by Zach Grant L1011jock » Wed Mar 01, 2017 5:21 pm
First, your electric fuel pumps should bypass the engine driven pumps. This is both good and bad and I will get to that later. If they do not join the fuel system after the engine driven pump (and have the integral check valves in them), you are not in compliance with the type certificate. The electric pumps should operate at a nominal 4.5 psi and have a flow rate at that pressure of over 28 gph each. The engine driven pump should be set to about 4 psi and should pump upwards of 65 gph at that pressure. So as you can see, any one pump should be able to support full pressure at full power flow rates. With this in mind, you have to look at the common link to all the pumps. The supply side of the equation has several possible culprits. Any flexible hose that is under suction can entrain air if there is a bad connection, bad hose, loose connection etc. Additionally, the fuel selector/strainer can have air leaks. I would replace a section of the hose from the junction of where the engine driven pump gets its supply with a temporary clear hose and run the engine on the ground, observing the operation and fluid in the line. Being that you can get the pressure to drop at runup power, you should be able to find a culprit fairly easily.
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
"Keep it above 5 feet and don't do nuthin dumb!"