Fuel / Pressure Issue

Equipment: RV-7A / Superior IO-360 / Airflow Performance FI / LW 15473 Dual Diaphragm Fuel Pump

Symptoms: Often, in-flight, I?ll notice a drop in fuel pressure into the yellow range around 17 ? 19 psi. I turn on the aux pump and pressure returns to the normal range, and after turning the pump back off, all will be well for an indefinite period, but normally anywhere from several minutes to well over an hour.

On a few occasions, if I have not ?caught? the low reading, it has dropped as low as 8 psi and the engine has sputtered. Again, turning on the aux pump returns pressure to normal for another indefinite period.

The second issue, not as unsettling as the above, is after starting a warm or hot engine, often after topping the tanks, the engine RPM will surge cyclically, at an even rate, by around 100 ? 200 RPM, at idle or very slightly above. Once out of the near-idle range, there is no issue, ever, and in-flight operation is normal.

Occasionally, I have seen a drop or two of fuel on or below the "witness tube" on the pump which indicates a loss of integrity of the diaphragm, but this is not consistent, and the pressure issue now happens most every flight.

As I did not build this airplane, I am hesitant to do any serious troubleshooting of the fuel system without a more thorough understanding of its operation.

Speaking with Tempest, it is recommended to replace the fuel pump after 10 years of operation. Mine is 13 years old so I will replace it. I've heard it can be tricky so any tips or tricks would be appreciated.

I appreciate any insights you may be able to provide.
Edit/Delete Message

Here is a good thread on replacing the fuel pump.

If your diaphragm is leaking fuel, you should not be flying it. You are having classic symptoms of a bad fuel pump.

Be aware (I believe it is discussed in the above thread) that there are dangers to not getting the pump installed correctly (lever inside the engine can be damaged...).

I've replaced mine again since that thread was started, and it was quite a bit easier than the first time! Biggest pain is getting/modifying an allen wrench so that it will go into the screw heads without a side bending load. I ground one down on one side where it interferes with the pump casting. A Bondhus brand ball driver would not go into the sockets on the bolts on my plane, just too snug.

Update: Not Solved

I replaced the mechanical fuel pump. No change. I replaced the Usher gascolator as I had one that had been recalled a while back . Mine was the old two-piece bowl and I replaced it with the one-piece turned bowl. I also pulled the filter and replaced the seals and inspected for contamination. There was no contamination.

The pressure is better but still dropped to around14 psi on the initial flights after the work which were no higher than around 6000 feet. The other day I decided to make a run for some performance figures and flew at several altitudes up to 13,500 ft. At the higher altitudes the pressure would drop to around 4 psi. and the engine would indicate it wasn't happy.

So now, at least some of the problem seems to be ambient pressure related. I have dismissed any sort of fuel feed system obstruction because it happens at any power setting and either tank.

I have also not suspected the vent system as each tank is vented separately.

I had suspected the gascolator for a while as Don at airflow performance said the FI system does not require one and if one is installed, it needs to be on the cold side of the firewall and not on the hot side (as mine is) and the fuel will begin to "boil" at 113? and vaporize. To address that issue, before removing the gascolator from the system, I insulated it with about an inch of "Great Stuff". No joy.

Any suggestions as to where to look next? What am I missing?

The worse condition at higher altitude seem to indicate vapor lock, the higher the altitude the lower the boiling temperature of fuel. check to make sure all your fuel lines are insulated, all of mine have fire sleeves.

It is not that uncommon for me to get pressure drops when above 8K' Never below 12 PSI and never an engine stumble. Many others report this as well. Not completely sure of the cause, but have theories. Bendix will work fine down to about 12 PSI, assuming the reading actually reflects the pressure at the servo (boiling in the pump will lower pressure at the outlet, where sender tee's off, even though downstream pressure is not being affected). For this reason, I took the pressure feed off the servo instead on the 10. Only a few flights above 8K, but have rock steady pressure at alttude. Too early to call it conclusive though.

Surging at idle is quite unlikely to be the fuel pump, assuming the engine runs well at full power. More likely due to an idle mixture that is too lean or boiiling fuel in the servo (not that uncommon shortly after hot shut down in warm wx).

Larry

Not yet mentioned is to make sure the tank vents are clear. Remove the filler cap and blow air into the tank.

Ensure minimal use of 90° bends in the lines.

FWIW, my 300+ hr -7A does this on either tank randomly. Mechanical pump was new at build, along with all the AFP parts. Nothing in the filters but I plan to pull the servo screen at annual and make sure it is also clear. Post build, I ran several gallons through the lines to the servo, which had to go through the AFP filter first, so I don't think I'll find anything.

Starlifter said:

I replaced the mechanical fuel pump. No change. I replaced the Usher gascolator as I had one that had been recalled a while back . Mine was the old two-piece bowl and I replaced it with the one-piece turned bowl. I also pulled the filter and replaced the seals and inspected for contamination. There was no contamination.

