RedLeader289

iTrader: (2)

Good morning ladies and gents. Strap in, cuz this is a long story.

Ok, 385 Fastburn small block 350 crate engine, HEI distributor, Holley 750 dp carb.

Recently my car had a stumble at higher speeds (essentially when the secondaries would kick in). It sounded like vacuum advance wasn't doing it's job, so I replaced that - no dice.

The stumble got worse, AND at idle it started having a hesitation when you would give it hard throttle (i.e. instead of VROOM!!! you would get V*-*rOOOM!!!)

To cover all bases I checked spark plug wires, spark plugs, and even readjusted the carburetor (via Sofa's method) just to end up at the same tune as before but hey, don't know til you eliminate it, right? -NOTHING.

This past weekend I bench test the Pickup Coil, it tests out right where it should be.
Bench tested the ignition coil - failure. Had the guys at AutoZone test it - failure. Upon further inspection I see that the button/spring under the ignition coil is bent so that when the coil is installed it wasn't making good contact, so it probably shorted out and there ya go. Bought a nice new MSD Blaster coil and installed it. The hesitation was now gone (SCORE!!) but the stumble at higher speeds and on quick acceleration (think passing someone) is still there.
You're probably thinking "what about the ignition control module?" -well, I replaced that around last November/December and the car has MAYBE gotten 300 miles of drivetime since then.


After replacing the ignition coil and getting everything back together (plus new spark plugs because my Dad had an extra set he had picked up and not used, so why not?) I'm driving home and she starts doing something....strange on the interstate.

I'm cruising along at about 75 mph @ around 3250-3500 rpms (yay 3 speed transmission!) and the rpms randomly drop to about 1500 for a couple seconds and jump back up. (this is just long enough for me to lurch forward in my seat, look at the tach, and see them jump back up). It does this a handfull of times and no matter how much throttle I give the rpms only jump back whenever they decide they want to (resulting in a backfire out the carb one of the times).
Then the kicker, it does the rpm drop and stays there. I throw it in neutral and coast, it doesn't recover (even trying to give it gas, etc.). then it just dies. Can't get it to start again until I come to a complete stop (about 2-3 minutes after the shutdown) and she fires up and runs just fine (except for the stumble is still there). So I get off the interstate and take surface streets home.

TLDR - new plugs, wires are good, new ignition coil, pickup coil is good, carb is good, fuel pump is good, timing is good, I THOUGHT the ICM was good but I'm thinking that's my culprit.

Thoughts/Questions/Comments/Snarcy Remarks?

Talk about frustrating.

Also, sitting in park and revving, idling, etc the car sounds just fine.

RedLeader289

iTrader: (2)

Found out my NAPA ICM has a 1 year warranty, so I'm going to do the free swap and see if it fixes the problem...

bigal55

iTrader: (1)

Have a cousin who had a similar problem, whenever the vacuum advance would move the pickup plate, the car would either die or drop RPMs just like you describe. The key point of that last sentence is about the pickup plate moving. Check the wires from the pickup to the ICM, his were almost completely broken. Something to look at. HTH and GL!

RedLeader289

iTrader: (2)

Thanks! checked and my wires are good. I swapped out for the new ICM and it fixed the problem. It's nice to be able to go WOT again

bigal55

iTrader: (1)

TTOP350

iTrader: (8)

GM ignition modules are the way to go.

RedLeader289

iTrader: (2)

Got a part number? Is it an actual GM part or an AC Delco?

I'm good for now but if I find a deal on some I'll try to pick them up, clearly it never hurts to have spares in these cars....