HELP Diagnosing- not sure where to start...

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MakeMineaMonte

Not-quite-so-new-guy
Oct 18, 2018
36
6
8
RI
I have an 86 Monte with a mostly stock 305 V8. It's been driving great for a while now. The other day, out of the blue it started misfiring here and there and just running a little off. I drove it the next day- same thing. The day after I was driving it to my studio to try to figure it out and it started running very rough. It stalled out at a light. I was able to start it back up and got another block away when it stalled again in the middle of the road. I couldn't get it restarted. The battery eventually died from trying. I had it towed. I checked all of my spark plugs and wires. The wires were good (relatively new) and the plugs had what looked like carbon build up. The distributor cap looked like it was getting good connections, etc. and nothing looked out of the ordinary. No vacuum leaks that I could find. I cleaned the spark plugs with a wire brush and started the car. It ran well for about 5 minutes and even drove well. I decided to bring it back home, but it started misfiring again on my way, finally stalling out in my parking lot. I wanted to make sure the electric choke was getting power, but the car won't start again at this point. I'm guessing it's a fuel or spark issue, but am not sure where to start. Carburetor (Quadrajet)? Distributor? Other? I'm pretty close to towing it to a shop, but really want to learn more and try to fix it on my own. Any ideas? What should I check and how?
 
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PVDave

Apprentice
Oct 28, 2021
52
34
18
Arvada, CO
I cleaned the spark plugs with a wire brush and started the car. It ran well for about 5 minutes and even drove well. I decided to bring it back home, but it started misfiring again on my way, finally stalling out in my parking lot.
Based on this description, it sounds like you have an electronic module that functions while cold, but fails once it warms up. Your ignition coil module is the prime suspect, but the issue could reside elsewhere.
 
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stew86MCSS396

Greasemonkey
Aug 1, 2022
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ICMs are usually all or nothing. Cold fine and warm/hot nothing. Not even a hint of a misfire. The symptoms you're describing that I've experienced was due to a failing ignition coil. don't know if it makes a difference but this was with a divorce coil small cap dizzy on a '87 Monte.
 
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PVDave

Apprentice
Oct 28, 2021
52
34
18
Arvada, CO
ICMs are usually all or nothing. Cold fine and warm/hot nothing. Not even a hint of a misfire. The symptoms you're describing that I've experienced was due to a failing ignition coil. don't know if it makes a difference but this was with a divorce coil small cap dizzy on a '87 Monte.
I like coil as well, especially since they warm up quickly and tend to run hotter than the other electronic components.
 
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MakeMineaMonte

Not-quite-so-new-guy
Oct 18, 2018
36
6
8
RI
Based on this description, it sounds like you have an electronic module that functions while cold, but fails once it warms up. Your ignition coil module is the prime suspect, but the issue could reside elsewhere.

ICMs are usually all or nothing. Cold fine and warm/hot nothing. Not even a hint of a misfire. The symptoms you're describing that I've experienced was due to a failing ignition coil. don't know if it makes a difference but this was with a divorce coil small cap dizzy on a '87 Monte.

Just to be clear, it wasn't starting at all at this point. The engine might crank once or twice and then nothing. Still sound like the coil? Regardless I went in to check out the ignition coil module and when I was taking it out one of the lead wires broke. So maybe that connection was already disconnected and leading to this issue? It tests fine for resistance, but not sure if that really means anything with regards to this issue. So now I need to either fix that connection or just get a new one anyway... Thoughts?
 

PVDave

Apprentice
Oct 28, 2021
52
34
18
Arvada, CO
Just to be clear, it wasn't starting at all at this point. The engine might crank once or twice and then nothing. Still sound like the coil?
Yes
It tests fine for resistance, but not sure if that really means anything with regards to this issue.
If it is failing when hot, it will pass a resistance test while cold. I'd repair the connection and buy a new coil. Fixing the connection may work until the coil heats, and then it will shut down again.
 
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bracketchev1221

Royal Smart Person
Jan 18, 2018
1,363
1,551
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2 areas that usually go are the module and the button from the coil to the rotor.
 
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oldsofb

Royal Smart Person
Supporting Member
Dec 7, 2007
1,360
3,910
113
Maryland
There is a little tube with a clear dielectric looking grease that comes with the module (or should) that gets smeared on the metal side that goes down when it bolts in. I have seen premature failures on modules that were dry when removed for replacement.

Not sure if that's there as a heat barrier or not but that's what I've observed.

Hutch
 
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Ernest

G-Body Guru
Apr 28, 2016
687
865
93
Texas

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