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Old 06-14-2010   #1
Fred91GTA
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Stalling at idle when hot

Ok, my GTA has just developed a new problem. When it's cold it will start up and idle fine. The car has plenty of power and runs really well. Now, a few days ago, I was at a stoplight and noticed that the car would idle a little funny. Watching the tach, the needle would fulctuate a tiny amount, maybe between 550 and 600 rpms, almost as if it was hunting for something. The car would also feel like it's about to die too. Once in awhile, it would really catch itself and rev to 800-900 rpms and then idle smooth for a little while and then act up again. Then once in awhile, while idling at a stoplight, it will stutter, feel like it's struggling then shut down. She'll start right back up and run fine for a little while and then the problem will come back a little while later. She stalled on me three times yesterday and it seems like the problem may be getting worse.

Some more infor on the car:
-Injectors were replaced with Ford 22lbs about 10k ago.
-Fuel Pressure is good
-I get no SES codes
-No vacuum leaks, I checked all over the place and have to clean up WD-40 now...
-Ignition control module was replaced with one from Standard about 1500 miles ago.
-Timing is set correctly

I had burned plug wires about 2 weeks ago, and I replaced both plugs and wires. Car ran great for a little while and then this problem popped up. I have a feeling that it's somehow related to a sensor for the TPI, I was just hoping somebody would have a little more insight. Thanks for any help you guys and gals can give.
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Old 06-14-2010   #2
FearThisGTA
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GTA Year: 1991
GTA Engine: 350 TPI Automatic
Re: Stalling at idle when hot

What about the IAC, ign coil, and do you have an aftermarket dist. If so, have you checked the pick-up assy inside as well?
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Cliff Jones
1991 GTA Trans Am
w/305,231 miles
on the original motor!!!!
Time of Death on motor, 4:07pm on 05/29/2010.
Hp:211.2@4200rpms/Tq.328.5@2700rpms
Best et: 13.959@97.05mph
Best mph: 98.3
Best 60' : 1.875 on BFG Comp TA's w/ 32 lbs of air in them
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Old 06-14-2010   #3
Fred91GTA
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Re: Stalling at idle when hot

IAC along with the housing was last cleaned out about 9k miles ago. I'm going to check on it tonight. The ignition coil is a brand new MSD blaster. I needed a new one for my T-RAM setup. The distributor is stock, I have not yet checked out the pick-up assembly, though it looked good the last time I had everything apart.
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Old 06-14-2010   #4
BigBadGTA
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GTA Year: 1988
GTA Engine: 350 TPI Automatic
Re: Stalling at idle when hot

I would check the idle air control valve, clean it off or replace it. I had something like this happen to me when my fuel pump went. It would idle erratically, hesitate, but it ran ok. Till one day it refused to start up, and when got it going it backfired all the way to the shop.
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Old 06-14-2010   #5
ThunderChicken
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Re: Stalling at idle when hot

As hot as it's getting and as well as we know Third Gens cool, it may be that you're getting mild vapor lock. If that's not the case, do check your IAC.
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Old 06-14-2010   #6
GTA88
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GTA Year: 1988
GTA Engine: 350 TPI Automatic
Re: Stalling at idle when hot

I figured it out. Its the FORD injectors you put on a Chevy motor and in a Pontiac!!!! LOL, sorry but I hate Fords! I do hope your car gets fixed asap and you find the real problem.
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Old 06-19-2010   #7
Gerry
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Re: Stalling at idle when hot

Quote:
Originally Posted by Fred91GTA View Post
I had burned plug wires about 2 weeks ago, and I replaced both plugs and wires. Car ran great for a little while and then this problem popped up.
When you burned a couple plug wires, did they actually get to the core and short the output of the coil out? I have had burned plug wires seem to cause a ignition module to subsequently get weak. At least that is that best answer I could come up with. After the plug wires were replaced, I had stalling problems and rough running. New ignition module, no more problem. I figured the two had to be related.

If you have a spare ignition module in your console ( like I do.... ) maybe its worth a shot?

HTH.

