My 91 Sierra 350 TBI has been intermittently not firing on occasion and more recently, it's cut out and shut off while I was driving. About a year ago, it first had issues not firing. I had a mechanic friend look at it and he thought he had it narrowed down to the coil. I replaced the coil and no go. He then told me to have the ignition control module tested at NAPA. I did that and it passed (tested several times). The guy at NAPA said to check my grounds, so I cleaned off the ground on the coil mounts and it started up just fine.

I drove it probably once or twice a week for a year and it worked just fine. Then after making about a 100 mile trip, it decided not to start again. I cleaned the coil mount ground again and it still won't start. I proceeded to check spark and fuel, and didn't come to any conclusion, then it just magically starts up and runs fine. So, I take it to town and as I'm driving home, it decides to die on me while I'm driving. I start it again and it dies again about a half mile down the road. This time, it wouldn't start again. I had to jump it to get it started because the battery wouldn't crank it any more. After that, I was able to get it home.

So, I've been reluctant to drive it anywhere because I'm afraid it will cut out on me. I've started it several times recently and used it around the farm without issue. Does anyone have any ideas on how to track down what might be causing these random problems?

 

Replace the ignition module. Put die-electric grease on the connectors that go into the module, and make sure they seat nice and tight into the module. They tend to not test as bad because they only fail when they get hot. It's good to have a backup module with you anyways. Make sure to apply a good amount of heat sink compound to the metal part of the module that mates to the distributor base plate because these ignition modules do not like to get hot.

Feel free to have it tested again, or when the truck dies again and has a crank no start condition again, pop a spark plug wire off and get a spark tester on there and see if you have spark. I'll bet you don't get spark when the truck acts up. Sounds like a classic ignition module.

 

Thanks for the advice. So I got all the stuff you mentioned above and put in the new module. Doesn't start. I check everything, then try the old module. Still doesn't start. No spark just like you said. I then tried putting a spark plug in the wire going from the coil to the distributor. Still nothing. I used my test light to check all 4 wires that go into the coil. They all pulse properly when I turn it over. I do have another coil, so I'm going to give that a shot when I get a chance. At this point, I'm thinking it has to be something at the coil, right? If I get pulses from all 4 wires (2 wiring harnesses) going into the coil and no spark coming out, it has to be something right there, no? It's gotta be either the coil, a bad ground for the coil or a bad wiring harness going into the coil. Is there anything else I'm missing?

 

Well I'd say you're on the right track. I figured it would be the module ... The module is working as long as you get 12V pulses to the primary side of the ignition coil (the side with two wires going to it) on the pink wire. Also check your IGN (F1) fuse in the fuse block in the dash right by your left knee.

 

If the fuse was blown out it wouldn't work at all. I would say it sounds like the coil. I jsut had to replace the coil on my 1990 with the 350 in it. It would just randomly not work every now and then. Replaced it and it runs fine now.

 

seen a lot of cracked magnets in the distributor shaft cause weird running conditions. Might be worth getting a rebuilt distributor??

The distributor uses a magnetic pick-up assembly located inside the distributor consists of a permanent magnet, a pole piece with internal teeth, and a pick-up coil. When the teeth of the timer core, rotating inside the pole piece, line up with the teeth of the pole piece, an induced voltage in the pick-up coil signals the electronic module to trigger the coil primary circuit.

 

Ok, the fuse was fine. I put the other coil I have in there and made sure the ground was good. Still no go. No spark again coming out of the coil. Is there a way to test these coils? The guys at NAPA didn't have a way to do it, so I don't know for sure if the 2 coils I have are good or not. I used my ohmmeter to test the connector from the ICM to the coil and both wires were fine. I'm not so sure that it's the distributor in this case because I'm getting signals going into the coild from all 4 wires on the 2 wiring harnesses. Anybody have other ideas?

 

Buy a new coil. they don't cost a whole lot, and you can always take it back.
And make sure you have the positive/Nagitive right on the coil wire hookups.

 

I'm wondering if the pickup coil in the distributor is sending a good signal to the ICM. If the pickup coil is bad, the ICM wont know when to pulse the coil.

 

I'm about ready to push this thing off a cliff. I got a new coil, put it in (along with the new ICM) and it still doesn't fire. Same symptoms. No spark coming out of the coil, but all 4 wires are pulsing with the test light when it turns over.

If it was the pickup coil in the distributor, the ICM wouldn't be sending pulses to the coil like what I'm seeing now, right?