Hi all, I have a 1990 Buick regal custom with a 3.1 L V6 I have not owned it lang but do know the previous owner. The car had the ECM replaced a year ago and I just changed out the TPS after testing it and finding it was out of the range it was supposed to be. The car shows no codes and yes I have checked it multiple times. I have also gon over all of the tubings looking for vacuum leaks and all seems fine. I did find a couple that were marginal and replaced them. I also unhooked the battery and stepped on the brakes repeatedly to drain the electricity in the systems, to reset the ECM after the TPS swap. The issue is the car runs great but on a cold start up the idle is up to 1500 and takes 10 mins to drop to 800 then after running it for a while it hits the 500-600 range all the time. If I let it sit an hour or so the high idle starts up again. No stalling and as I said runs smooth and great once it gets past fast idle. Any clues on where I should look at this point? the car has just 56,000 miles on it sat in the garage most of its life was not used regularly.

Thanks for your help

 

Have you ha a chance to check out the coolant temp sensor?
If it is not reporting correct info to the ECM it will not throw a code for that. , but will affect cold start if not reporting the correct info to the ECM.

 

I ran the electrical tests and found the Idle air control was just slightly out of its range so I replaced it, reset the ECM by disconnecting the battery. I started it up and for the first time, no fast idle took it for a ride smooth as silk. Came out this morning and started it up and damn if it did not stall. jump up and down off of fast idle and then sit at fast idle for 8 mins before it started to settle down. Stalled two more times getting out of the driveway. I beginning to think the car is haunted. The previous owner had the ECM replaced about 3 months ago how would I find out if the ECM has all the updates on it? I ordered a new temp sensor and figure what the heck last of the sensors to be replaced. I also talked to the mechanic who worked on this he replaced the fuel pump and the fuel regulator because of stalling issues said it cleared it right up. I replaced the fuel filter today hoping tomorrow the gremlins are gone. Any other ideas welcome thanks for the help.

 

Perhaps throttle position sensor? Maybe talking about same thing, would determine speed through gas pedal and if erratic may cause conditions as stated when motor is cold. Wait, 1990 may still have throttle cable, single point fuel injection and throttle body? If so- same difference, used to all be done mechanically and now electronic......

Once in awhile even answer years old posts lmao.

 

At this point after several IAC controls swapped out all ACDELCO. Ran the full by the book retrain on the ECU with the same results the car runs well after all the work then reverts to the fast idle on startup. I have replaced the TPS twice and the temp sensor once both on the meter read spot on. The IAC is where the fluctuation occurs I did check the tree lines for possible shorts and all show clean from the sensor to the ECU no continuity to the engine or the frame. So I have ordered another ECU and will give it a try. I would like to ask has anyone found an issue with the chip and can it be replaced without having to reprogram? None of the Buick dealers in the area has any equipment or techs with knowledge on that, so they say. All refuse to try and service it at this point. Did find a local mechanic but he no longer has the equipment as well.

 

Roger that on repairs and thorough troubleshooting. Wouldn’t have mouse nest or hoard under air filter? I did but not to the point of any troubles. Found my old timing light and dwell meter up on top shelf in garage, alot of dust after 30 years. Believe cars from the ‘90’s may still be able to use some of the more conventional test techniques; can’t recall for sure or even know if Buick.

If engine had a hard time with air flow, would auto idle faster perhaps? Everything looked at would be past air entry before filter so far it seems.

 

If this is of any help,
The ECM has what is called a PROM chip in it. PROM= Programmable read only memory. We don't program them , they come from the supplier already built.
The integrated circuit chip is removed from the old ECM and installed in the replacement ECM. The replacement does not come with the chip because the chips are programmed according to what vehicle they will be installed in. Each vehicle has a PROM code and the test equip will always tell you what code chip is presently in the computer (ECM).

