92 3.1L V6 runs rough/stalls only after a hot soak.

[sjohn274]

Please help. This is an odd one.

92 Lumina 3.1 V6 130,000 miles

My vehicle is running rough, sputtering and stalling ONLY after a 20-30 min hot soak. When cold it starts and runs fine and even when hot it never has any issues until you turn it off and let it sit. If you go back to start it it starts and will start to run rough and sputter (no poping). I can usually feather the pedal and eventually clear it up but if I just let it idle, it will stall and likely not restart until 1) I let it sit until cold or 2) I bleed the pressurized air out of the fuel rail. What? Yes, as the vehicle sits, pressure builds up in the fuel rail. When the engine is shut off hot, the pressure remains around 30 psi. 30-60 seconds into the hot soak the pressure climbs to 40 psi and sits there. Iam assuming anything above that is being bled back into the tank through the regulator. If I hit the bleed on the gage or the shrader valve on the rail, ALOT of fuel mist sprays out of there with lots of force for about 20 seconds. It is more air than fuel and once you do this, the pressure builds back up again. Each time you bleed it after the first attempt, there is nothing but air in the rail and it sounds like you are letting air out of a tire. After a couple of bleeds, the engine will start up and stay running but the fuel is extremely aerated. If you let it sit overnight, the pressure bleeds off on its own.

Fuel pressure when running normal- choppy between 20-35 psi
Fuel pressure when hot after a bleed- 25-30 psi and not nearly as choppy
Fuel pressure KOEO cold or hot- Right around 30 psi. When hot it creeps to 40

I have just replaced the fuel filter, pressure regulator and blew through the return line to the tank with no change to the concern.
Fuel tank cap on or off makes no change
Good bright white/blue spark when it wont start.

I am about ready to replace the fuel pump but before dropping that tank and putting another $100 part on this thing, I was looking for any other suggestions?????

Thanks,
Seth

[richtazz]

I'm thinking you have a problem between the fuel pump and the sender (the part the pump mounts in with the fuel level sensor and lines). There is a short piece of rubber hose between the pump and the metal line that has probably sprung a leak, or the clamps retaining it have come loose/broke. Regretfully, I think your only option is dropping the tank.

[jeffcoslacker]

I remember Fords had a problem similar problem in the late 80's, there was a deflector that protected the fuel pickup from from the return line's disruption and aeration...sometimes it would break off or sag, and the pump would suck aerated fuel once the tank got about halfway full...made them hard to start hot and sometimes stall...

Rich's explaination sounds likely...the inside the tank is usually the only place you see odd stuff like that happen. Used to be you'd see that on old mechanical pump carb systems when the line cracked between the tank and pump, on the suction side of the system. Now the suction side is completely contained in the tank...

[sjohn274]

Thanks guys. I was afraid of that but I have a bit more confidence now. I'm dropping it this weekend. We will see how it goes.

[sjohn274]

Sorry for the delayed post but I could not get on for some reason. A new Fuel pump did the trick. I also replaced the Pulse dampner while I was in there since this was not a fun task. Runs like a champ now. Does not have any more power than it did before but the Fuel pressure is a steady 40 PSI all the time now and no more hot soak issues. Thanks for the help and hopefully this helps someone else out. Definately one of the wierdest things I have ever seen a fuel system do.