Help PLEASE !!!

[george_01]

put new fuel lift pump on. getting VACUME not pressure at water bleedoff from fuel filter also VACUME at air bleedoff at fuel filter. I also checked power to fuel lift pump pryor to changing it. It was 11.5 volts at idle. thats what led me to believe it was the lift pump. However its still missing and has the vaccume at the filter. WHAT CAN IT BE. HELP PLEASE!!!!

[DmaxMaverick]

Is the "vacuum" occuring while the engine is running? 11.5 volts would not indicate a bad pump, and is normal, if not better than most. The injection pump (if reasonably healthy) will suck enough fuel to run the engine at idle and slight power demands. If the lift pump is not operating, the inj. pump will create a vacuum in the fuel lines/filter.

If the vacuum is while the engine is running, I suspect the pump is not operating. The most likely candidate is the OPS. If you have 11.5 volts at the pump with engine running, the OPS is fine. Could be a bad ground, even as rare as that could be with these trucks (sarcasm).

Try applying battery voltage to the pump directly. If the pump pumps, check all electrical connections/grounds. Even try this on the old pump.

If you don't already know how, you can "test" the pump with the engine off: Apply parking brake, turn key to "on" (no start, engine NOT running), place gear selector in any other position other than "P" or "N", turn key to "start". The engine should not crank, but the pump should work. This bypasses the OPS.

Check out all the above and get back to us. The fuel systems of these engines are reletively simple, as are the cures for most problems.

[HowieE]

Sorry to suggest this but check to see if the pump is installed in the right direction.

[Peter J. Bierman]

If the lift pump does not get fuel it can't pump it.
Sounds like you have a (partialy) blocked fuel line.
The IP will suck harder then the fuel line permits and creates a vacuum.
Try to have the lift pump suck from a canister and see what happens.

Peter

[george_01]

checked voltage to pump once again. voltage from 10v - 13v. however voltage was not constant, it would have voltage for awile then quit then start and stop again. Is this common if ops is bad or does it indicate a short or bad ground? Also checked both pumps direct from battery new pump and old pump both run OK. Thanks Guys :confused:

[george_01]

Checked voltage to pump once again. voltage from 10v - 13v. however voltage was not constant, it would have voltage for awile then quit then start and stop again. Is this common if ops is bad or does it indicate a short or bad ground? Also checked both pumps direct from battery new pump and old pump both run OK.Is ops next? Thanks Guys

[kevin77]

THe OPS is bad - I did the same thing - replaced the lift pump and I had a vacumm in the line. I then replaced the OPS and now fuel will come out of the filter and the water drain line while the engine is running.

kevin

[george_01]

Thanks Kevin. I been fighting this thing for days. Very frustrating for an amature like me.

[kevin77]

When you replace the OPS - you will want to get the special GM ops socket to get it out - I think it was about $12 at the local auto store. Some people have taken a socket and ground part of it away - you might want to try seaching the forum for "ops replacement"

Kevin

[george_01]

Replaced OPS. Everything runs ok now. Tried to make wrench. Didn't work. Borrowed socket from local GM dealership mechanic. Had to sign aggrement to give up first born virgin grand daughter if I don't return it. LOL....The right socket made it simple to replace. Took loose fuel filter canister. Removed wireing harness bracket. Used thick pad to lay on motor. Made it quit comfortable.Took this novice diesel mechanic about 1 hour to complete. Thanks to all you Guys. !!!

[george_01]

PS......Tried to turn on a couple of people to our WEB site. Our web site was blocked on the posting and we were called a scam web site by The dieseldoctor web site. What idiots!!!