Help! 89 350 tbi won’t run with timing advance plugged in.

[BeatUp98]

I had a weird intermittent problem with the truck randomly dying then sometimes restarting and sometimes not. I’ve pulled the distributor and replaced the icm and pickup coil and it already had a fairly new cap and rotor. Shaft look good and gear had no wear on it. Reinstalled now and timed to 0 degrees without the timing advanced plugged in but now it won’t run with it plugged in. Runs great without it plugged though which leads me to think I have a bad icm out of the box?

It’s an o Reilly’s accel part I believe. Should I just try at least 2-3 more to be sure? I’m limited on my ability to diagnose these engines.

I should add it’s a rebuilt motor with maybe 30k miles and it has new plugs and coolant temp sensor and before I pulled distributor ran great besides the weird occasional dying issue.

 

[PlayingWithTBI]

It’s an o Reilly’s accel part I believe. Should I just try at least 2-3 more to be sure?

It sure does sound like the ICM. How old is your ignition coil? A bad coil can/will take out the ICM too. With the EST Bypass wire disconnected, you're taking the ECM out of the loop in spark control so, the pickup coil sends a sine wave to the ICM which converts it to digital and grounds the coil causing spark.

At the same time, the ECM gets the signal from the ICM and fires the injectors. If it runs with the bypass wire disconnected, we have a pretty good idea the ECM is getting signal but, the ICM may not be getting it back from the computer. Check your wiring to and from the ICM too.

Here's an example of how the ICM is wired except it shows a 7 wire setup (without ground) instead of your 8 wire one.

You must be registered for see images attach

 

[BeatUp98]

I have no idea how old the coil is. I don’t wanna parts cannon this thing but is it worth getting a new coil and another icm to see if maybe it did hurt the new one?

 

[Schurkey]

I have no idea how old the coil is. I don’t wanna parts cannon this thing but is it worth getting a new coil and another icm to see if maybe it did hurt the new one?

It's not the coil. At least, the problem where it won't run with the EST wire connected is not the coil.

It might be the ignition module.

My money is on the computer, or the wire harness between distributor and computer.

 

[Jonathanhicks]

I agree with planinwithtbi I had the same problem on my father in laws 93 about a year ago

 

[PlayingWithTBI]

It's not the coil.

I agree, I was saying the coil can/will take out the ICM.

 

[BeatUp98]

Replaced the icm they didn’t have a coil in stock and same story. New issue however driving there if I let the truck idle very long it will stumble and die even with the timing advance unplugged. How do I diagnose the coil or computer?

 

[BeatUp98]

Another update. Put old icm back in and it won’t even start so I’m not sure new icm is the issue. I also replaced pickup coil at the same time is it possible pickup coil could cause this or perhaps the old ignition coil is messing with the new parts or vice versa?

 

[PlayingWithTBI]

How do I diagnose the coil or computer?

The coil can be Ohmed out and may prove it's bad but, it won't prove it's good.

Put a scanner on it and watch to see if it's commanding RPM feedback. Ohm out the wire from it to the ICM.

Put old icm back in and it won’t even start so I’m not sure new icm is the issue. I also replaced pickup coil at the same time is it possible pickup coil could cause this or perhaps the old ignition coil is messing with the new parts or vice versa?

Who knows, it's starting to sound like a wiring issue. Each time you replace parts you're disturbing 34 year old wires and connections.

 

[BeatUp98]

What ended up fixing it?

I agree with planinwithtbi I had the same problem on my father in laws 93 about a year ago