lift pump operation

[rfuntime]

Does the lift pump keep operating until a specified pressure is reached or does it shut off when the glow plug controller shuts off? Mine seems to shut off when the controller quits, it doesn't seem right. Paul

[DmaxMaverick]

Some vehicle info would be helpful here. What year, model, engine, 8th digit of VIN, mods, etc.

The LP is not pressure regulated. If the circuit is hot, the pump should be on all the time. There are several circuits that operate the LP. Depending on year model, these can be different. Most of the circuits are fail-safes, but vary by year.

[Hubert]

Something is probably wrong.

The lift pump is NOT pressure regulated or anything. Its on or its off. Suppose to be on with engine running all the time.

Depends on your year truck and transmission to exact logic. Early trucks it ran on start later trucks lp works on run or wait to start for ~ 15 seconds then a fail safety stop controls it.

After engine is running OBD-I trucks the LP is works unless oil pressure drops below ~ 3psi on the oil pressure switch (or the switch goes bad).

I think OBD-II does this also but ...
Recently read the OBD-II trucks the ECM also senses the crank rotating.

Either way some mechanism controls shut down in case of accident and driver can't cut off ignition switch.

After truck is running idling do the open water drain to test flow. Wether they make much noise, vibrate, or pump is somewhat independant or can be.

-----

On edit yep what he posted while I was typing.

[rfuntime]

Sorry DMax....you're right. More info would be good. 96 1500 2wd with the 6.5. It is an "F" series engine.

I think Hubert hit the nail on the head. After so many seconds, it quits. Must be the fail safe mechanism working. What should minimum lift pressure be? I am down to 3lbs. Is this enough?

Thanks again for your help!!

Paul

[Hubert]

Where are you sensing fuel pressure and under what conditions?

I have an electric 0-15 psi guage Tee'ed between filter and IP. On wait to start I get ~ 7-8 psi, idling I get ~4-6 psi. Driving it dances around a lot. Under WOT acceleration high rpm it will read 0-1 psi and momentarily drop to 0 psi. Read others read 0 psi while towing grades. The OEM lift pumps are not all that good at supplying fuel under high demand (another reason to change fuel filter often). The IP can suck fuel if the rest of the system is ok but its best to have positive pressure flow all the time.

I think the official GM test is a flow test not necessarily pressure. Something like a pint in less than 15 seconds at idle out of the water drain.

[DmaxMaverick]

Realistically, .1 (point one) PSI is all that is needed. Problem is, you need it ALL the time. The key is to keep the IP from having to draw fuel, which it will do to some degree if the LP falls behind (or fails). Even at 0 PSI, the LP can supply volume, and prevent cavitation. Pressure means absolutely nothing, outside of diagnostics. A healthy pump and system will perform just like Hubert describes his is doing. GM's minimum recommendation for fuel pressure is 4 PSI at idle. A new healthy pump will produce 7-10 PSI, but I've seen brand new pumps out of the box put out 5 PSI, and they were OK under load. Much of the pressure depends on how the rest of the system performs. If you have higher than normal return, you will have lower pressure. Like I said, the key is just to keep the IP from cavitating.

[rfuntime]

Everything was just like you said!! Installed a new lift pump and fuel filter..........5 lbs at idle , 9.1 lbs max. Less smoke at startup. Thanks again for your help!! Paul