What parameters control idle speed on a cold motor before going into closed loop? I am chasing a high idle on a cold engine. Motor is a1989 L98 with mods listed in my signature. The car idles fine after it goes into closed loop but before it warms up it will idle around 1,200 RPM and it cranks for a long time before firing. A warm restart idles perfect (~750) and starts right up. I am wondering what inputs the ECM looks for when idling cold. Is it just CTS or is MAF input involved? I have replaced a ton of ignition components and sensors throughout the vehicle but the issue remains. I don't have access to a wideband to check AFR but will track one down if it is likely a fueling issue. I do have an ALDL cable and the ability to datalog.

 

CTS and MAT (same function as IAT) are the primary inputs, The MAT in the stock location is on the underside of the plenum. That's not very convenient. Moreover, the sensor can be artificially heated by the plenum, so restart sensor readings can be way off.

My '86 is similar, and I moved the sensor location by abandoning the original in place and adding a second IAT and extension harness to mount it in the intake air duct.




It was an easily accessible connection to the wire harness.

 

I do have a MAP style intake with the intake temp sensor mounted in it. I may try out the extension wire and see if there is a change. I assume you leave the old sensor in place or find a threaded plug of some sort? I will also see what those sensors are reading at startup with the ALDL cable.