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Low fuel pressure

brayski98

Well Known Member
I have an RV-6A with an O-320. Yesterday I went to go fly and after starting engine started, I turned off the electric fuel pump and the fuel pressure went to basically zero. The EIS gave a warning and the fuel pressure was between zero and 1. When I turned the electric back on it went to about 5 PSI. I taxied out and got gas and both fuel pumps seem to be working. I flew with no issues. I switched tanks and chalked it up to the fuel selector valve. But I had the same problem today. Switching tanks does nothing and with the electric pump on it?s fine. So to me everything points to the mechanical fuel pump. But looking for opinions from others. 2 fuel filters and no gascolator. Engine has just under 1000 hours on it and the plane is 25 years old. Thanks.
 
I have an RV-6A with an O-320. Yesterday I went to go fly and after starting engine started, I turned off the electric fuel pump and the fuel pressure went to basically zero. . . . .When I turned the electric back on it went to about 5 PSI. I taxied out and got gas and both fuel pumps seem to be working. I flew with no issues..

Check for a bad ground on the FP sender or perhaps you have done some rewiring that has caused a ground loop. Those circuits employ tiny currents. A slight change in ground potential between the sender and gauge will make the indications crazy and erratic. Sometimes they will look normal, other times what you see.
 
If the fuel pump is 25 years old, I'd change it before flying again. Lots of posts and threads about diaphragm material breaking down much before that time. Even if it is one third that old I'd change it. There is a long thread on changing pumps here.
 
Low fuel pressures

If the fuel pump is 25 years old, I'd change it before flying again. Lots of posts and threads about diaphragm material breaking down much before that time. Even if it is one third that old I'd change it. There is a long thread on changing pumps here.

Thanks. I plan on it. Took the old one off today but wanted to make sure I am not over looking another issue
 
The fact the electric pump brought pressure back up to norm pretty much rules out a sensor problem. The new mechanical pump will most likely return good pressure readings.
 
Per Lycoming

If not alarming in cruise ? per Lycoming ..... .05 psi seems to be acceptable. Per an A&P friend, takes little to no pressure for a carb engine 6A. Carb is nearly downstream/gravity. You are keeping a bowl and float happy. I was getting errors on departure of questionable flow. I changed out pressure sender and all’s good. 1000 since new with an original pump might warrant a change, but if nothing coming out of “witness hole”, is it really broke?. I would be surprised if the diaphragm has gotten weak and no longer develops needed pressure. I’m not an engine guru, but I am logical. The above is my logical view. Good luck. Maybe reduce alarm threshold to less than .5 and fly. No issues at cruise, electric pushes things up....just an option.

MAX 8. DESIRED 3. MINIMUM .5 Aka 1/2 pound

The above is from page 27 of Lycoming O320 operators manual.
 
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Just replaced FP sender

My FP sender was less than a year old (as well as my mechanical fuel pump) and was chasing erratic fuel pressure readings that would get better with one tank or with the electric pump on. On the ground everything would test out fine. GRT sent a replacement sender and all is good. I know your frustration!
 
Fuel pressure sensors do go bad. I had symptoms like yours and traced it to the sender. Cheap replacement I would start from there before changing mechanical pump.

 
I second Vlad's comments.

Here were my symptoms. I would pass 1000' AGL and turn off the pump while still climbing at full throttle. The SkyView would complain about low fuel pressure and when I would look at it, it would be reading 0 to 1 PSI. Turn on the electric boost pump and it would come right up.

Here's the thing, I could continue to climb at full power with the boost pump off and the PSI reading zero. I'm talking about a Vx climb under full power with my O-360 and the fuel flow reading over 15 GPH and he engine never stumbled.

It became pretty obvious that the issue wasn't the fuel pump.

The first thing I did was to remove the wires at the forest of tabs and at the sensor and sand them down with some 1500 grit sandpaper. This did not solve the issue.

Three days ago I replaced the fuel pressure sender and now have 1.3 hours of hard climbs without the boost pump on. So far, so good. No more low fuel pressure warnings.

At the same time this was going on, I also was getting an occasional high oil pressure warning when I would throttle back. Needless to say, that sender was also changed at the same time.

I was able to locate both VDO senders on Amazon for a few bucks cheaper than any of the normal aviation sites.
 
Similar vintage - it was the fuel pump...

I had the exact same symptoms in a very similar vintage engine. I replaced the fuel pump and all is well again. Thankfully, they are a number of threads on how to do this properly. The lacing to hold up the pin method worked for me. Thanks to all that previously posted.

Cheers, Sean
 
All fixed now

Well I went ahead and replaced the fuel pump today. It?s not a fun job as you can imagine but I will survive. Pulled it out and did a test run and low and behold, I have fuel pressure once again. A little lower than before but it read 4 or 5 psi. Much better than reading zero. So I think it?s all fixed. Just finished safety wiring it. (Yet another pain). I couldn?t do too many of these without a blood transfusion. I will cowl it?s up and hope to fly in the next couple of days.

Thanks everyone for your help. Hopefully it will last another 25 years.
 
fuel ??

I Can't believe this thread is here now as the last three days my RV-6A 0-360 550 hour 2 mag cs has been acting up
Take off climb out and climb to 8,000 feet normal. No boost on at all. Get to 8,000 feet or so climbing full throttle 2200 rpm leaned as much as possible 100 knots steep deck angle. engine starts to stumble and would quit if I let it. Now if I put on boost pump engine regains composure after a second or two. repeatable situation. most times. nose down anytime and it runs great all the way back home. Maybe it's homesick.
But level off engine smooth, pitch up keep climbing 12,500 no problem.
Thought it to be engine pump, still sorta stumped. Gonna pull cowl for pump inspection. Crazy some of us have these problems at same time. Maybe it's got a Corona virus. Or maybe the Costco premium has the virus.
Art
 
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