'92 GTP won't start, help troubleshoot

[crazyd]

I've got an intermittent no-start/stall condition on my '92 GTP LQ1 that started about a week ago.  What I know so far is that it's not fuel related, as I just put a new fuel pump in last fall and it gives good pressure when forced at the diagnostic connector plug by the airbox.  Instead it appears to be ECM related, because the SES light that is supposed to come on before you start the engine is NOT coming on, and is NOT flashing when put in diagnostic mode at the ALDL.  The fuel pump is also not powering on through the ECM and the pressure doesn't decrease during cranking. 

 

I had to have it towed home last weekend when it stalled in town, but then a couple days later after I was messing with it, it started and ran just fine until yesterday.  I have checked ALL the fuses, including the one for FUEL/ECM at the underhood electrical center (UEC), and I replaced it and also replaced the FP relay right next to it just in case.  The car will actually run without that relay as I discovered, it can also power the fuel pump off the oil pressure switch.

 

I rechecked and reseated the four plugs at the ECM and the two fuel injector harness plugs.  They were fine.  All I've done lately is charge up the A/C and replace the vapor hoses at the fuel tank.  I also checked and reseated the fuel pump plug at the tank.

 

I'm working my way through the FSM diagnostics, but I'm getting a little confused by it as to where these actual circuits are.  If this is a well-known problem I haven't been able to find it by searching the archives yet, so if anyone else has already been through this and can offer suggestions that would be helpful.

 

I also just moved to a house on the sea in March, and increased corrosion is a potential issue from the increased salt levels in the air.  I've had issues with that UEC before with corroded fuses due to the high humidity, so I just replace them if they look like they're starting to have problems. Both that 10A FUEL/ECM fuse and the FP relay did - but it didn't help this time.  Being an intermittent problem, it certainly seems like corrosion somewhere might be a factor, but I'm not sure where else to look.  I'm also thinking it's time to use some electrolytic grease on all those exposed UEC terminals.

 

Anyway, here are the troubleshooting diagrams from the FSM that I've been trying to figure out.

 

 

[Imp558]

Find and check all the grounds in the engine bay and on the engine. also Don't Unplug or plug-in a computer unless you have the battery disconnected. it sounds like you have a power delivery problem but if you rolled out all the fuses and everything power related ground is what's left

Edited May 28, 2017 by Imp558

[55trucker]

 Instead it appears to be ECM related, because the SES light that is supposed to come on before you start the engine is NOT coming on, and is NOT flashing when put in diagnostic mode at the ALDL. 

Are you absolutely certain that the check engine light bulb is functioning? If it IS burned out then you'll not get the display of codes that may be set. The only other way that you can accurately read the ECM is with a scanner.

 

You've got the factory manual, did you go into the section for troubleshooting diagnostics for stalling/intermittent start?

 

For the DOHC it should be section 6E3-B-1 3.4L Driveability & Emissions.......section B symptoms.......Contents

[digitaloutsider]

I agree, make sure the MIL lamp is actually functioning.

 

After that, the very first thing I would do is ohm out the injectors with a multimeter. 

[crazyd]

The SES light is a symptom, not the problem.  It is definitely working.  If it comes on I know the car will start.  I also have a scanner but it cannot connect to the ECM over the ALDL port.  The injectors are also a symptom, not the problem.

[55trucker]

OK,

 

go to page 6E3-A-14 3.4L (VIN X) (LQ1),

 

Chart A-2

 

*NO ALDL DATA or WON'T FLASH CODE 12*

 

go thru the diagnostic procedure on those 2 pages for troubleshooting the ALDL symptom, note that there is a reference in there for chart A-1,

 

let us know what you find

[Imp558]

No. No ses light, no fuel pump the ecm is not powered. If you're sure it's got power then the problem has to be a ground. That's all there can be to it.

 

there are two grounds for the computer one is down by the starter on the bellhousing and the other is on the frame on the front left of the engine bay (driver side)

Edited May 29, 2017 by Imp558

[crazyd]

That was it!  My messing around with charging the A/C must've made an okay ground into a slightly iffy ground.  Tightening up the front left frame ground brought it back to life.  Thanks Team W!

[Imp558]

Yup, go team!

[crazyd]

Wanted to mention for anyone else who may face this problem in the future that I also found a deteriorating connection for the small gray wire that connects at the battery terminal post just above the battery itself, this is also a potential culprit as that's apparently the ECM's main power wire.  The original was hanging by a thread and broke when I went to tighten the terminal post nut, so I crimped on a new ring terminal connector.