Sudden fuel pressure drop

Hi.
We are having an issue with the fuel pressure in my rv7. The engine is an io360 with an fm150. During the climb the fuel pressure is steady but drops gradually and then when running lop the fuel pressure fluctuates between 22-24psi. After about an hour the fuel pressure takes a very sudden drop to under 15 psi. The electric pump increases the pressure to a stable 28, increasing in the descent. The fuel flow and egt also fluctuate so I’m assuming it’s not an indication. The vents seem to be clear, fuel lines are all firesleeved fwd of the firewall and there is a cooling shroud on the mechanical fuel pump.
Has anyone had these issues or know of any fixes?
Thanks

A couple of questions,
- Have you tried switching tanks when the pressure drops?
- If you turn the electric pump off, after you have turned it on to recover the pressure, does the pressure drops again immediately also.

It would be good to check for air leak as that can cause pressure drop. If there is an air leak, it is most likely be between your electric pump and mechanical pump which is simpler part to check.

Tank selection doesn’t seem to affect the pressure. When the electric pump is switched off the pressure drops to the previous level, around 22-24. You can see from the chart when the electric pump is switched off and on. The gradual pressure drops and rises coincide with climbing and descending altitude.

mmcelrea said:

Hi.
After about an hour the fuel pressure takes a very sudden drop to under 15 psi. The electric pump increases the pressure to a stable 28, increasing in the descent.

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I have found it common for fluctuations to occur late in the climb and attribute to mild vapor issues due to the reduced cooling in climb attitude. Getting a drop to 15 one hour into cruise is likely your fuel pump telling you it is near it's EOL. Disruptions in fuel delivery could do that, but the boost pump should experience the same issue. Typically if the mech pump struggles but the boost pump fixes it, something is going on in the mech pump and vapor issues in the mech pump should not be occurring one hour into cruise, assuming temps are normal. I have seen blips on the pressure in cruise, but always come back in a few seconds. A sustained reduction is a clear warning. The dropping EGT means that it was not just pressure, but the flow volume also was reduced; Another warning.

If it were me, I would replace the pump.

I would also look into the fuel lines proximity to hot exhaust pipes, maybe some heat shields could help.

FUEL PUMP

Try another fuel pump, new , not rebuilt

don’t you think it could be the engine driven fuel pump, loss of diaphragm integrity? only takes a pinhole, I’m told…

MacCool said:

don’t you think it could be the engine driven fuel pump, loss of diaphragm integrity? only takes a pinhole, I’m told…

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I do and I suggested replacement in my response. His pump is barking warnings that should be listened to.

Larry

Fuel pump

I had the same symptoms in my rv-10. Mechanical fuel pump was only 100 hours since new. I looked at everything. Did extensive leak down pressure testing of the fuel lines/system to look for leaks that would be drawing in air to the mechanical pump. Put a thermo-couple and absolute pressure sensor in the fuel line at the mechanical pump to check if fuel was vaporizing. Pulled apart all the fuel lines to look for debris, etc, etc. Found nothing other than the several second pressure drop occurred randomly throughout the flight. I finally replaced the mechanical pump out of desperation and have not had a pressure drop since in more than 50 hours. I pulled apart the old pump with the issue and found nothing out of the ordinary. I can only speculate that one of the check valves may have been hanging open briefly under certain conditions, but could not recreate on the bench.

Bryan

A1AVIATON47 said:

Try another fuel pump, new , not rebuilt

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Why does it matter? Internals are new in rebuilt.

I had similar symptoms. Step change drop in supplied pressure from ~~23, 24 down to 16, 18. High altitudes are where it would become apparent. Seemingly randomly and it would occur for a couple of minutes at a time. Just replaced the pump with Tempest rebuilt/reconditioned, and symptoms are occurring seemingly more frequent and at lower altitudes. Only 4 hours on it though. Sigh.