4.9L Cadillac Intermittent starting problems

**sanderson231** · 01-17-2023

92 4.9L Cadillac in an 88 Fiero - I have three PCM's for the car. Let's call them Abel, Baker and Charlie. Abel and Charlie have stock PROMs. Baker has a PROM reburned by Sinister Performance to disable the VATS and few other things. The car is also wired with a Baker Electronix signal generator to disable the VATs.

Abel was in the car for a couple of years with intermittent driving. I jumped in the car one day and it started then quit. Try to restart and it would fire and quit. Put a noid light on one of the injector connectors and had the wife crank the engine. The light blinked once and quit, Multiple retries and the same thing. So I say it must be a VATs problem. Could be the Baker Electronix box or the PCM. Let's give Baker a try. He has the VATs disabled. The car starts right up and I drive it infrequently for a month. This week I get about 400 yards from home and the car quits. Put it in neutral and try to resart. Fires once and quits. Fortunately, I am on a small down hill incline and so I coast in the driveway - dead stick landing. Abel is sitting in passenger seat and the console is off. So let's give him another try. Same problem, fires once and quit. Very curious two PCMs with same problem. Charlie is resting in the garage so I grab him. Swap Charlie in and the car starts right. Being the curious type I put the Baker PROM in Charlie and the car starts right up. Being even more curious, I swap Abel back in and the car starts right up. Then Baker and the car starts right up. Then Charlie and the car starts right up.

Grab an analog ohm meter and test light. Check all the ground wires on the PCM plugs and get about 0.5 ohm - doesn't sound like a ground problem. Put a test light on wires that supply power to the PCM and it lights up brightly - doesn't look like a bad connection on the power supply. For good caution I grab a can of QR electronics cleaner and spray the PCM pins and plugs and then work them in and out a few times.

So after rethinking the symptoms, I am wondering if it is cam sensor or ignition module problem I think the car should have started off the limp home circuitry on the memcal even if the PCM couldn't execute the machine language instructions stored on the PROM.

Any other ideas?

**1project2many** · 01-20-2023

I would also consider an intermittent connection on power, ground, or another circuit and that the connection is affected by physically moving the wire harness to connect or disconnect the pcm. I'd also locate the OEM chart for diagnosis as they're usually pretty good at eliminating systems which are not the problem. For example, the OE test may direct a technician to crank the engine while watching for RPM on the scantool. If no RPM is displayed then the technician is directed to begin diagnosis of crank sensor and circuit but if RPM is displayed the crank sensor is immediately ruled out.

With the minimum number clues available I'm not convinced we should be trailing Professor Plumb through the mansion.

**sanderson231** · 01-21-2023

I have the factory service manual and there are a troubleshooting trees. First step is verify spark. I originally dismissed a spark problem since the car would fire and then quit. If the problem happens again I will use spark tester to verify that the spark continues while cranking and also verfify that the tach reads while cranking.

I hate to throw money at parts but I like it even less getting stranded on the road. So will install new ignition module and coil as a preventative measures.