Fuel Pressure

Scott Hersha · #1 03-06-2009, 07:56 AM

Question for A&P's, engine gurus, gear-heads - What is normal fuel pressure limits for the engine-driven fuel pump on an O-360-A1A with a carb? I set my warning limits on the EIS on my former RV-6 (N601RV) at 8 psi for the upper limit, and the owner is seeing 8.1-8.2 and a warning light. I think this is OK, but not sure, and I don't know what the upper limit would be. He can re-set the warning light threshold to get rid of the light, but we need to know what's normal. I replaced the fuel pump and required by and AD a few years ago, before I sold it, but don't think I ever had this warning. It is now based in Reno, so is normally flying at a higher altitude - not sure if that affects it or not. I would check my Lycoming overhaul manual if I was at home, but I'm stuck in NJ for a few days and would appreciate any advice/info.

Thanks,
Scott

rocketbob · #2 03-06-2009, 08:18 AM

The range is 0.5-8.0 psi. A blocked crankcase vent can cause the pressure to go up, since the fuel pump is vented to the inside of the crankcase. If the fuel pressure is too high, fuel will get past the float needle and flood the carb.

Sam Buchanan · #3 03-06-2009, 08:22 AM

Quote:

Originally Posted by Scott Hersha

Question for A&P's, engine gurus, gear-heads - What is normal fuel pressure limits for the engine-driven fuel pump on an O-360-A1A with a carb? I set my warning limits on the EIS on my former RV-6 (N601RV) at 8 psi for the upper limit, and the owner is seeing 8.1-8.2 and a warning light. I think this is OK, but not sure, and I don't know what the upper limit would be. He can re-set the warning light threshold to get rid of the light, but we need to know what's normal. I replaced the fuel pump and required by and AD a few years ago, before I sold it, but don't think I ever had this warning. It is now based in Reno, so is normally flying at a higher altitude - not sure if that affects it or not. I would check my Lycoming overhaul manual if I was at home, but I'm stuck in NJ for a few days and would appreciate any advice/info.

Thanks,
Scott

The low-pressure senders used on many of our engine monitors are well documented to sometimes be less than totally accurate. I would consider the sender as a possible suspect if the plane has a long history of reliable fuel pressure.

videobobk · #4 03-06-2009, 04:59 PM

I fought this problem a little over two years ago (just before my first flight.) I had five A&Ps tell me a fuel pump couldn't fail to a too-high situation, but mine did. It overflowed the carb. The fuel pressure will never go higher than what it takes to force fuel past the float valve, something around 9psi, at least on my carb. We replaced the float, float valve, everything. The only thing that worked was bypassing the fuel pump and using the electric pump. I replaced the FP and all was well--and has been for over two years. DO NOT overlook this. Check with a calibrated gauge. An overflowing carb is bad enough on the ground.

Bob Kelly