Hi everyone,

I've got a problem that I haven't been able to diagnose, and it's driving me nuts! Hoping for some friendly advice.

The car: 1991 Camaro RS, LH0 3.1L V6 MPFI (VIN code T), bone stock, 161k miles. Last year had the heads rebuilt, installed new water pump, timing chain, distributor, alternator, radiator, oil pan, starter, and had injectors tested and cleaned. 4L60 automatic.

The Problem: I was driving it and it just died. After maybe 30 or 40 minutes it restarted and ran fine. I made it a few miles and it died again. Had it towed home. Several hours later, it started up and ran fine again, as I moved it into its parking spot.

Now in the driveway, it starts right up and runs just fine, for maybe up to a minute, then dies. On restart attempts, about 25% of the time it'll idle really slowly and badly for up to 30 seconds before dying. It might do this several times in a row. The rest of the time, it might fire a few cylinders but won't stay running. After several hours, I can go out, it starts and runs fine again, then dies after 30 seconds or a minute.

Diagnosis attempted thus far: Checked fuel pressure at fuel rail, checks out ok at 41psi. Gas tank about 1/4 full, added another gallon for good measure. Changed the fuel filter just in case (or out of desperation). Put a noid light on injector connector, injectors are getting signal pulses. Since the car runs fine at times, and the injectors were recently cleaned and tested, for now I'm assuming they're ok.

Put a spark test light on each plug wire, all plugs are getting spark. Visually inspected spark using an old plug grounded out to the engine, looks good and strong. Swapped the coil for an old one I had in the garage, no effect. Looked around the engine bay at night to see if any stray sparks were jumping around, and there were none.

Check Engine light comes on at startup then goes out like normal. No codes are displayed when I jump the ALDL test terminals (other than 12, which is expected).

IAC and TPS are fairly new. I dont think the problem is restricted to idle, I can't get the engine to do anything different by opening the throttle plate (tried quickly and slowly).

I don't have access to any scan tools. I'd love to see what's going on inside the computer, but haven't picked up the connector/software as of yet. I've always been suspicious of the ECU, but I don't trust computers at all really, so it may be unfounded.

Alright, so I have fuel, air and spark. Runs fine sometimes then doesn't. Please help!

Thanks in advance,
-Patrick

 

I know you said it had a new distributor a year ago , and that you say you see spark , but , what you describe is pretty much textbook ignition module failure . Remember , brand new parts can and do fail , sometimes with alarming regularity , and just because you see spark is no indication the spark is sufficent to run the engine . Are we talkin a bright blue lightning bolt of a zap that'll jump a 1/2 inch gap in free air , or a crackly lil fizzle that gets lost in the compression of the cylinder ? Did you check for this spark both before and after it dies to see if the spark is weaker after it's run a bit (then died) ?

 

Well, it zapped the snot out of me, but that's not much of a diagnosis, more of an observation. The spark I observed was a bright blue spark, observed while the engine was barely running. I've got a relatively new ignition module on the shelf I'll try swapping in. I'll have another look at that spark first though. Once it stops raining, that is. Thanks for the suggestion.

 

Well, I ran out and swapped the ignition module, but she still won't start. Then the rain picked back up, so I didn't get much more chance to poke around. Any other thoughts?

Side note- The module I had on the shelf was in the car for about a week before I replaced the distributor. It worked at the time.

 

Well there goes the easy diagnosis .... Anyway , another crazy thought that popped into my mind , while reading your description , may be a failing cat. converter partially clogging the exhaust ? You mentioned 160 K miles and I wonder if the cat has broken up inside and is back pressuring your engine to death with each startup ?

PS , got friends in Florida and they tell me you folks are seeing record rainfall these past days . I totally understand the frustration of troubleshooting between downpours !