Tach jumping/miss

[gasburner]

Hello,
I'm narrowing down a miss/jumping tach problem. 1993 C3500, 454. Fresh rebuild on motor (<300 miles). New coil, distributor, wires, plugs, nothing half-a$$.
While driving the tach jumps as much as 400 rpm and there is a definite engine surge/miss with frequent pop in the exhaust.
I've replaced both connectors at distributor-heat shrink terminals, spiral loom, done properly.
My latest clue is that if I unplug the tan/blk timing wire, the miss disappears and the tach is smooth.
I'm unclear if I have an ESC on my truck, where it is, or if the timing is all in the ECU?
I suspect that my problem is one of the two. Does it sound like I'm on the right track here? Suggestions appreciated!

 

[1998_K1500_Sub]

Hello,
I'm narrowing down a miss/jumping tach problem. 1993 C3500, 454. Fresh rebuild on motor (<300 miles). New coil, distributor, wires, plugs, nothing half-a$$.
While driving the tach jumps as much as 400 rpm and there is a definite engine surge/miss with frequent pop in the exhaust.
I've replaced both connectors at distributor-heat shrink terminals, spiral loom, done properly.
My latest clue is that if I unplug the tan/blk timing wire, the miss disappears and the tach is smooth.
I'm unclear if I have an ESC on my truck, where it is, or if the timing is all in the ECU?
I suspect that my problem is one of the two. Does it sound like I'm on the right track here? Suggestions appreciated!

There was a fellow that had a very similar problem earlier this summer.

He had a problem with the distributor cap and/or ICM:

Post in thread 'Bad running after manual swap'
https://www.gmt400.com/threads/bad-running-after-manual-swap.66935/post-1408658

 

[gasburner]

There was a fellow that had a very similar problem earlier this summer.

He had a problem with the distributor cap and/or ICM:

Post in thread 'Bad running after manual swap'
https://www.gmt400.com/threads/bad-running-after-manual-swap.66935/post-1408658

I have seen that thread. The fact that the coil, distributor, tach, and ICM all work great when the timing wire is unhooked makes me believe that all these new parts are working properly, or at least trying to.
I've seen several other threads related to coils, pickups, and wiring with similar symptoms. I have wiggle-tested everything everywhere. The tan/blk timing connector absolutely fixes it every time. It has to be related to that wire and what it goes to, in my brain anyway. Thanks for the reply!

 

[1998_K1500_Sub]

I have seen that thread. The fact that the coil, distributor, tach, and ICM all work great when the timing wire is unhooked makes me believe that all these new parts are working properly

Don’t forget, when you connect that timing wire, the ICM then derives the ignition timing from a signal from the ECU.

So the ICM could still be at fault…

Or perhaps the ECU has an issue.

Less likely, the wiring itself is a factor.

 

[Rock Hard Concrete]

Sounds like ICM. It interprets the signal from the tach. When est wire is unplugged, it is bypassed. They are known for being bad from the factory, in increasing numbers these days. Use lots of heat sink grease if you do end up replacing it. Heat can also make them go bad quickly and they are in a spot that gets hotter than ****.

 

[gasburner]

Don’t forget, when you connect that timing wire, the ICM then derives the ignition timing from a signal from the ECU.

So the ICM could still be at fault…

Or perhaps the ECU has an issue.

Roger that. Prior to the engine rebuild this was an issue. Fuel line was hitting old ICM and broke a piece off the connector shield.

Sounds like ICM. It interprets the signal from the tach. When est wire is unplugged, it is bypassed. They are known for being bad from the factory, in increasing numbers these days. Use lots of heat sink grease if you do end up replacing it. Heat can also make them go bad quickly and they are in a spot that gets hotter than ****.

I've had bad new parts before. One of these is used from the old distributor. The second came in this new distributor and got replaced by the one in it now. There is currently another entire new distributor in the motor. Four different ICM's with the same symptoms has to be like lottery odds. This is part of why I'm leaning towards ECU.

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[1998_K1500_Sub]

As I remember, there are four wires that lead to the ICM. Do you have the means to check the voltages / signals on those wires? DVM / ‘scope?

Swapping an ECM wouldn’t be too tough.

 

[gasburner]

I have DVM, might be able to get an old MODIS going to graph voltage. There is also a 2 pin connector that runs to the coil.

 

[Schurkey]

1993 C3500, 454.

if I unplug the tan/blk timing wire, the miss disappears and the tach is smooth.
I'm unclear if I have an ESC on my truck, where it is, or if the timing is all in the ECU?

The engine will have "ESC". Early engines had a separate ESC module, later ones had it integrated into the computer.

I don't know when the change took place.

Download the service manual set for your vehicle from the links in the Sticky thread section of the Engine forum.

 

[gasburner]

The engine will have "ESC". Early engines had a separate ESC module, later ones had it integrated into the computer.

I don't know when the change took place.

Download the service manual set for your vehicle from the links in the Sticky thread section of the Engine forum.

Studying manuals now, thanks!