Lift Pump Question

[Steelkilt]

I have a 98 suburban. my understanding is, when i turn the ignition to run position, the lift pump operates. this is what it did last time i changed the fuel pump. turn the key, lift pump operates until you turn off the key again.

but this no longer seems to happen. when i changed the fuel filter today, the bowl did not refill. i checked the lift pump and it wasn't running; and it was only four months old. i checked the circuit and no power. finally, on further testing, i discovered it was only coming on for about 3 seconds and then shutting off. in fact there was power, but only for about three seconds. it cycles this way each time i trigger the key.

i'm wondering why this change has happened.

[DmaxMaverick]

A better question might be, "What fixed my lift pump?"

The way it's working now, is the way it's supposed to work. It should only "charge" the system at "key-on", then run continuously once the engine is running. Why it stayed on before could be a number of reasons. Bad OPS (stuck closed), stuck relay, PCM hiccup, bad ground somewhere, underwear inside out. I dunno.

[Steelkilt]

i honestly like it better having the lift pump run for longer than 2 or 3 seconds when i put the key in the run position.

i tried putting in a new fuel pump relay. now it runs like an old champ again the way it always did. must have had a failing relay.

so what would cause the relay to go bad?

[DmaxMaverick]

i honestly like it better having the lift pump run for longer than 2 or 3 seconds when i put the key in the run position.

i tried putting in a new fuel pump relay. now it runs like an old champ again the way it always did. must have had a failing relay.

so what would cause the relay to go bad?

Time, use, or anything under the sun. Sometimes they last forever. Others fail in 10 minutes. Most of the time, they lose solid contact due to corrosion or contamination, internal or external. Often, they get replaced when they didn't actually "fail".

If your fuel system is healthy, it will prime in less than a second, so more pre-pumping isn't necessary. You might have a look at the system fail-safe. Take away the OPS signal to ensure it doesn't stay running when it shouldn't. Just disconnect the connector at the OPS while the engine is running, and the fuel pump should stop (engine shouldn't die, though). If the pump continues to run, the circuit is compromised. Either by failure, or someone bypassed it.

[Steelkilt]

maybe i need a circuit diagram. what is an OPS?

[DmaxMaverick]

OPS = Oil Pressure Switch. An open/closed switch to signal the PCM (on OBDII trucks) oil pressure is present, and the engine is rotating. On OBDI trucks (94-95), it actually provided power to the lift pump. On earlier models, the lift pump was mechanical. The pump itself was the fail-safe (engine rotates, fuel pumps).

http://www.thedieselpageforums.com/tdpforum/showthread.php?t=20598