Ok, my 1999 Suburban is sidelined with problems, so I went to get my 1996 C2500 out of storage. I hooked up the batteries, and the wait to start light cycled on/off instead of the usual solid on. Truck wouldn't start. As I sat in the truck with the ignition off, the lift pump kept tapping away. I sat there for ten minutes and it never shut off. I had to disconnect the batteries. What the heck have I done to irritate the Diesel gremlins? I will be waiting for warm weather to replace my fuel tank, lines and lift pump on my Suburban before attempting a dealer warranty IP replacement. I really need my pickup running! Any tips appreciated. Maybe I should just trade these lumps for one good truck..... Arrggg. Hal

maybe the relay stuck

Where is it hiding? Thanks.

I would think it is the Oil Pressure sender, or the harness for it......

96 should be controlled by the pcm relay is in the power center under the hood

Either ground problem like bare wire or the LP is bad.

Does it pump fuel out when running? if not, then you know it is bad.

The lift pump is fairly new, and I checked it today, it was pumping clear fuel. It was not staying on like this when I parked the truck a month ago. Is there a place where all these part abbreviations are explained? Thanks.

How's this? I took a long trip to pick up a bike. My Suburban had stalled a couple of times before I left. I was concerned about the ECM, so I 'borrowed' the one off the pickup, to have as a backup on the trip. It turned out that the stalling was a loose wire on the back of the fuse/relay terminal box. Hmmm, maybe if I took the ECM out of my tool box, and put it BACK ON MY PICKUP, it would start? Oh man, I must of had too much of the seventies. I don't know for sure, because I don't remember a lot of it! Ha! Ok, I'll put my foolish butt to bed now, now I can sleep, knowing why 'Old Faithfull' wouldn't start. I just hope I didn't cause any damage. Signing of. KnuckleHal

Seeing that SES light come on with just the ignition on is always the first thing to verify the PCM is functioning.

Thats still odd that the LP was running constant. If its like -10°F that would be why, meaning whichever type of oil you have is like mollasses when cold. The cold oil wont run, so when you crank the oil pump drives oil pressure into the OPS which will power the LP as a backup to the PCM controlled relay. The OPS has a hot battery feed and just needs oil pressure to close the contacts.

Believe it or not, the OPS could be simply mechanically stuck. Happened to me in the off position, i.e. no power to the LP, the IP was sucking its own fuel. Gave it a few taps and it flipped power to the LP on. I have the OPS output wired to an indicator light on the dash so I can tell when there's no power to the LP.

You OPS may be stuck on - give it a few taps and see if it changes.

Btw, that was the revered AC Delco brand OPS only four/five months and 8k mi down the road.

Usually when something that is relay controlled doesn't shut off when the power is turned off, it is the relay itself that is bad. The OPS probably doesn't even have power when the key is off, so thats probably not it.

Is it normal for the lift pump to keep pumping after the key is off?? On the '97 K2500 I noticed today that the lift pump keeps pumping for about 30 seconds after the key is turned to off (even when the key is removed) so it definitely has power to the relay with the key off. The truck seems to run great, the lift pump turns on while I am waiting for the glow plugs to warm up and cycles off just before I start the truck so it seems like it is functioning properly.

If the oil pressure switch is "stuck" the fuel supply pump will continue to run even with key off.

Residual oil pressure will cause ops to remain closed & pump to continue running for a bit after shut down . Longer if oil cold & thick ( start up & imediately shut down ).

Like, said before, OPS has hot battery feed, and runs off oil pressure until pressure is gone. It is common if you crank when it is very cold, and you dont start it that it will stay on because the oil doesnt flow.

I'm surprised the OPS has battery power. I dont see any reason for it to have battery power as opposed to switched power.

Mine would do this on cold starts. If the truck wouldn't start it would just keep pumping away. Now when it warmed up and I shut the truck off it went away.

my pump runs for 30 seconds or so when the key is turned of also.

Residual oil pressure will cause ops to remain closed & pump to continue running for a bit after shut down . Longer if oil cold & thick ( start up & imediately shut down ).

Makes sense since the temperature was in the upper 20s (after being 18 overnight) and I only moved the truck about 100 feet to loaded it full of wood that I had just cut up, then another 500 feet to unload the wood and a couple of hundred more feet to park it, so it never ran long enough to warm the oil up much.