I recently acquired a used 2001 Century with 252,000 KM (or 156,00 Miles). It is good shape and runs very well.

However recently I noticed a problem with car continuing to accelerate on its own even though my foot was NOT on gas pedal (just like it was in cruise control mode). The car can maintain the same speed for many miles with my foot off the brake pedal. When I brake to slow down, the car would again accelerate again on it own after I release the brake. I tried manually activating cruise control ON and OFF to see if this would correct problem but no effect. I noticed same acceleration problem occurred at even at slower city street speeds. When arriving at my destination and puttng car into PARK, I noticed idle was revving high.

This problem has happened twice in same week. At other times car drives normally. I don't think my problem is the same as "sudden acceleration" problems that plagued Toyota since my car does not lurch forward.

I see on internet that others have similar "auto acceleration" problems like mine but I have not seen any solutions mentioned.

Can anyone suggest a solution?

You need to have a certified mechanic look at this before driving the Buick again.

With that said, the first thing I would do would be to remove the floor mat. Start simple.

I agree with Michael, do not drive it till the issue is found, and fixed.

A lot of carbon buildup could be causing the throttle body butterfly valve to stick a little. get some throttle body cleaner and see if that helps, if you drive it afterwards to test it, I'd stay off of any main roads, and not go too fast, as if it is not fixed, and gets worse, then you could end up getting in a wreck, so be SAFE!

Check the IAC also, remove it, and inspect its pintle for carbon buildup, if dirty clean it.

If that does not help, I'd check for any possible vacuum leaks, listen for any hissing, and inspect all the vacuum hoses.

id like to agree w/the guys, specially WilliamE, specially in regards of IAC. If u dont know what IAC is, its an idle air control motor, it sits on your throttle body. It happen to me, but much more violently then u described. I was driving my Lincoln Mark VII when it went crazy. It decided to open on me as i was approaching a red light, car basically went from 40 to 70 and i had to shut it off on a road to stop it.

Another thing, I do not mean to to get off topic, but I noticed your mileage, at that mileage, if you have never had the lower intake gasket changed, I'd suggest planning to do that in the future, your 3.1L LIM gasket is made out of the same known to fail nylon 6,6 just as the 3.8L nylon 6,6 LIM gaskets fail, replace them with Fel-PRO'* aluminum framed LIM gasket.


Let us know how it runs after cleaning the throttlebody and IAC.

I agree with everyone so far. Check for vacuum leaks. If you find nothing out of the ordinary, I suggest taking it to the dealer and having them run a diagnostic. Usually about $75.

I can only think of a couple things what would cause a high idle condition..
A. IAC-could be sticking too far open
B. Vacuum leak
C. Faulty TPS
D. Throttle plate not closing all the way.

When you are in Park or neutral, what does the engine idle at?

Thanks for all your suggestions. To answer Mike1995 question, when car was in PARK with this problem I didn't take note of the rpms but it was reving noticeable louder.

Before seeing all your comments I decided to I clean the throttle body with throttle body cleaner. I did not remove the throttle body but just cleaned as deep as I could with throttle body cleaner and rag. The front of throttle plate looked pretty clean but the back of the plate and behind was quite dirty and black. The throttle cables seem to be in good shape. I did not remove or check the IAC but after seeing your comments I will look at that tomorrow. After cleaning throttle body I drove the car around the block a few times and car seems to be running normally.

I think I would feel more comfortable taking to a dealer to analyze. Does the dealer have special diagnostics tools to analyze this problem and identify IAC or vaccum leak? I'm worried that they won't be able to replicate problem.

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2001 Buick Century Limited

they can use their tech2 to actuate the iac but you can take a look same as they can. and for vacuum take carb or tb cleaner and spray around suspect area with the engine running and listen for the engine to change pitch

Do I have to remove throttle body to get to IAC? or can I remove IAC only. I cleaned throttle body without taking off because I didn't have a new throttle body gasket. I have to order from dealer and apparently it will take a couple of days to get delivery.

Also using a process of elimination if I remove the fuse for Cruise Control will this disable only the Cruise Control function or will it affect anything else? If the acceleration problem re-occurs without the fuse then can I reasonably assume that cause of problem is not caused by a defective or intermittently working Cruise Control lever?

should be screwed into your tb.

as far as cruise i doubt that has anything to do with it but they still had cables for the actuators until the 3500 so on yours you could just remove the cable from the tb