I don't know at this time because I cannot see the vehicle, however, the system is designed to add fuel (increase injector pulse width) according to how much air is coming into the engine. A vacuum leak that diminishes when the engine gets warm could be one cause. If the IAC (Idle Air Control) valve is not working correctly when the engine is cold could also be involved. You didn't state if there was any check engine light.. If there is, need to get the code(s) stored in the computer. Easy to do , only need a paperclip to do it.

I still own a 1990 Buick Century 3.3l engine . One of the best Buicks I ever owned. Had many before this one
Compared to the newer Buicks, it is a GEM.
Rich B.is The guy to go to , he does a heck of a job on this forum, Thank you Rich.

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ok, let me start with the basics of what has been done to the car in the last three months. Fuel pump, sending unit and screen all replaced (ACDelco) new fuel filter (ACDelco) Temp sensor, TPS (twice ACDelco) IAC 4 times (ACDelco) ECU replaced with a Cardone remanufactured, New control module (US made) 3 new coils Plug wires and plugs all (ACDelco) and a new Fuel Regulator. The car has never shown a code I do have the OBD1 hardware from GM. We used Propane to check for leaks found a slight leak in the vacuum line to the climate control and repaired.

The car after each change out the ECU was reset following the GM procedure battery disconnected 3 mins reconnected the car is started put into drive car runs til radiator fan cycles on and then off Car is shut down and repeated. It runs fine no racing no stuttering smooth as if new. Next morning come out try to start hard starting and rough idle then off to the races RPM up over 1500 on Tach. settles a bit shifted into reverse or drive car stalls restart hard starting then off to the races again 1500 RPMs. Verified wires from the IAC were not pinched or broken or shorting out the meter says they are clean. Car running initially looks fine then next day fluctuations in IAC meter reading coming from IAC as if bad or ECU is running to find a proper setting.

Moving on new ICU from GM due in tomorrow. If this does not do the trick I am tempted to get a new EPROM just concerned about getting one that will work. I can buy a blank the issue is reprogramming it to the car specs if the original is bad. Was hoping someone had tried that if so with whom did you go to?

 

Thanks Hanky but we know you probably forgot more than I ever knew. Can’t come up with any better than your overview. Recall my C4 Corvette was able to upgrade with a PROM. Was like they call a tune these days back then. Chrysler in ‘84 had to change complete computer. One thing I was never sure of was with that, had to install a 160 degree thermostat that came with program. Allowed engine to run cooler? I also understood engines should run hotter for maximum efficiency. Know back to road runner days before that even the 160 wouldn’t let the engine get hot enough on winter days, no heat lmao.

Forgot, if even one of those PROM tiny legs/pins is bent, not seating- the thing probably won’t work. And yes, they only go in one way; like being polarized, but will fit either direction. Usually have dots, indentions for direction.

Dont know either when OBD 1 was upgraded to OBD 2. May be using wrong tester?

 

Hi all, Thank you for your help thus far here is an update. I ordered a new ECU this one did not come from cardon and is a genuine GM remanufactured. I put it in and then ran the full reset started the car put it in drive with my foot on the brake and ran it about 10 minutes till the cooling fan ran and shut off at the radiator. Shut it down and restarted it and it was better ran it tor 7 mins till the cooling fan ran and shut off. It seems way better but tomorrow will tell. I did get a stutter when I drove it after the reset and it now stutters as I give it gas just a bit then runs like a champ. Did I say this car has only 60,000 miles on it. I know I replaced the fuel pump and the fuel regulator because the pressure would not hold in the rail after I shut it down it would slowly drop. I thought I had that issue resolved but know I think I may have missed something. I am leaning to either the pump is a bad one (it was new ACDelco) or should I be looking at a bad fuel injector. Any help or suggestions would be great. If anyone can read their manual mine is old and well has some grease on it and I am not 100% sure what the pressure should read at the rail port when running. If you have one and can take a look and post it that would be very helpful. I will keep you updated. Thanks Again