Hello Everyone!
Hoping to please receive some assistance with an idle issue I'm having on my 1989 Pontiac Trans Am GTA. It has the L98 5.7L in it and everything is stock/untouched/unmodified (46k miles if that info is important). Only thing done to it was a light tune up of spark plugs, wires, and an air filter a week or so after purchase. It's been driving perfectly smooth for the year or so I've owned it, but it's now developed a weird idling issue. Upon start up, the car sits at 1k like it should but then after about a minute of warming up it drops to around 500 and then bounces as if its revving itself, the needle bounces from 500-750ish RPM. I have noticed on a couple occasions while out driving it'll suddenly get stuck at a high idle and I'll need to go to Neutral to let get down to normal again. Basic research has led me to believe it could be an Idle Air Control Valve, TPS sensor, or a vacuum leak. I know for sure it's not a vacuum leak so that leaves me guessing at the IAC and TPS. Both are original to the car and have never been changed to my knowledge and look their age. Would swapping them out for new ones get it back to normal and is a certain procedure required to set them or program them? Apologies if this is well known info but I'm a bit on the younger side at 25 years old and my knowledge is surface level at best, but I have a strong sentimental love for these 3rd gens. Any assistance/advice on this issue would be immensely appreciated as it bothers me incredibly that it doesn't drive as beautify as it did before and me not knowing what to do makes it worse. (I can attach a video if that helps).

Thanks Everyone!

 

Video:

 

Ordinarily I'm not in favor of throwing parts at a car without doing proper diagnosis. I'm assuming you're not getting any trouble codes or the SES light coming on.

But in this case, if you're convinced the IAC and TPS are original from 1989, it probably wouldn't hurt to change them. Even if they're not the issue, it'd be preventative maintenance for the future.

For setting the IAC and TPS, simply follow the "How-To"

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

If it ends up not being either of these two, I would look into getting the ability to scan the ECM. Either an OBD-1 scanner (which are getting harder to find), or getting a ALDL to USB cable, and then using Tunerpro with a Windows laptop.

There could be numerous things going on... bad injector(s), other bad sensors, etc... You'd be relegated to guessing without being able to see the running data from the ECM.

 

Thanks for the insight! I too prefer not to just throw parts and money at it, especially since I work to pay all of the bills I have and the car was to rekindle childhood memories with my dad and his old TA's, so not able to throw high amount's of money like it's no big deal lol. And they do indeed look original to the car so I hope swapping 36-year-old idle valves and sensors for brand new ones can help solve the issue quick and easy. Thanks for the "How-To" link as well. Missing a couple of tools listed so need to get my hands on a volt-meter and possibly a scanning tool.
Thanks again!

 

The Idle Air Control solenoid and Throttle Position Sensor aren't consumable tune up items. They either work right or they don't. All mechanical parts will wear out over time with use though. Your car just has 46,000 miles so they shouldn't be worn out. I've had TBI and TPI cars and trucks with over 250,000 miles and the original OEM GM assembly line installed IAC and TPS. The IAC was removed, cleaned and lubed and the IAC passages cleaned in the IAC housing and throttle body and a new IAC gasket and throttle body gasket installed. At 46,000 miles the throttle shaft bores in your throttle body probably aren't worn out and in need of being bushed.

The TPS either swings smoothly through its voltage range without skips and jumps or it doesn't. Unless you can prove through testing that something is wrong with your IAC or TPS they don't need replacing.

Most likely the throttle bores and throttle plates in your throttle body need cleaning and /or the IAC solenoid, IAC passages in the IAC/Coolant housing and throttle body need cleaning along with replacing the IAC solenoid gasket.

I'm ready to eat some ice cream and watch a movie. I'll add to and edit this some more later on.

 

Ah I wasn't aware of that. Since the post I've had the car actually tested for vacuum leaks via a smoke test and it passed. As a precaution a couple of lines were changed just to prevent a future leak. Seeing as how when it comes down to idling issues, research says look at the IAC. Didn't realize parts like that should last a long while and not just go bad from age. I'll look into what the cleaning procedure looks like and if I'm able to get the stuff on it cleaned up. Because otherwise, I'm so thrown off at the bouncing low RPM's.

Thanks for the insight!

 

I just now attached the video to get a better visual of what I mean.
Thanks Everyone!

 

If you have rust dust under the distributor cap, replace the distributor. Took me 10 years to find my idle problem.

 

Woww you're serious?? Dust under the distributor cap caused an idle issue for you? Taking care of that solved it?

