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Intermittent Low Fuel Presssure

C

ChrisMcC

Member
My fuel pressure is dropping (<12 psi) approximately every 30-60 minutes in flight. I can turn the boost pump on and it returns to normal (25-30 psi). It’s not just a bad sensor, because the engine has stumbled before when this happens. This is a fuel injected Aerosport Power IO-375 with 400 hrs.

I have done the following troubleshooting:

1. Checked every fuel fitting dozens of times. No loose fittings or fuel leaks observed.
2. I talked to Tempest at OSH and they said, “Our pumps done fail like that. They work or they don’t.”
3. Cleaned and inspected the fuel servo filter. No issues found.
4. Cleaned and inspected the AFP filter. No issues.
5. Talked to Don at AFP and he said, “Turn the fuel selector valve off put 10 PSI air pressure at the firewall fitting. Bubble check all connections going back to the selector valve.” No leaks found.
6. Both tank vents are clear and working.

This seems to only happen when the tanks are full. But it also happens on both R and L tanks making me think the issue is somewhere from the fuel selector forward.

Before I start replacing parts, what else can cause this?

Thanks,
Chris
 
I had that those same symptoms in my IO320 a couple of years ago. It was a failed engine-driven fuel pump (second one in 400 tach hours).
 
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Experienced this on a friend’s new RV last week. Turned out the fuel vents were cut square with a tubing cutter and not cut on a 45 degree angle to produce a little ram air pressure in the tanks. Issue resolved.
 
ChrisMcC said:
SNIP

2. I talked to Tempest at OSH and they said, “Our pumps done fail like that. They work or they don’t.”

SNIP
Click to expand...

This is false - there are numerous threads detailing exactly your symptoms that have been rectified by a new engine fuel pump. I'm at a loss to explain why, but the observations are hard to argue with.

How many calendar years old is your pump?

Prudence says to replace every 5 to 7 years or so.
 
Thanks everyone. The airplane has been flying for 3 yrs, 9 mons. It would be a shame if my fuel pump only lasted that long.

My vents are cut at 45 degs.
 
Ordering pump now. Can someone please suggest what sealant to use for the crank bolts that secure the pump and the fuel line threads?

Thank you!
 
ChrisMcC said:
My fuel pressure is dropping (<12 psi) approximately every 30-60 minutes in flight. I can turn the boost pump on and it returns to normal (25-30 psi). It’s not just a bad sensor, because the engine has stumbled before when this happens. This is a fuel injected Aerosport Power IO-375 with 400 hrs.

I have done the following troubleshooting:

1. Checked every fuel fitting dozens of times. No loose fittings or fuel leaks observed.
2. I talked to Tempest at OSH and they said, “Our pumps done fail like that. They work or they don’t.”
3. Cleaned and inspected the fuel servo filter. No issues found.
4. Cleaned and inspected the AFP filter. No issues.
5. Talked to Don at AFP and he said, “Turn the fuel selector valve off put 10 PSI air pressure at the firewall fitting. Bubble check all connections going back to the selector valve.” No leaks found.
6. Both tank vents are clear and working.

This seems to only happen when the tanks are full. But it also happens on both R and L tanks making me think the issue is somewhere from the fuel selector forward.

Before I start replacing parts, what else can cause this?

Thanks,
Chris
Click to expand...

A remaining mystery is the full tank observation. You said "seems" to only happen with full tanks. This would be counter to a flaky fuel pump, one would think.

If you do choose to change the pump, this thread has lots of good information, in case you haven't seen it already: https://vansairforce.net/community/showthread.php?t=15148
 
I have an Rv7a with an xpio360 engine. I have seen fuel pressure dropping below 20 about once every 5-10 hours. It goes back up on its own, or immediately back up with the boost pump. The engine never stutters.

I replaced the fuel pump at about 200 hours. It made no difference.

I wonder if a bubble is getting in the system briefly ?

David
 
AlexPeterson said:
A remaining mystery is the full tank observation. You said "seems" to only happen with full tanks. This would be counter to a flaky fuel pump, one would think.
Click to expand...

Yes, agreed. Maybe it's just coincidence, but it seems to happen more often until 5 gallons are burnt from each tank. I flew yesterday with ~25 total gallons, and pressure never stumbled (1.2 hrs). After I landed, pressure dropped while taxiing in.
 
Chris, a couple of things. I have a 7 and noticed a similar cycle of my pressures so tried to find the causes. I recommend air pressure tests of the fuel system from the fuse bulkhead up to the servo. This will find all leaks including suction side. The smallest suction leak adds bubbles that expand with low pressure and temperature.

The manual pump creates intermittent low pressure and bubbles take longer to dissipate than they do to form.

The fuel, from the valve to the manual pump, gets warmer. I was surprised how much the rise was in the tunnel to the firewall (tested). I added extra insulation on the firewall forward side (radiation shielding) and ensured the line in the tunnel did not touch the floor. I have a 4-into-1 exhaust and added some insulation over that. Just a 1/16" fiberfrax can do wonders. Even hand wraps with aluminum foil is an adequate/safe test method.

Take care not to over tighten the fuel B nuts as it can crush the 3003 alloy lines and result in a failure later.

I'll post test results if the temps can be located.

I measured 50F rise from bulkhead to mechanical pump. This was reduced to 30F with a barrier between my exhaust and the floor.

Lastly, it is assumed the fuel flow sensor is located after the mechanical pump and insulated.
 
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Last night I:

1. Did a bubble test from the fuel servo back to the fuel selector. Nothing found.
2. Ran the boost pump with servo line in a clear container. No air bubbles were noted.

So, I ordered a new mechanical pump. Will post here if that fixes my problems.
 
The new pump is installed, and all seems to work fine. Lessons learned for this community:

1. The solution is often the most obvious.
2. A fuel pump CAN fail intermittently, despite what Tempest says.
3. A fuel pump CAN fail in <400 hrs (as many others have reported here).
4. Follow the guidance (linked earlier in this thread) for installing a new pump. I used the safety wire trick to hold the pin up, and it took me <2 hrs total (including de-cowl, a run-up, and cowling)

Thanks to you all!
 
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