Low Fuel Pressure - Troubleshooting

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kyleb

Final Approach
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Jun 13, 2008
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Marietta, GA
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Drake the Outlaw
(Also posted on VAF)

On the way out to the runup area today, I had a low fuel pressure warning. The digital gauge showed 0 or even slightly negative fuel pressure, but the engine continued to run. I turned on the electric fuel pump and the pressure came up to the normal operating range of 5-6 PSI. I turned the electric pump off, and the low pressure indication returned. Restarting the electric pump raised the pressure to normal again. I returned to the hangar, pulled the cowl, and looked for any obvious issues - blue fuel stains, etc. No joy.

The engine is a 160 HP, carbureted O-320 with an electric fuel pump and mechanical pump in series.

Here are the possibilities I've considered:

-Sender/gauge problem: Since the electric fuel pump raised the fuel pressure both times I turned it on, I'm going to assume the sender and gauge are not the problem.

- Intermittent failure of the low pressure mechanical fuel pump. I've seen reports of this problem before, but it was not clear how the problem item was identified.

- Leaky fuel fitting or line, allowing air to enter the fuel system somewhere upstream of the mechanical pump (but after the electric pump?). Should I be able to test this (at least the connections between the electric pump and carb) just by turning on the electric pump with the engine shut down and looking for leaks and/or seeps?

- Collapsing fuel line. The fuel lines are stainless braided with a Teflon liner and should be good for a lifetime, but there are no guarantees in life.

This is a potentially dangerous problem and I'd rather identify and fix it with certainty before I fly again.

So I'd appreciate any thoughts people have on the subject. In the meantime, I'm going to dig through my Sacramento Sky Ranch book...
 
GlennAB1 said:
Sounds like normal operation for everything I've ever worked on.
Click to expand...

The mechanical pump has sustained 3-5 PSI during taxi and flight for the life of the engine and aircraft. Something has changed.
 
Kyle, our RV-8 has done this since Day One. It drove me nuts at first, trying to find a problem, but in two years and hundreds of flights I have found no problem, other than the erroneous fuel pressure read-out.

I have concluded that it is an instrumentation or calibration error somewhere, since you can't have sustained NEGATIVE fuel pressure with the engine running. The engine has never skipped a beat, no matter what the stupid digital readout has indicated.

As with yours, when you turn on the boost pump, pressure goes up.

I have the GRT transducer, going to the EIS 4000. This, in turn, sends the data to my GRT Horizon HXr EFIS. Somewhere in there, something isn't talking to something else. What system do you have?

Sometimes, I long for a good old dumb gauge. :)
 
I've had issues with the mechanical diaphragm pump/electric pump combo.

A few times I've looked down to see 1-2 psi on the mechanical pump only(mainly during a climb), which would go up to 5-7 when I turned the electric pump on. Leveling out seemed to bring the pressure up(on the mechanical pump only - perhaps because of the slightly higher RPM? IDK).

First time it happened in a rental, we actually turned around and went back to the airport... now I'm not so sure that it's even something to actually be concerned about. So sorry for the partial hijack but I would like to learn something:)
 
Jay Honeck said:
Kyle, our RV-8 has done this since Day One. It drove me nuts at first, trying to find a problem, but in two years and hundreds of flights I have found no problem, other than the erroneous fuel pressure read-out.

I have concluded that it is an instrumentation or calibration error somewhere, since you can't have sustained NEGATIVE fuel pressure with the engine running. The engine has never skipped a beat, no matter what the stupid digital readout has indicated.

As with yours, when you turn on the boost pump, pressure goes up.

I have the GRT transducer, going to the EIS 4000. This, in turn, sends the data to my GRT Horizon HXr EFIS. Somewhere in there, something isn't talking to something else. What system do you have?

Sometimes, I long for a good old dumb gauge. :)
Click to expand...

I'm flying behind GRT too. But, in 15 years with this airplane, I've never had a low fuel pressure warning (the airplane has a master caution light for engine parameters), and fuel pressure has always been 3-5 psi in normal taxi operations. Since turning on the boost pump resolved the low pressure warning today and that "solution" was repeatable, I'm inclined to believe there is something unusual going on in the fuel system - possibly a failing hose, a failing mechanical pump, or possibly an air leak somewhere.