 

I see in more than one thread you have that you have replaced the sparkplugs, sparkplug wires, and air filter. But no mention of the distributor cap and rotor button. Have you replaced these or took the cap off and looked inside yet?

After watching your video there is a slim to almost 0% chance this is causing your idle speed hunting issue. But it for sure should be looked at.

I removed the distributor cap off my friend's 27k mile 1 owner 1990 Formula 350 that you could eat off of top to bottom, inside and out and underneath, and the inside of the distributor looked like it had been recovered from the bottom of the ocean right next to the Titanic. A huge heaping handful of white fluffy corrosion mixed in with green corrosion and rusty AF inside.

The original assembly line installed Delco-Remy distributor cap and rotor button were ate the f*ck up with just 27k miles on them. The distributors for our cars are poorly designed due to the fact they have no "good" way to vent the ozone and moisture out of the distributor cap. They have two fine mesh screens on the bottom of the distributor to vent the ozone and moisture out of the distributor cap but most of the time they are clogged up and never did work worth a damn to begin with anyways. The cap itself should have been vented.

There is an improved rotor button for our distributors that greatly helps with this problem that was used on marine and HD applications.


ACDelco D461 GM Original Equipment Ignition Distributor Rotor Button.

This is the improved marine and HD rotor button that came in the distributors for those applications.


ACDelco GM Original Equipment D447 Ignition Distributor Rotor Button.

This is the crap rotor button that came in the distributors on our cars.



GM Small Cap HEI Distributor Magnet Cracks.

I haven't forgotten about my post above concerning the IAC and TPS. I been busy doing other things and I'm lazy. I'll get back to it.

 

Good deduction skills Wolfe! I actually have not replaced the distributor cap and rotor button. Maybe that's something I should look into as a precaution so it can just have a fresh one. The only thing I know about bad discaps is you see them physically aged or like misfires/rough start which the car never had so I assumed I was in the clear. But now wanting to do the cap and rotor as your insight reveals a lot. I really appreciate the visuals and descriptions thank you!

Also, as for the IAC, with the advice given here and some research on google/YouTube, first I'll try some cleaning. Going to stop by Harbor Freight to get some Electronic Connections Cleaner, and Throttle Body cleaner. Simple enough for my young-inexperienced *** to try and do first lol. Going to disconnect the IAC plug and spray with the electronics cleaner and then spray up the throttle body/air intake areas and lastly, if I have the proper wrench size, remove the IAC and spray clean the pindle. If I have no success after a good cleaning, then a new IAC is my only next move.

Any other tips and knowledge is immensely appreciated! Thank you!

 

Give me a few seconds and I'll send you links to the CRC products I use.

"CRC" Brand

QD Electronic Cleaner, Throttle Body & Air Intake Cleaner, Mass Air Flow Sensor Cleaner, Technician Grade Dielectric Grease

I use the Electronic Cleaner & Dielectric Grease for the connectors. The Throttle Body & Air Intake Cleaner for the TB and IAC cleaning, and the Mass Air Flow Sensor Cleaner for guess what?

Probably have to go by an auto parts store to get it all in one place.

 

Thank you!!

 

I updated my reply above.

Do just the throttle body bore cleaning first. Don't get your fingers pinched in the TB by the blades while you have the throttle at WOT with one hand. Engine off of course.

Clean with a lint free rag and or soft toothbrush. Get the bores and blades clean as best you can. The car will run like azz for the first little bit when you start it afterward.

Remove the MAF sensor and spray the hot wire inside with the MAF cleaner. Inside the MAF sensor will be a small round opening that looks like a gunsight with a little bitty wire inside. That is your target for cleaning. That little bitty wire.

Sometimes this is all it needs to fix your problems. Try this first step and see how it turns out. Report back results.

 

Took note of the cleaners in the reply above thank you!

And noted, I'll start off with the throttle body cleaning first and the MAF sensor cleaning, and I'll report back.

Thank you very much AirWolfe!

 

I was specific when I said 'rust dust' under the cap & rotor. My $50 distributor from rockauto, car runs amazing.

 

Distributor cap and rotor are definitely in my future then.
Thanks!

 

We have an English communication problem. DISTRIBUTOR
The entire assembly.

 

Quick reply lol, noted - thanks!

 

46k on a third gen? I'd recommend taking a fuel sample also. Check for water and contaminants. The fuel tanks are made of steel and will corrode from the inside out if left to sit for too long.
After watching the video I doubt that's it but it's an easy check and worth a shot.