Unsolvable? 3 mechanics and no answers


1993 Buick Regal 3800 engine:

1. If you start it cold, it will idle at 1500 (the usual cold idle speed). Then after some time passes it will idle down to 950 or so, the way it should. If you DON?T drive it, everything will usually function in normal fashion.

2. However, if you put it in gear immediately (before it warms up much) after starting it and then drive it around the block (or sometimes, just rev up the engine), the engine will USUALLY continue to idle at about 1500 RPM no matter how long or how far you drive it. If you take it out of gear (no transmission pull down), it will idle at 2000. And if you ?kick down? the gas pedal it will idle even faster. One way to get it to idle normally again is to go to number 3 (below).

3. If you turn the engine OFF and restart it AFTER the car has run for a FEW MINUTES, the car will again idle at normal speed (at 950) . Shutting off the car and restarting it ALWAYS works. And it will idle correctly until it completely cools down again, i.e. if you do errands all day it will idle like a new car.

4. To sum this up: The car idles normally only after it has reached a certain warm up temperature (a few minutes) and is restarted, OR according to one mechanic, as soon as the computer is plugged in it also idles down to normal again. That is why he said he can’t figure out what is causing the problem. He thinks there may be a problem somewhere in the wiring harness.

The throttle position sensor was replaced in January of 2006 and 7500 miles before the current problem started in October of 2006. That replacement cured a previous problem, i.e. an extremely high idle that would not stop no matter what we did. An internet mechanic said that maybe the TPS wasn?t calibrated correctly when installed. The mechanic who put it in checked it recently and said it was fine.

So in January or February of this year (about 500 miles ago):

I replaced the idle air control valve. Another mechanic checked the TPS and replaced the ECM (that was a dumb decision on my part, because I spent money when we were not sure of the problem). I replaced the PCV. The problem has gotten worse since then. It used to do it once out of every 5 or ten cold starts. Now it does it almost every cold start. Needless to say, I?m pretty frustrated with the car. Here?s hoping SOMEONE can figure out the problem. THANKS in advance!

P.S. It has been checked and rechecked for vacuum leaks.


Darnit… I had your solution in mind until the last paragraph; I was going to suggest the IAC valve.


Man, I was hoping you would have the answer. I have a similar issue with my 89 3800. Intermittant high idle. Usually after its hot. Turn it off, go through a few key cycles, and back to normal. Get intermittant code 22 for TPS but have replaced it twice, and the IAC, and the connectors, and MAF as a last resort. Swapped ECMs too. Really can’t figure it out so if you find out let me know. I also posted this about 6 months ago but no one had much to say that I didn’t already know. I’m ready to try the air sensor and engine temp sensor out of desperation.


If you had a scanner, it would be nice to see what the readings show? I’m sure the mechanics hooked up a scanner at least. No codes?

On a cold start-up make sure everything is turned off when you start it, no lites, heater or anything, see if it makes a difference.

If it is temp related, you might check the CTS and circuit. If you already checked it, I guess I missed it in your post.


Here’s a thought. The next time there is a high idle, try blocking off the air intake, see if it makes any difference. It should make a big difference in the idle, if it doesn’t, you definitely have unmetered air coming in from somewhere.


So, how was it checked for vacuum leaks?
The only proper way is to connect a vacuum gauge and see what’s going on.

You do realize that you can even have a vacuum leak in the dash don’t you? This could be through vacuum operated vacuum pods that control various doors in the heater/A/C box, and the leak can even be dependent on where the mode control is set; vent, DEF, A/C, etc.


I think you’re going down the wrong path. If it was a simple as a vacume leak or air flow, one of the mechanics would have seen that. Its more complex than that if it is anything like mine which I believe its the same problem.

On mine, I can go into diagnostics and see that the IAC is being commanded at a high idle level. I can even go into over-ride and command the idle down to normal. Coolant temp looks normal also, as well as the temp at the air sensor. If it were a simple air leak, that wouldn’t be the case. So I think in both cases, faulty information is being fed to the computer, that then sets the IAC. The question I think is what sensor would do that.


Two things.

  1. 950 is the WRONG idle speed for this engine ANYWAY! The 3800 (first series) should idle 700, give or take 50.

  2. This, as well as the other things you’ve told me, lead me to beleive that the Coolant Temperature Sensor (CTS) is your problem. Go ahead and replace it. See if that takes care of it.



The sensor is only about 12 bucks at Advance Auto. It’s a snap to replace. Definitely worth a shot.

-Matt (again)


There should be a couple of coolant temperature sensors, one for the fan, one for the gauge and one for the ECM. Its the one for the ECM you want to replace.

But, did your mechanic check for vacuum leaks around the intake manifold gasket? Spray some carburetor/fuel injector cleaner or wd40 around the intake manifold gasket to see if the idle drops, or kicks up depending on the product used. Any change indicates the intake manifold gasket has a leak. Some GM v6 engines have issues with this gasket, but I don’t think the 3800 is one of them, but it wouldn’t hurt to check.


The 3800 was not part of the infamous intake manifold gasket issue family. At least, not this incarnation of the 3800. This is a Series I engine.

Also, on the series one, there’s only one coolant temp sensor. It’s the ECT (Engine Coolant Temperature) sensor and it’ll be either in the cooling passage at the intake manifold or near the thermostat housing. It reports temperature directly to the PCM, which uses it to calculate mixture and control the fans. The PCM also reports this sensor’s reading to the BCM (Body Computer) which controls the dash gauges.



Is the throttle body clean? Many idle problems are caused by gummed up throttles and IAC passages.