M923 Prime Loss - Air Bubble

[fuzzytoaster]

I've got an atypical issue I've not had before and looking for input. I've got a m923 that keeps getting a small air bubble in the line near the lift pump when the truck is off. It doesn't drain down to the filter, it just forms a bubble enough to stall the engine from a start. I've swapped all the lines from tank to pump, rebuilt the hand pump, tried capping it off, pressurized the tank and fuel lines for leaks (none seen), and it still looses prime to stall out. After another quick crank (maybe 3-5 seconds) it fires up and all is good in the world. I know the air has to come from somewhere but where..? Thoughts?

 

[Shoprat 19th Engr]

Try taking out the pickup tube it may have a crack at the top of the tube

 

[fuzzytoaster]

Try taking out the pickup tube it may have a crack at the top of the tube

I did this as well when I replaced the line, someone stripped out the 90 degree elbow so all that was replaced.

 

[MtnSnow]

you might check your "Return Fuel Check Valve" If it wont seal it will allow fuel to run back to the tank out of the injection pump.

 

[silverstate55]

you might check your "Return Fuel Check Valve" If it wont seal it will allow fuel to run back to the tank out of the injection pump.

Also check return lines in tank; mine were rubber & swollen/disintegrating to the point that they were choked shut. I had some solid tubing brazed onto 90-degree return elbows, problem solved. I had same problem as you, and did same repairs you did, but problem still persisted...until I changed out the return lines in tank/tanks (I have 931 with dual tanks).