Low Fuel Pressure - Troubleshooting

[Kyle Boatright]

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. Sam Buchanan had this problem once upon a time, but as far as I can tell, his posts didn't explain how he diagnosed it.

- 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...

[Sam Buchanan]

Kyle, my engine would randomly lose rpm in flight, quite suddenly while climbing, and would instantly pick back up when the electric pump was switched on. Loss of rpm was inversely proportional to rise in pulse rate.....

After this happened three times (at first I thought it was carb ice) I decided I'd had enough and replaced the mechanical pump. Problem solved, the pump had ~700 hrs on it.

[Jrskygod]

Check pump pressure with a mechanical gauge. I had same problem- thought it was a sender problem - turned out to be a bad mechanical fuel pump. Replaced the pump and all is well. Pump had less than 500 hrs. on it.

[Kyle Boatright]

Originally Posted by Sam Buchanan

Kyle, my engine would randomly lose rpm in flight, quite suddenly while climbing, and would instantly pick back up when the electric pump was switched on. Loss of rpm was inversely proportional to rise in pulse rate.....

After this happened three times (at first I thought it was carb ice) I decided I'd had enough and replaced the mechanical pump. Problem solved, the pump had ~700 hrs on it.

How'd you determine it was a pump issue and not something else?

BTW, the Fokker looks nice.

[Sam Buchanan]

Originally Posted by Kyle Boatright

How'd you determine it was a pump issue and not something else?

BTW, the Fokker looks nice.

Thank you, Kyle.

Because the engine immediately regained rpm when the boost pump was engaged I assumed the mechanical pump was failing. I have had zero issues of this sort in the 500 hrs since the pump was replaced.

[Jesse]

That is what I would suspect as well. If the gauge reads normal with the boost pump on, then I would assume you have a bad mechanical pump. On the ground the carb will likely draw enough fuel from the tanks to run, but not in flight probably. I would check the mechanical pump.

[Randy]

I have had intermittent fuel low fuel pressure problems also. In the process of chasing this down I have replaced all my fuel lines etc, eliminating many restrictive fittings and elevation changes etc. I thought I had it nailed when I replaced the Kavilco pressure sender and afterwards did not see the problem again, until now...

I even temporarily replaced the fuel line between the Red Cube flow meter and the servo and ran the engine while observing,,, no bubbles.

After explaining all that I have done to solve this problem to the folks at Aerosport they have offered to send me an overhauled Tempest fuel pump and it is on the way.

Talking to the fuel pump tech experts, they all tell me the mechanical pump cannot fail this way, that they either stop pumping or they are OK. I have now heard of several instances where they have failed on an intermittent basis.

We will see how things go after changing out the pump but I may have to wait quite a while to feel certain I cured the problem as it has been hours and hours of flight time between the failures I have experienced.

Mine is on an IO-375, the mechanical pump will normally maintain 25 PSI and the backup pump bumps it up to 32 PSI.

The fuel injection folks clued me in to the fact that looking for other engine parameters like fuel flow or MAP etc to drop along with the pressure does no good as the servo adjusts for pressure up to a point. I think I have always been too fast in switching on the backup pump so in looking at graphs of engine parameters from my Dynon, I can see the fuel pressure drop down to about 13 PSI in cruise. They tell me if this happened on a take off run I might see decreased power and high EGTs etc as the fuel demand is much higher, but in cruise conditions, the problem is hard to detect other than the fact that "Betty" says into my headset, "Fuel Pressure", and my hand goes very swiftly to the fuel pump switch when that happens.

I have wanted to repeat the problem at altitude and make myself hold off on turning on the back up pump but whenever I am ready for this, of course the problem will not appear.

Just passing this on in case others get the same message I have from the "experts" that say this can't happen.

Randall in Sedona

[n82rb]

I know your airplane has been flying for a while, has there been any other pressure problems up till now? sounds like a flaky pump to me. lycoming says that you really only need about 1/2 psi to run at idle. people saying that they do not fail that way are wrong, I have seen them fail in a lot of different ways. my last one would pump fine at power, and idle when cold, but pull it back to idle on landing and the engine would surge then quit. pressure gauge stayed solid until it quit running. I am willing to bet your pump is headed south. how many hours is on it?

bob burns
RV-4 N82RB

[Kyle Boatright]

Originally Posted by n82rb

I know your airplane has been flying for a while, has there been any other pressure problems up till now? sounds like a flaky pump to me. lycoming says that you really only need about 1/2 psi to run at idle. people saying that they do not fail that way are wrong, I have seen them fail in a lot of different ways. my last one would pump fine at power, and idle when cold, but pull it back to idle on landing and the engine would surge then quit. pressure gauge stayed solid until it quit running. I am willing to bet your pump is headed south. how many hours is on it?

bob burns
RV-4 N82RB

Never had problems until yesterday. The pump, engine, airframe, and everything else have ~1,000 hours. I'll get to put one of those fancy tags on the prop next year at Osh. ;-)

[Kyle Boatright]

The obvious follow-up question is "If I do buy a replacement pump, which brand do I buy? Tempest or Lycoming?" Looks like about a $100 difference at Aircraft Spruce.