Hey y'all, I recently purchased a 1989 Pontiac Trans Am GTA and drove it home 7 hours no problem on the highway. However, as soon as I put the car in park, it sputters and struggles to stay alive, rpms sometimes getting as low as 400, constantly surging in random patterns, usually dying unless I keep giving it gas when it's about to die. On a cold start, it's much worse. The car won't stay alive more than a few seconds after starting unless I rev it. Shifting is rough, the car dies or almost dies every time unless I apply gas immediately after the shift. Basically the car sounds like its cammed but its not. Sometimes after the car is warm, it will idle no problem around 800-1000 RPM, on one occasion after driving through town it idled for over 10mins in park, shifted and drove fine.

I hadn't moved the car until today, I adjusted the idle screw and raised the idle, which seemed to alleviate the problem, but now I'm worried the RPMs are too high for shifting, especially because it's still erratically surging to even higher rpms at times. My temperature gauge never gets above 160 degrees, which seems pretty cold. Could it be a bad coolant temp sensor causing the weird idle? Also the previous owner also installed a cold air intake, which could have messed with it.

In order for it to shift on it's own with no throttle application, the car has to have been warmed up with my assistance on the gas for at least 3-4 mins, and it's only 45 degrees Fahrenheit. Car seems to idle better after it's been driven through towns, rather than highway speeds, which is why I think it's a temperature thing. Seems abnormal, let me know what y'all think.

I attached two videos below, one is of the car on a cold start, idling rough, you can hear it surge. (it died after the video ended)

The other video is of the car after I raised the idle, and it had been warmed up for a few mins.

 

Ok so I did the paperclip test and got a code for coolant temp sensor, circuit 'low or open'. according to https://mechanicbase.com/trouble-code/obd1-codes/

So should I replace the sensor or is it something electrical

 

Use a ohm/multimeter to check the resistance of the sensor first. Could be bad wiring, could be a bad sensor. On the far side, could be an ecm.

 

Does your car have an EGR valve? Disable it & see if it makes any difference. Been there & done that.

 

Not sure, i'll check tomorrow.

Here is a video of the idle surging after a couple mins of running. https://streamable.com/zvn7oi

 

I had similar surging idle/stalling issues with my 1992 Z-28.
Two items that fixed my problem.
1) Changed the fuel filter under the car, below the left rear passenger seat.
2) Replaced the vacuum switch in the HVAC dash control. ( I could hear mine leaking when moving the control levers)
After both were replaced car returned to good health.