1989 Trans Am GTA 350 TPI

Hello all, I've been having an issue that's gotten progressively worse over the last several months. When I start the car from cold, it will idle up to 2,000 momentarily before dropping to 5-600, and then surging up to 1500-1600rpms. It will do this over and over, the surge lessening over the course of 1-2 minutes until it has 'balanced' itself at 1,000 rpm.

The next issue is one that's been bugging me since I bought the car in late 2021-- When the car is finally up to temp and placed in park, it idles around 1400rpm and sticks there. Only does it when warm.

The car is also slow to start. Takes 3-4 seconds of crank and a tap of the gas pedal before it roars to life.

Any input greatly appreciated, car runs just fine in drive and at stop lights.

 

Have you checked for any vacuum leaks? Has anyone messed with or improperly adjusted the idle stop screw on the throttle body? A smoke test will be your best bet IMO, here is a simple and inexpensive one that I used after seeing this video, I placed the tube over a vacuum port on the plenum. Some ppl have had success using carb cleaner to find vac leaks but be careful on hot surfaces or an unlit propane torch could be used but once again be careful.

A few other ideas after vacuum leaks, any check engine lights? Cleaning the throttle body won't hurt either and inspect the throttle bushings for excessive play, check timing, a bad Throttle Position Sensor or maybe a dirty/broken Idle Air Control valve? One last idea is your Coolant Temp Sensor not reading correct. A scan tool would made checking the sensor readings much easier but you can check a lot with just a multimeter and referencing the shop manual values which a google image search should produce.

 

Will do, thanks for the detailed checklist. Question: How could a faulty idle screw adjustment be the cause of the problems described? Doesn't make much sense to me because the idle is both too slow and too fast depending on temp.

Thanks!

 

There is a special procedure to adjust the idle/minimum air speed on these cars, but if someone just adjusts the screw it affects the amount of air entering through the throttle body and can screw idle up. It really should not have to be adjusted, especially on a stock or slightly modified car. Easy way to tell if its ever been touched is to see if the little block off plate(its about the size of an eraser head) that covers the idle/air adjust screw is still intact. GM put that block off to keep ppl from messing with it.

I doubt that is your problem but just wanted to throw it out there in case someone had messed around with it, years ago on a fuel injected 2.8 V6 I foolishly tried adjusting idle that way and it caused high idle after start up etc and keep the idle up too high.

I really think you have a vac leak or your IAC is out of whack/dirty or TB really needs a good cleaning.



Check this out, good info on TPI start up sequence:

https://www.corvetteactioncenter.com...opriate%20time).


Also this may be useful, just remember if you perform the minimum air adjustment be sure the engine is up to operating temp:

https://www.thirdgen.org/tpimod2/