Since our weather has turned cold with lows just above 0ºF and highs around 10ºF, and we’ve had 9" of snow this week, I have been driving a seldom used 1994 LeSabre rather than my “nice” car, an '08 Grand Marquis. There’s no point in exposing the newer car to salt and slick streets. At least the interstates are completely cleared off. The LeSabre runs great despite its 177K miles. The only issue I have with the Buick is that it has died repeatedly after a 20 mile run up or down the interstate. I lift my foot or tap the brake pedal to turn off the cruise control, and the engine dies. The first time it did it, I pulled over, stopped, put it in park and restarted it. Now that I know what it’s doing, I keep rolling, put it in neutral, and restart it before getting to the bottom of the ramp. I does it ALMOST every time after a long 65 to 70 MPH run. It was not doing it the last time I drove it at more bearable temperatures, in the 40s or 50s so it may not be a problem next week. It does not seem to do it going from stop light to stop light, even at low temperatures. It has the 3.8 Liter V-6.
Your symptoms describe a dirty throttle plate or an idle air control valve getting balky. The IAC is what keeps the idle speed where it should be when you start the car or are just sitting. It also opens wide when you are cruising so that the sudden closing of the throttle plate when you pull your foot off the gas doesn’t kill the engine. I don’t think you can see the thottle plate on your engine, if I recall there is a screen at the throat to smooth out turbulence over the MAF. Anyway, try a cleaning of the IAC or just replace it, see if you can get to the throttle plate to clean that too.
It’s located on the throttle body, I’m sure you know what that part is, right? Held on by 2 torx head screws, T20 I believe. It’s essentially an electric motor that pushes a plunger in and out to modulate air flow into the engine. I have a feeling that all the cold air is making that valve a little sticky. That and being 20 years old. It is easily tested with a scan tool but I don’t know how you’d check it otherwise.
You’re welcome. Try spraying some cleaner into the hole the IAC mounts in. The car may be a little hard starting after you do that, though. Hope it works out for you. Funny, I looked up the part to get a picture, and I thought at that price why bother cleaning it, just replace it.
IAC is definitely the prime candidate by a longshot, however if changing that doesn’t help, try putting a clip on the rubber line to the brake booster and see if that helps. Since it’s only happening when coming off the interstate, it’s possible that the vacuum spike of deceleration is pulling excess air through a leak in the booster assembly. It’s a longshot, but it’s free and very easy to test. In a car that age anything is possible.
Glad you got it sorted out. But tell us, how did you fix it? By cleaning the IAC or replacing it?
I’ve about had it with all you cheap SOBs here. Cleaning parts instead of replacing them…said the guy who rebuilt his 17 year old dryer for $70 in parts and told his wife it will last another 10 years now.
BTW, having a 20 year old car doesn’t mean you’re cheap. If the car does what it’s supposed to and is reliable then keep using it. Heck, my wife has been married to the same guy for 17 years and doesn’t plan to replace him soon (looking over my shoulder…)
Besides, as an MG mechanic you should be used to and enjoy tinkering.
It’s not worth squat, but it’s still a good backup car once in awhile
By the way, I cleaned an IAC this morning at work. I had replaced a fuel pump on an Explorer that came in on the hook. It started up readily enough, but it occasionally wanted to stumble. I cleaned the IAC and all was well again.
Since the truck wasn’t capable of starting when it came in, I had no idea what kind of idle it would have when I got it started