Lift pump diagnosis???

[maverick2]

I have a '95 tahoe with a 6.5 and 140,000 miles that recently developed a fueling issue. In recent week, rig has started to stumble and hiccup under load, so I changed the fuel filter as it felt like it was getting starved for fuel. When I pulled the filter, noticed the filter housing was only 1/2 full of fuel. Replaced the filter, and things ran smooth for 100 miles or so (full tank of fuel) and then stumbling started again, and it eventually died idling across a parking lot. Cracked the filter housing to see if I had fuel in the filter, and had to crank things over for 10-15 seconds to purge air from the filter before I had fuel again. Once the filter housing was full of fuel again, engine restarted and runs fine. Started scratching my head a bit, and realized I'm no longer hearing the lift pump once the engine shuts off (normally hear the lift pump for 4-5 seconds). Pulled the relay for the lift pump, jumpered across the terminals, and lift pump seems to run fine. Started looking deeper, and am finding that the voltage to the lift pump relay seems faulty -- not seeing any voltage to the relay at startup or shutdown (I believe that lift pump should run right before cranking over, and again after shutdown until the oil pressure bleeds off...) That makes me think the lift pump and oil pressure sending units are OK, but am wondering why I'm not getting good voltage to the relay (voltage differential I see is in milliVolts rather than Volts when that lift pump should be running at ignition and shutdown...) Any ideas and suggestions?

[DmaxMaverick]

You are in luck. Unique to the 1995's, is an easy way to test the lift pump independent of the "running engine" system. Set E-brake, turn key to "run" (do not crank/start), move shift lever to any position other than P or N, turn key to start (crank). The starter will not engage (because the tranny is in gear), but the lift pump will run. This is also handy for priming after a fuel filter change. If the pump runs during the test, but doesn't run while the engine is running, the problem is most likely the OPS (Oil Pressure Switch). The OPS supplies voltage to the lift pump only while the engine is running, or while there is sufficient oil pressure (the reason you hear it after shut down, while the oil pressure bleeds off).

Replace the OPS (use ONLY GM part, no aftermarket) and, if funds and time/ability allows, install a relay to power the pump (relay powered by OPS), or install a pump controller, such as sold by kennedydiesel.com. This will relieve the load on the OPS (most common reason they fail), and ensures full, clean power to the lift pump (helps prevent premature pump failure).

[maverick2]

Interesting. Ran the test you suggested -- as you forecast, the lift pump runs while the key is in the cranking position, indicating the oil pressure sensor may be the culprit (which is re-inforced by the fact that when the engine is running, I no longer hear the lift pump running for a few seconds while the oil pressure bleeds off once it's shut down.) If that OPS is indeed bad, how am I getting fuel to the filter and injection pump when I'm running -- once I bleed the air out of the filter, the engine will run flawlessly for a couple hours before hiccuping and dying, which I wouldn't expect if it were the OPS... Does the OPS tend to fail slowly (ie., requiring higher and higher oil presures to trigger the lift pump) or what???

[DmaxMaverick]

What you are seeing is almost normal (normal for having a dysfunctional OPS). The injection pump will draw fuel. However, it is not very efficient at it. This usually leaves you with a power loss at higher engine loads. Also, if you have any leaks in the system between the pumps, what would normally be an external fuel leak under pressure (if it leaks fuel at all), becomes a suction air leak. A suction air leak may only suck in air, but not be large enough to leak fuel out, or have a "check valve" effect. This is why it runs OK for a while, then eventually loses prime. If your system had no leaks, it would continue to run, although handicapped, until the additional stress on the fuel transfer pump (inside the injection pump) causes the IP to fail. The OPS is a common point of failure on 94/5 models because the OPS (in OEM form) carries the entire fuel lift pump electrical load. Adding a system using a relay to power the pump drastically reduces that load on the OPS, and ensures full, clean power to the lift pump. It's a win-win, with only an initial expense to install it. Realistically, it will pay for itself within a year or two if you buy the most expensive option.

[maverick2]

Thanks -- will replace the OPS tomorrow, and put a relay on next week.