Ok I've been a mechanic for quite a while now and one thing has me a little puzzled. 85 IROC 5.0 TPI.

Car runs great when I start it up, after a little while (not quote to closed loop) it starts to misfire. It's a random misfire and loses Rpms from 900 to anywhere down to 500 and sometimes dies at the light by dipping too low if it's really warm out. It immediately fires right back up no problem and continues to misfire.

While messing around today I unplugged the wire that is disconnected from the distributor to set timing and the misfire completely stopped. I plugged it back in (ofcourse the Check engine light came on as soon as I unplugged it) and no change, it had the smoothest idle with no problem. If i shut the car off and restart it then it starts to misfire again, but if I unplug the timing wire and plug it back in then it runs perfect.

Anyone ever dealt with this before and have any suggestions?

 

Im not a big fan of the 870 ecm, your ecm controls the timing with the est connected. It will advance the timing with the est connected at idle . you would have to watch what its doing to cause the miss. This could be a bad icm but Im thinking the ecm maybe to blame. The 1227165 swap is worth the effort.

 

Thanks tuned performance ill look into it. Just got my aldl in the mail so ill be data logging to see what exactly the timing is doing and approach it from there. I have a spare icm from my old car so ill swap them n see of it makes a difference. Don't know the ohm specs of it or I would test that way. M
Thanks again

 

i not aware you can test a icm with measuring resistance . Another possibility is false knock counts retarding timing with the est connected .

 

I've tested icms in the past by doing a ground check. Seemed to be around 5-10 ohms was showing poor ground once it got heated up and was my sign it needed replaced.

Might be different on this one but I've done quite a few lt1 icms like this as well as others.

I hope to have something jump out at me once I get a data log