I have a third gen with a '94 F-body LT1. It's been starting unreliably for some time--sometimes it'll start right up, sometimes it'll crank and only start firing when I let off the key, sometimes it'll just crank and do nothing. Once it starts, it always runs well and doesn't stop until I turn the ignition off. When it's been sitting for a while and the engine is cold, it almost always starts immediately.

I did quite a bit of poking around, but to make a long story short, I tested for spark and injector pulses while it was cranking but not firing; spark was present (pulled one plug wire off, stuck a screwdriver in the end, and set it on top of the cylinder head where I could see the spark while cranking), but the PCM wasn't firing the injectors. The fuel pump is turning on, though, and VATS is programmed out. I can't think of what would make the PCM decide to not fire the injectors and only do so during cranking......; if anyone has ideas and/or experience with this, it would be greatly appreciated.

 

You did check the fuses.

When the fuse(s) is blown or out, the PCM shuts them down. NO SES. But a code is present.

 

Double-checked the fuses, and they are good. The only codes on the computer were 15 (coolant temp input low) and 42 (EST grounded). The coolant temp problem turned out to be a bad connection at the sensor--tweaking the contacts a bit fixed that. However, the problem's still there. I noticed that if it cranks and doesn't fire, waiting with the key in run and trying again after a few seconds does nothing, but if I turn the key off, wait 5 seconds, and turn it back on again, it fires right up.

As for the code 42, I've had that for quite some time--it sets immediately upon starting. When I first checked codes a few months ago, I had my old GM opti on the motor, and there was a low res pulse failure code in there as well. That code did not set immediately like the code 42, but I'd also had the engine stumble hard a few times, and I thought both codes were from an opti on its way out. Now I have an MSD replacement opti (P/N 8381) on the motor, and the stumble hasn't reoccurred. Seems like the code might be something to do with the ignition module, but I don't see how it could be related to this problem, since I always get spark even when the motor isn't firing.

 

A loss of the low res signal from the opti can cause the PCM to shut down the injectors. But it cuts out the fuel pump also. Perhaps you could monitor both the FP relay and the injector pulse to see if they both drop out at the same time or not.

 

Fuel pump is definitely on during the "cranking but not firing" episodes (put a meter on the power wire from relay to pump), and the code 36 and stumbling haven't come back since I put the MSD opti on. Doesn't seem like that's the problem.

 

check for bad grounding on the injector wires. had a jeep with this problem...everything but the injectors worked fine because the wires were cut.

 

They aren't grounded, our injectors are "grounded" by the PCM. They have 12 volts going to them at all times and the PCM fires them by "grounding" them. If the wires are cut to the injectors, it throws a code right away. Shuts down the suspect injector but will not set a SES.