High idle and stall on 1990 3.1 TBI

I have a 1990 3.1 Camaro that is hard starting, stumbles on acceleration and stalls when stopping. However, in very cold weather (10-40F). The car will start with 1 click of the starter when the key is turned and run like a top. Basically, the car runs great in cold weather.

In hot weather (above 50-60f) it is a different story. When starting the car in the morning, it may take 4-5 seconds for the car to start. Generally the car starts as designed, but the idle will be 1500 rpm and generally increase to about 2200 rpm. If I place the car in drive with my foot on the brake, the car will stumble/hesitate, but will usually not stall.

If it is really hot outside or after the car was warmed up or driven within a few hours, the car will only start if I press the gas pedal to the floor. Then, the car starts like it is flooded and then run at the 1500-2200 rpm. If I don’t press the gas pedal (assuming I need air), the car will not start.

If I get the car to move, the car runs great. The car has great pickup and acceleration. When I come to a stop light or stop sign, the car will mostly have a very low idle (400 rpm) and begin to surge to 800-1000 rpm and then repeat the cycle or stall. So, now I have to drive with 2 feet – one on the brake, and one on the gas to maintain the rpm to fight the stall. There are times when I let out of the gas at 60 mph and the rpm stays at 1500-1800, and then in the same spot the next day when I let out of the gas the rpm could be 1000 rpm. I don’t notice any problem with the car at higher than 15mph because I believe the rpms are high enough not to stall.

I took the car to a local mechanic and he diagnosed the problem to be a leaking fuel injector. The estimate was $500 to find and replace one injector. I decide to look at the injectors myself. I also have a rear engine seal leak which may make this a terminal car if I cannot fix the rpm issue. I removed the injectors from the engine and reconnected the fuel lines to the housing after disconnecting and inspecting the injectors. The injector manifold was full of gas. I turned the ignition key several times to refill with gas and pressurize the injector manifold. After one hour, no gas had leaked from any of the injectors. I reinstalled the injectors in the engine. I believe the injectors are good.

Two years ago (6,000 miles) I performed a major tune up (wires, plugs, cap rotor, pcv, air filter). I also replaced the IAC motor because I was experiencing this same type of issue, but that did not fix the issue. A week after the tune up, winter came and the car ran great in the cold weather. Last summer I had a few stalls, but not nearly as much as I have had in the first 2 months of spring/summer this year.

I have removed and cleaned the IAC and TBV housings. I also read that the coolant temperature sender could cause these same symptoms, I replaced it but the problem persist. I do not have any trouble codes. I am a weekend mechanic, with very little knowledge about cars.

Can you offer any more simple suggestions that I could try? I really thought the close loop operation was the problem, but the very high rpm on startup that would lead me to believe it is something else, or I have multiple problems. Thanks again for your help!

well first off i would like to correct you for claiming not to know alot about cars, as youve done quite a good job diagnosing and working on the car thus far. and came up with a pretty good way to test the injectors as well, not usually how i would have done it, but maybe better.

In regard to your problem, it sounds a bit like a bad ICM module, your ICM controls your ignition timing and has been known to cause poor low idle and stalling without setting a code.

also the difference in air temperature has to deal with air density which makes me think it might be a bad MAP or Manifold Absolute Pressure sensor which is used to determine air fuel ratios. as the air gets warmer it gets thinner and if the MAP isnt correcting for this, it may be causing an over rich condition in warm weather which would cause the problems you describe, as you open the throttle more air would come in, leaning out the mixture and it would run better, but at idle there wouldnt be enough air and it would run like crap.

i would try replacing those two items, they should not be expensive, maybe 120 combined price or a bit more. I hate to give advise on swapping parts without reason, but without any error codes, my guess is as good as yours.

could be a bad computer also (ECM). when I bought my car it would not start (it was winter) without many attempts and the gas pedal to the floor. The rpms fluctuated just like you described. I ended up doing all the same stuff you did, plus replacing the ECM (about $90).

I was doing the exact same stuff (2-footing the pedals), idling the car up to keep it from dying at stop lights (with the idle screw).

The only things that I did that you have not done, are replace ECM, had GM reset the timing (they said it was off a bit), and replace O2 sensor with factory one. I wouldn't rule out the O2 sensor, if it is bad, and sending a signal to the computer when it shouldn't be, this could affect starts. Bad sensors don't always throw DTC codes either (mine didn't).

This is what I would do (in order):
1) spray the interals of the IAC with wd-40 and make sure the valve is moving freely. if not better then...
2) replace MAP sensor... you can return this if it does not help, if not....
3)get factory O2 sensor
4) replace ECM


I'm pretty confident that your problem is with the O2 sensor (1st choice) , ECM (2nd), or the MAP sensor (3rd).

By the way... did you mean 3.1 MultiPort Fuel Injection? I have never heard of a 3.1 TBI engine.

maybe thats his problem......

Thanks for the replies. I actually looked at the IAC units today (I have two - one from 2 years ago.) What do you mean by move freely? In order for me to get some movement from the actuator, I have to turn the stem. Is this correct? I could only turn the stem about 1/4 inch on both units. Does that seem correct?

