very fast idle

This is 2000 century 3.1 engine. Quite frequently it will assume a very fast
idle, guessing 2000 rpm or there abouts. No engine lights or warnings of any
kind. Will happen cold or hot at random times.
It is in tune and has plenty power , starts easily and otherwise is normal.
Visual checking of hoses did not uncover any thing. Where should I start on
this one? Thank you

Welcome to AF.

The idle cold and warmed up is controlled by the PCM module. The sensor is the Idle Air Controller or IAC. It is located at the top right hand side of the throttle body (TB). Soft carbon builds up. You can remove it and clean the pintle with carb or throttle body spray cleaner. Also, clean the seat where it mounts to the throttle body or the whole inside of the TB. The air orifice gets clogged. If this doesn't work then the IAC is flaky.

BTW - May not give a SES/CEL or set any DTC error codes.

Thanks for your reply to this. Sounds quite possible to be the trouble and when
it cools down some I'm going to do what you suggest. Its seems to went into
remission since my post but Im sure it will return.
Im in Ok and we are having a heat wave. Over 100 in my garage right now. Some things can wait. When I fix it I will post back.

Well I did the service to the idle air control unit. It was dirty and I cleaned it
as best as I could but next day problem returned. Apparently that wasn't
enough so Im thinking of just replacing it. Thanks for reponse and Ill post
back if it gets fixed. Ive also got a problem with a soft brake pedel which
Im going to post in another thread. I dont know if this is connected. All vacuum hoses look good to me.

Instead of replacing the IAC, try cleaning it properly first. That same varnish, carbon, and dirt that can restrict the air passages in the throttle body not only contaminates the cone of the IAC pintle, but accumulates inside the IAC stepper motor as well. Furthermore, like any grease that is exposed to prolonged heat and age, the oils in the grease dry up, leaving the soap and wax bases, resulting in a sticky paste instead of a lubricant. The carbon and varnish that mixes with the grease doesn’t help. Between the dried grease and contamination, the IAC can be slowed or rendered almost useless. Fortunately, the stepper motor coils and gears seldom fail, so they can be restored to normal operation. You’ll discover that if you proceed.

You need to remove the IAC from the throttle body. You may want to apply some penetrating oil to the two attachment screws and allow it to do it's job before attempting to remove the IAC. The screws can fuse to the aluminum throttle body and may twist off without proper precautions. That would make for a very bad day, indeed, but you need to do this regardless of whether you try to clean the IAC or decide to replace it.

Once the IAC has been removed, remove the pintle from the stepper motor. The pintle does not unscrew, and you should not attempt to rotate it since it is keyed into the stepper motor housing by slots in the shaft and corresponding plastic keys molded into the stepper motor core nose. Rock the pintle back and forth as you gently pull it outward. It takes about 90 seconds to work the pintle out fully to the point where it will release - Longer if the IAC is really dirty internally.

http://72.19.213.157/files/IACDisassembled.jpg

Once it is apart, you can examine the pintle shaft and see for yourself that it is not threaded, but has concentric rings. To complete the cleaning:
Clean the pintle shaft including the rack gear portion with a spray type carburetor cleaner and small soft brush if necessary; Clean the interior of the IAC stepper motor (since there are mating gears on the stepper coil solenoids) with the spray cleaner; Clean the spring and allow everything to dry completely; Retension the spring to a free length of 2¼"; Apply one or two drops only of clean engine oil to the pintle rack gear; Assemble the spring in place, align the flats in the pintle shaft with the keys in the stepper motor core nose, and work the pintle back into the core by the same rocking motion while gently pushing it inward against the spring.

It takes only a little bit of varnish on the rack gears to completely "kill" an IAC. As long as the stepper coils are intact, and if the IAC is working at all, you can usually restore original operation, speed, and responsiveness by cleaning in this manner. The top example is an IAC that was diagnosed "dead" and was completely restored as shown in the lower photo:

http://72.19.213.157/files/IACCompare.jpg

Because the mounting screws tend to carbon fuse into the aluminum throttle body housing, you may want to apply a coating of anti seize compound to the screws before you reassemble the IAC to the throttle body. I'm sure I always do.

Personally, I would not waste any time going into a tear down of the IAC and any in depth cleaning/maintenance unless you have nothing better to do or on a limited budget. Based on my experiences, there is a high probability you will have the same problem especially if there is an electrical problem with the stepper motor or you fail you put it back together properly.

It really doesn't cost that much. AZ has the Wells brand for around $40, PN: AC107 and places like GMPartsDirect.com has it for around $62.00. GM PN: 17112350. Also, make sure the IAC to Throttle Body o-ring is is good condition.

I tried the recommendation from Blue Bowtie, and it worked. I didn't go quite as in depth as he described, but a removal and cleaning seemed to do the trick. Thanks so much. :smile:

I would never attempt, to take an IAC apart, you only create more problems....

Just clean the pintle, and the mating passageway, and also the throttle plate and bore.......also check the male contacts in the harness connector for tightness....on the old 3.1's, in some cases, you would have to perform an "idle learn" procedure....

Thank TechII. Would this 2000 Buick Century (3.1 L) need the idle relearn procedure? If so, how is it performed?

Thanks.

No it won't....