QuintenMcKracken

I've been going through a lot of troubleshooting to figure out why my '94 is having trouble with starts. Sometimes it'll be just fine but other times it cranks for a while. I've been shot-gunning parts at it to no avail. I did, however, notice today that I don't hear the fuel pump prime every time. I swapped around the AC relay with the fuel pump relay and that didn't help. They both checked good as well jumping them with the battery. Looking at the fuel pressure at the rail, it is around 30PSI when off and doesn't drain more than a couple PSI in an hour. However, when I go from OFF to RUN, I don't always hear the fuel pump prime; maybe 3 out of 10 times it primes. Is it supposed to prime every time? When I jump the relay the fuel pump turns on every time. Just curious why a seemingly healthy fuel pump and relay wouldn't prime every time.

QuintenMcKracken

Alright, did some more troubleshooting this morning. When I originally tested the fuel pressure, I checked it after it had been running and my fuel rail doesn't drop below 30psi for quite a while. However, when I noticed it wasn't priming I got a remote trigger starter (since I don't have any friends) and decided to see what happens when I enter this hard start deal.

- First start of the day, the pressure was at 0psi (it had been sitting for, like, 12hrs). Did not hear the fuel pump prime with ignition ON and fuel pressure was still at 0psi after ignition ON.
- Cranked it and saw the fuel pump kick on and pressure went up to 40psi and started no problem.
- Drained the pressure in the rail via the gauge and ignition back to ON and it primed and started fine 3 times in a row.
- Turned on ignition again, it primed, I drained the rail, and tried to start it and it ended up with the hard start; sounds like its missing and doesn't want to rev above 3k. Pressure in the rail was 30psi when running but, I decided to bleed the rail and out came a bunch of air and it smoothed out! Did that a couple times and got repeatable result.

Whenever it primes it doesn't have the problem but it seems to be air is getting into the line in between the pump and the rail. On successive starts, there must be a bubble it can't clear without priming. I don't see any fuel leaks so I don't know where its being introduced and also why my fuel pump isn't priming every time.

Does anyone know if this is a pump issue? A PDC issue? I'm glad to know what is causing it but I have no clue how to fix it at this point without shotgunning more parts.

Jim Malcolm

The pump itself is what prevents the fuel from draining back into the tank. If there was a hole large enough to allow air into the system after the pump, you'd almost certainly be seeing fuel leaking out. I'm sure crazier thing have happened, but... It could be leaking in the short section of hose between the pump and the steel line that goes up and out of the tank, but if you're going to go through the trouble of dropping the tank to check it, I'd be replacing the pump while in there.

Off-the-cuff, I would say the only reason the ECM wouldn't prime the pump is if the ASD relay was still "on", the ECM thinking the restart is falling within that few second window after the ignition is shut off and the ASD relay shuts off power to the ECM. I can see the computer not priming the pump if it thinks you just shut it off and you're starting it again. In your above description, it absolutely should have primed the pump the first start of the day. The ECM is in charge of doing so, but that's not to say that an issue with the pump, relay or PDC couldn't be the problem... I'm not sure what the spec is, but if the fuel pump is getting up there in age, it's rarely a bad idea to swap it out...

In your step #4, where your primed it, and then drained the rail, the response you got is absolutely what you'd get doing so... Your fault on that one, don't do it again... :-)

QuintenMcKracken

Yep, totally agree draining the rail was my fault but I was chasing after a failure case since the pump was, uncharacteristically, priming every time. I was relieved that it gave the exact same symptoms I've been fighting for a couple months. I was initially convinced it was fuel problems until the pressure measured good the first time I did it. Then I redid grounds, a new starter, new TPS / IAC, and none of it solved it. I was about to go down the CPS route until I figured I'd buy the remote trigger for $10 and try the fuel again instead of replacing another sensor. Glad I did.

Thanks for the info on the ECM logic. Typically when I didn't hear it prime, it was after a lot of key jockeying so that makes sense. I did swap out the ASD relay as well just in case that was the problem. At this point, might just get a new fuel pump and change the rubber lines and hopefully it'll solve this problem.