The pressure is better but still dropped to around14 psi on the initial flights after the work which were no higher than around 6000 feet. The other day I decided to make a run for some performance figures and flew at several altitudes up to 13,500 ft. At the higher altitudes the pressure would drop to around 4 psi. and the engine would indicate it wasn't happy.

So now, at least some of the problem seems to be ambient pressure related. I have dismissed any sort of fuel feed system obstruction because it happens at any power setting and either tank.

I have also not suspected the vent system as each tank is vented separately.

I had suspected the gascolator for a while as Don at airflow performance said the FI system does not require one and if one is installed, it needs to be on the cold side of the firewall and not on the hot side (as mine is) and the fuel will begin to "boil" at 113° and vaporize. To address that issue, before removing the gascolator from the system, I insulated it with about an inch of "Great Stuff". No joy.

Any suggestions as to where to look next? What am I missing?

Click to expand...

gascolator is bad for FI. It holds almost half a pint of gasoline in a high thermally conductive container in 180* ambient air. It will definately add heat to the fuel going into the pump. I would remove it, but at a minimum cover the bowl with a heat reflective material. Carb systems use much lower pressures and don't typically see issues with them.

You can try making an air block to prevent the hot air coming from the heater box from directly hitting the fuel pump. You can also experiment with turning the heat on full (all hot air goes into the cabin) and see if the problem goes away.

Thanks....

....for all your thoughts. As I posted, I have insulated the gascolator with at least 1" of Great stuff. Unless I can find a definitive cause that points elsewhere, I'm going to remove the gascolator.

I know that some have directed air at the mechanical pump for cooling, but I'm not convinced that's the issue as I never have a problem on a ground engine run with the aux pump off and the throttle almost full.....even at high ambient temps, which is the worst condition if heat is the issue.

I used a shop vac at the vents but they have fairly fine mesh screens over the ends so if there's any obstruction, I can't get it out. And I'm not about to blow from the external end as that would just send anything in the lines into the tank. I tried everything I could to hook up a line to the vent inside the tank, but no joy. But, again, I cannot believe both vents or tanks could have the same obstruction and the symptoms remain the same when I swap tanks in flight.

I had the same issue with fuel pressure dropping with the IO-320 in my 9A, so yours may be different. After trying everything to shield and cool the fuel system, I researched vapor lock with 100LL and concluded that was the problem. By design, the mechanical fuel pump is a foot higher than the fuel lines on the floor of the cabin, so it needs to exert negative pressure on the fuel to pull it up to the pump, making the problem worse.
So, I installed the 6 psi facet pump from Van's catalog in the fuel line between the fuel valve and the auxiliary pump to push the fuel up to the mechanical pump. First flight after installation was up to 17,700 msl where I loitered for a half hour with no pressure drop. I have now flown 200 hours since installing the facet pump, much of it cruising at 15,500' or 16,500' with no drop in fuel pressure. With the facet pump on, the fuel pressure is rock steady at 27 psi.
As I said, the IO-360 in a 7A might be different but this worked for me.

Vacuum check system

After reading this thread, no one has mentioned vacuum testing your fuel lines. You might find that air is being sucked into the fuel supply at the Fuel Selector valve ( the most actuated/wearable component ). Cap off the fuel lines at the wing roots, then remove fuel supply line to fuel pump and attach a vacuum pump w/gauge. See if you get any leak down in the system.

KC10FE said:

After reading this thread, no one has mentioned vacuum testing your fuel lines. You might find that air is being sucked into the fuel supply at the Fuel Selector valve ( the most actuated/wearable component ). Cap off the fuel lines at the wing roots, then remove fuel supply line to fuel pump and attach a vacuum pump w/gauge. See if you get any leak down in the system.

Click to expand...

Thanks, Erik. Sounds plausible.

Mike P.
Retired C-141 Flight Engineer

Ranger said:

I had the same issue with fuel pressure dropping with the IO-320 in my 9A, so yours may be different. After trying everything to shield and cool the fuel system, I researched vapor lock with 100LL and concluded that was the problem. By design, the mechanical fuel pump is a foot higher than the fuel lines on the floor of the cabin, so it needs to exert negative pressure on the fuel to pull it up to the pump, making the problem worse.
So, I installed the 6 psi facet pump from Van's catalog in the fuel line between the fuel valve and the auxiliary pump to push the fuel up to the mechanical pump. First flight after installation was up to 17,700 msl where I loitered for a half hour with no pressure drop. I have now flown 200 hours since installing the facet pump, much of it cruising at 15,500' or 16,500' with no drop in fuel pressure. With the facet pump on, the fuel pressure is rock steady at 27 psi.
As I said, the IO-360 in a 7A might be different but this worked for me.

Click to expand...

@Ranger any updates on this install? So you essentially have two electric boost pumps now? Are they both switched or is the faucet always on? I have similar intermittent low fuel pressure and appreciate any assistance you can provide.