Gerry-
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Old 06-21-2010   #8
Fred91GTA
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Re: Stalling at idle when hot

So I think I figured it out. It looks like I have an EGR that may be sticking open. An EGR sticking open acts like a vacuum leak and causes the car to stall out when decelerating. I changed out my ignition control module yesterday and the car runs fine when given normal power, but will stall out and die at idle. Yesterday was really bad... I went to pepboys for the new module, and the car cranked and started and died immediately twice. Then it finally ran when I started it a third time and held the gas pedal down but at every stoplight, I needed to put her into neutral and give her gas as I braked or the car would stall out.

Thinking it was the fuel pump, I hooked up a fuel pressure gauge and drove around for a little bit. Expecting to stall again, I kept it local, but naturally this time, the car didn't want to stall, but instead idled wierd again. The fuel pressure did not drop once, even during the wierd idle when the car felt like it wanted to die. Did some reading this morning and ran across the symptons of a bad egr and it turns out that my car is doing exactly what was described in that article so here goes nothing...
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Old 06-25-2010   #9
ThunderChicken
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Re: Stalling at idle when hot

The pressure won't drop at vapor lock, either...but EGR is one I hadn't thought about. Vacuum EGRs rarely go bad...so it's more likely that your EGR TUBE is clogged up and needs to be cleaned out. Simple fix with a coat hangar and some carb cleaner.
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Old 06-30-2010   #10
Fred91GTA
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Re: Stalling at idle when hot

OK, I cleaned out the EGR Tube and changed out the EGR. Car runs much better now, but still stalls once in awhile when braking. I'm thinking it may be a vacuum related problem. The fluctuating idle only comes on now when the car is in gear and I have my foot on the brake. Also, the car will stall if I let off the brake and stab the gas pedal. Any other suggestions?
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Old 06-30-2010   #11
FearThisGTA
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GTA Year: 1991
GTA Engine: 350 TPI Automatic
Re: Stalling at idle when hot

I tend to agree with the vac idea.
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Cliff Jones
1991 GTA Trans Am
w/305,231 miles
on the original motor!!!!
Time of Death on motor, 4:07pm on 05/29/2010.
Hp:211.2@4200rpms/Tq.328.5@2700rpms
Best et: 13.959@97.05mph
Best mph: 98.3
Best 60' : 1.875 on BFG Comp TA's w/ 32 lbs of air in them
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Old 06-30-2010   #12
Fred91GTA
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Re: Stalling at idle when hot

Something I noticed, there seems to be a swoosh sound when the brake pedal is pressed while the car is off. It also makes the sound when the car is running, but the brakes still work. I read somewhere that this is indicative of the diaphragm in the booster rupturing but I still have full power brakes. is that possible?
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Old 07-06-2010   #13
Fred91GTA
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Re: Stalling at idle when hot

Ok, so I finally figured out everything with the car and she runs better than when all this even started. Here's what happenned:

I did the flow chart procedures for troubleshooting in the GM Service manual and found that I had a sticking EGR. I changed out the EGR and the idle and driveability improved but she was still stalling every once in awhile and was still hunting for idle. I removed the IAC and found that the EGR sticking open had probably gummed up the passages. I cleaned out the passages and put in a new IAC and the car was idling better and not stalling as much but still had a miss between 700-1100 rpms at light throttle.

Pulled all the plugs and found that #7 had a cracked porcelain insulator and was arcing. Replaced plugs and idle improved and was stable until the car got hot. Thinking it was ignition related, I pulled my coil and had it tested. It tested out fine when cold, but put out the wrong voltage when hot. Mechanic speculated that the arcing plug may have fried the coil so I put in a new one.

Now, the car was running perfectly again, until I got it out onto the highway and noticed it bucking forward at certain RPMs. Re-verified all readings with scantool and found that the TPS was giving wrong voltage. Must've failed while I was troubleshooting this whole ordeal. The TPS was the first thing i checked when all this nonsense began. So replaced TPS and the car's running beautifully again with one small exception, I used a TPS from Wells instead of AC Delco, so the really light throttle settings are sending a wrong signal to the computer and causing a slight miss. I need to switch out to an AC Delco unit. With the new TPS though, she feels like a totally different car, and it shifts a lot smoother too, probably because the computer is getting the correct TPS reading now and not locking the converter at the wrong time.

Hope this helps people out who have a similar problem in the future.
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Old 07-07-2010   #14
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Re: Stalling at idle when hot

Great that you finally found the problem! And what a good write-up!
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