I will replace the map sensor if you would be so kind to tell me the location. I assume it is on the manifold, but do you mean exhaust?

Is the ECM and ICM module the same thing? I also forgot to mention that when I had this problem 2 years ago I did replace the O2 sensor with no change.

And yes, MultiPort Fuel Injection. I have no idea why I wrote TBI!
Thanks again for your replies...

ICM - Ignition Control Module, it controls your ignition timing. its located inside your distributor.

ECM - Electronic Control Module, which is the brain of your car, it is the computer that all the sensors report to. its located in the dash assembly on the passenger side.

your MAP sensor should be located on your intake manifold. connected to vacuum lines and the intake manifold both.

from what I remember, there is a spring inside the IAC and it pushes on a little valve. spray the area around the spring and valve... if you are unsure, just spray the whole inside... it won't hurt anything. Then push the valve in and out to make sure it can move without getting stuck.

I might be confusing the chevy IAC with the IAC on my '02 Ford Escape. but nonetheless, just spray the moving parts with wd-40.

The MAP sensor on the 3.1 is actually located on the firewall, to the right of the a/c canister (if you are standing in front of the car). it has a green connector plugged into it.

how did you end up fixing this problem a year ago? and was the O2 sensor you bought, the factory unit?

Two or three years ago, I decide to tackle the problem of my car not warming up in cool/cold weather. For years (8-10), my temperature gauge would read very low in very cold weather (110). It also would stay around 130 in the summer time as long as I were driving. At a light, the temp would slowly increase until the fan came on and go back down to 190. When I started driving, the temp would go back to 130. I decide to replace the radiator cap and then the thermostat, and silly me, thought I had to take the plenum off. I replaced the thermostat, but did not replace the plenum gaskets (I new one had tore in half). I did not realize how important the seal was for the plenum. So, I created a very bad running car.

I knew I created the problem, but when I looked to see the last time I performed a tune up, I notice that I never had performed one (125k and no tune-up!) So, I replaced the plugs, wires, cap, rotor, fuel filter, air cleaner (not the PVC valve). The car still did not run correct. I replaced the IAC unit and O2 sensor (not factory) with not much change. Then I decided to replace the plenum gaskets, and the car ran much better but not perfect. The last item I replaced was the PVC valve (but the weather that day had drop severely from 65F to 30F), the car ran like a top.

So I really do not know what "fixed" the problem. I can say that the problem I have now would occur 2-3 times a month (even in winter), but I pretended it did not since the other days it would really start/idle/accelerate and drive great.

I did clean the IAC unit(s) yesterday, and it does not appear to be the problem. Both units operate identical in the car and I verify both units move with a signal applied. Just for the record, I cannot push the plunger in by hand.

I did spend about 1/2 hour yesterday looking for the MAP on the intake manifold, but could not find. I will look at the firewall tonight.

I know have 138K on the car.

Thanks again for your comments!

Just to get you up to date.... I purchased a MAP sensor and installed it after work. The car started funny 2-3 times, then it ran as though it was new. I drove it around for 20 minutes, and I was very happy with the cars response to the gas pedal.

After dinner, I went out to start the car, and IT'S BACK! Same old stuff!!!

Just a follow up to my problem with the high idle.

I think it's fixed. After reading many other web sites (it had to be a 1000 high idle questions over the last month), I found one entry that set a light bulb off inmy head.

The entry read something like this:
If you replace the IAC, you must reset the ECM module by doing the following:
1.) unplug the IAC unit
2.) jumper A and B on the diagnostic connector
3.) start the car and let it run for 30 seconds
4.) remove jumper
5.) stop engine and wait 10 seconds
6.) plug in IAC
7.) start car several times.

With the IAC unit unplugged, and pressing the gas pedal to the floor to get the engine started (and idling at 2000 rpm), it took a good - I mean good 30 seconds before the ECM throttled the car down to 700 rpm. The car did not idle well, it rocked back and forth like a race car, but the idle stayed at 700. I let the car run for another 30 seconds in this ruff idle state.

When I reconnected the IAC unit, the car started really rough for the first 3 or 4 times. I actually had to use the gas pedal to get the engine to start. I would let the car run for 1 minute and stop for one minute. Around the forth or fifth time, the car started and ramped up to 1200 then within 30 seconds, came back down to 700. That is the way I remember the car acting when I bought it new in 1990.

Two years ago I do not believe I reset the ECM to the new IAC module. I gues the stem was close enough to make the car run but not run correctly. So that is why the past 2 summer I have been having such a problem. When I swapped out the IAC last month with the old one from two years ago, I know for a fact that one stem was all the way out and the other was almost close.

Anyway, tomorrow morning will be a good test, but today I started and stopped the car 30 times and went for a 20 minute ride and the idle acted the way I expect with good response from the gas pedal.

I still think I need a tune up, but the car did not want to stall and ran really well.

In the future, if a recommendation to a newbie like me is considered to replace the IAC unit, please remember my problem an attempt to convey the above procedure to them. But, I know, for the real gear heads out there, this should have been obvious!

Thanks for your responses earlier, synergy is always the best way to solve a problem!