I’ve got a 1986 IROC-Z28 with the 305 TPI in it and i’ve just about rebuild half of it in the last year or so and have had zero idling or engine issues. But recently I’ve been dealing with a little high operating temp for my likings. So I replaced the heater core after finding a leak in it but now the car has a very puzzling idle issue. Depending on some unknown factor the car will either idle at 1500 rpm’s in park and 700 in drive. But when it’s put into drive and i let my foot off the brake it begins to accelerate very fast on its own. Accelerating fast enough that it will shift all the way up to third gear (25-30mph) without ever touching the accelerator. OR it will idle at 700 in park and somehow at 350 and will steadily sit there without cutting out. I’ve checked for vacuum leaks multiple times and confirmed the MAF, timing, and IAC operate as should. Please help I can usually figure out issues with either this forum or my service manual for the car but this one has me stumped. Thanks
(Also forgot to mention it has a custom dongle for the ECU that deletes the EGR valve and a few other things such as the cold start injector.)

 

Verify the CTS signal is getting to the ECM. Without an accurate temperature reference, the target idle will be higher.

Also verify the tachometer is correct, or in other words, don't count on the factory tach being accurate.

 

I will take a look at the CTS that’s a good suggestion. The tachometer was rebuilt a year ago done myself so it isn’t that. I’ll have a look at the CTS and report back. I have replaced it twice already due to the first replacement being faulty.