Now about the pump plumbing....Look at the parts manual page 2H11, ( and 2H7 though a 180 with single electric pump it shows the routing of the hoses more clearly), it is obvious that the supply side of the engine driven pump is not the pressure side of the electric pump, but in fact shares a common supply from the firewall fitting. The pressure side of the electric pumps join with the pressure side of the engine driven pump prior to the carb. The fact that the engine driven pump operates at a slightly lower pressure than the EDP causes a stagnation of fuel in the EDP when the electrics are running, and has led to vapor lock in hot weather that was found when the electric pumps were turned off after takeoff and the pressure wasn't monitored. Solution is to turn on the electric pumps just long enough for pressure recovery and then off, and allowing the vapor to work through the system until the EDP pics up the full flow. Thats the bad part. The good part is that when the diaphram comes apart in the EDP and plugs the fuel flow, the electric pumps can still supply full fuel flow to the engine to get you safely to an airport instead of whatever you are over at the time of the failure!

-Zach

Andrew

Andrew, maybe in the end, you are right and things are normal. I'm not comfortable though at this point when the needle is nearly on zero. I do not have a fuel flow gauge. as mentioned above, my fuel pressure always lived in the happy middle of the green prior to overhaul. so something has changed. hopefully we are not chasing a fake gremlin.

Andrew as for my overhaul, see a former post of mine under Engine Breather Tube. The conclusion to my overhaul was a very good outcome overall. I did have to replace my crankshaft which cost me an additional $8000 for an overhauled one with the full flange and could be ground again. My original didn't pass muster due to grooving in one or two journals and some heat cracking. it was caused by the old style rods that had been installed on the previous overhaul apparently. those rods had a narrower face and wore the journal. So when I got all done for my entire firewall forward including my prop I spent $48,000. About $10,000 more than I had hoped to spend , but I have confidence in everything up front.....except the fuel at the moment.....but it's fixable I'm sure.

Other Lycoming setups have a more sensitive large face round pressure activated gauge. Fuel line right into the cockpit - like a Lake LA-4. The fuel pressure gauge is a sensitive pressure type that was directly connected to the fuel. That needle flew all over the place. If the carburetor bowl was full the pressure was very high and when the needle valve opened to call for fuel the pressure dropped right off down to 1 psi and then bounced right back up to 8 psi when the needle closed. During cruise flight the pressure flopped back and forth between 3 and 10 psi. These pressure fluctuations are normal and due to the way the plunger style diaphragm pump works.

But the resistive type automotive gauges smooth all of that out and provide a sort of average reading. Do you have the original resistive gauge or has it been replaced with something more sensitive? An electronic engine monitor maybe?

Also, if there is air or vapor bubbles in the fuel it will still indicate the pressure properly. The bubbles are at the same pressure as the fuel they are in. Makes no difference to the gauge. Bubbles in the line may indicate that there is a restriction and leak somewhere upstream of the pump pulling air in (which is bad) but the pressure is the pressure whether there are bubbles in the fuel or not.

Fuel injected is different - but we are talking carburetors here, as I recall.

Heritage Aero traced it to a mis-seated O-ring in the fuel selector that was acting as a check valve and allowing some air into the system at high fuel flows. Unfortunately this was after I assumed it was the engine-driven pump and replaced that...

Thanks for the follow up. It makes perfect sense.

Zach

This discussion has brought another question to mind. Assume a 250 with a carburetor and one failed (unknown to the pilot) electric fuel pump on a takeoff roll at low altitude. After liftoff, and after their is too little runway left for landing, the engine driven fuel pump quits. Would reducing the throttle somewhat stop the surging and reduced power because of insufficient fuel flow? Would such a move actually increase the total power output over time? Where in the sequence of immediate actions would you put this?

Hank
Henry A. Spellman