Bled the line more, then hooked transducer back up. Fuel pressure on the gauge came up to 4 LBS. All is well again. Don't know what caused the problem. Hope this might help with someone who has had this happen.
 
Low Fuel Pressure Issue

Lots of good suggestions here, but here's another. Check the fuel feed lines for air leaks. The mechanical pump on the engine won't pump properly if air has been introduced into the fuel lines. Check all AN connections, the gascolator, AND the fuel selector. I've seen versions of Van's fuel selector that leak air past the valve stem seal. (Easily fixed with fuel lube...).


albertaflyer said:
Just wondering what would cause a fuel pressure drop. I have an o360 with the stock mechanical pump and a faucet boost pump. What would cause it to lose pressure (down to .5 psi) even on the slightest of nose up attitude (without boost pump on)? Even with full fuel it still will drop unnervingly low! If i turn the boost pump on it will climb back up to about 5 psi. The engine has never stuttered or missed but I would just like to know what may be causing this. There is a tee after the mechanical fuel pump, one goes to carb the other to a remotely mounted sending unit on firewall... Any ideas? Is it because the mechanical pump is sucking the fuel from quite a distance and through quite a few restrictions (fuel selector, elbows, fuel pump that is not on)
Click to expand...
 
Recovery?

agough277 said:
make sure you use boost on take off. My new o360 quit with full power on take off (no boost).
Click to expand...

Were you able to get the engine back by flipping on the boost pump?
In my Rv7 the sensor is about 2 ft above the carb. If the line were filled with water (heavier than gas) the pressure at the sensor will read about 0.8 psi less than at the carb. So for gas it probably is 0.5 psi lower reading. If that line were not primed and left filled with air, I am thinking it will read the pressure more accurately. My physics may be wrong here, comments? If the line is primed, will an increase in G force cause a low reading as well?
 
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