Fuel Coming out of Fuel Pump Overflow

JackinMichigan · #1 09-05-2017, 07:42 AM

The plane was setting on 3 scales (approx 4-5" tall). Removed scale from front wheel which means the nose pointed downward maybe 5 degrees.

As soon as the nose dropped fuel started running on the floor. Perhaps a cup or more of fuel. We traced it to the (engine mechanical) fuel pump overflow tube.

We had turned on the electric pump a few times that day. FYI, the pressure was around 27 psi which sounds high to me.

I have the fuel injected version of the 540.

Does anyone have an explanation? I hope I don't have to replace the fuel pump.

CORRECTION...CORRECTION...CORRECTION....

The fuel was coming from the drain in the air cleaner, NOT THE FUEL PUMP... the question remains...

vic syracuse · #2 09-05-2017, 08:08 AM

27 PSI is normal fuel pressure for an injected engine. If it was truly coming out of the fuel pump overflow, most likely you have a bad fuel pump diaphragm and need to replace the pump.

Turn the boost pump on and see if you can duplicate it.
You can also get fuel out of the FAB if you turn the pump on with mixture in the rich position, as you will flood the cylinders and it will run back down the intake tubes.

Vic

WrightsRV7 · #3 09-08-2017, 05:32 PM

The only way for fuel to come out that tube is a bad diaphragm, if you turn on the power and run the electric pump, you now have hot wires and fuel in in the same general area.....be careful as you do this. Bummer, better now than later in the air though....

Phil · #4 09-08-2017, 07:00 PM

Yep. What others said.

There are two diaphragms in the pump. The lower one is the one that comes in contact with the fuel regularly. Then there is a cavity and another diaphragm on top of the cavity. If you look closely, you can see your drain port is plumbed into this cavity.

That upper diaphragm is there to keep oil out of the fuel. By putting the drain between the two diaphragms, you can see when one of them has ruptured by looking for a streak on the belly. Then you know you've got to replace it; but the fuel and oil still doesn't mix. This drain hole is a tiny little thing, so you don't dump loads of fuel overboard and onto your exhaust. It's just supposed to leak enough that you pick up on the visual clues.

Most likely, your pump is good and the fuel came from the drain on your FAB. But you can confirm it by pressuring up the system (Throttle
And Mixture aft) and watching the drain line from the pump. If it's dripping fuel, you've got a replacement or overhaul on your hands.