Fuel pressure isn't something I can leave to the fates...
 
kyleb said:
I'm flying behind GRT too. But, in 15 years with this airplane, I've never had a low fuel pressure warning (the airplane has a master caution light for engine parameters), and fuel pressure has always been 3-5 psi in normal taxi operations. Since turning on the boost pump resolved the low pressure warning today and that "solution" was repeatable, I'm inclined to believe there is something unusual going on in the fuel system - possibly a failing hose, a failing mechanical pump, or possibly an air leak somewhere.

Fuel pressure isn't something I can leave to the fates...
Click to expand...
Well, if you find the glitch, let me know. It has so far eluded us.

Given that the result of uber-low (or negative) fuel pressure would be engine failure, and the engine is running great, we have learned to live with this instrumentation problem. I will follow your search for a solution with great interest!
 
Last edited:
I didn't see fuel flow mentioned. Do you have a way to monitor it?
 
Jay Honeck said:
Well, if you find the glitch, let me know. It has so far eluded us.

Given that the result of uber-low (or negative) fuel pressure would be engine failure, and the engine is running great, we have learned to live with this instrumentation problem. I will follow your search for a solution with great interest!
Click to expand...

Is your engine fuel injected or carbureted?

You might want to check the impedance on the pressure transducer and also make sure whoever set up your EIS4000 dialed in the correct set points for the sender you have. IIRC, there are two factors you have to enter correctly to create the "map" for different senders.
 
kyleb said:
Is your engine fuel injected or carbureted?

You might want to check the impedance on the pressure transducer and also make sure whoever set up your EIS4000 dialed in the correct set points for the sender you have. IIRC, there are two factors you have to enter correctly to create the "map" for different senders.
Click to expand...
Carbureted.

We checked the EIS parameters when we put the Horizon EFIS in, but it's been on my to-do list to recheck them for quite a while. I've got to replace a broken EGT probe this week, so that will be a good time to do it.
 
Put on a mechanical pressure valve and compare. Could be the measurement system, could be the pump, could be the lobe that drives the pump.
 
I've had this happen on an 0-320. It was the mechanical fuel pump. I had to buy two before I got one that worked.
 
kyleb said:
This is a potentially dangerous problem and I'd rather identify and fix it with certainty before I fly again.

So I'd appreciate any thoughts people have on the subject. In the meantime, I'm going to dig through my Sacramento Sky Ranch book...
Click to expand...

This scares me. I was the designated wrench once for a 1934 Ford Fire truck, and the rubber diaphragm in the mech. fuel pump tore. It filled the crankcase with mogas.

Check your oil quantity just to be sure. I don't know if the design of our "modern" fuel pumps are fail-safe for this particular fail....

-Skip
 
Skip Miller said:
This scares me. I was the designated wrench once for a 1934 Ford Fire truck, and the rubber diaphragm in the mech. fuel pump tore. It filled the crankcase with mogas.

Check your oil quantity just to be sure. I don't know if the design of our "modern" fuel pumps are fail-safe for this particular fail....

-Skip
Click to expand...
thats why AC aircraft pumps are dual diaphragm pumps.

bob
 
I vote mechanical fuel pump. The airplane pumps should vent out the tell tale tube instead of the case.
 
Capt. Geoffrey Thorpe said:
Put on a mechanical pressure valve and compare. Could be the measurement system, could be the pump, could be the lobe that drives the pump.
Click to expand...

Could be the spring in the pump that actually creates the pressure. If it breaks, the pressure falls off.
 
Partner took our Warrior up to RDU. Began the return trip back and after take off he clicked off the electric pump and the engine "hiccuped" as he said. Turned the electric pump on and all was good. Turned it off and same thing. Turned around and landed. Did a runup on the mechanical pump only and could not get more than 21-2200 out of the engine. Mechanical pump replaced, all good. He never mentioned any fuel pressure numbers.
 
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