2000 GMC Sierra 5.7 Has fast idle while braking


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2000 GMC Sierra 5.7 Has fast idle while braking

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homes4you
New User

Jun 16, 2011, 2:20 PM

Post #1 of 4 (3503 views)

2000 GMC Sierra 5.7 Has fast idle while braking	Sign In

My 2000 Sierra 2500 5.7 with 160 k miles, has developed a minor problem. When starting the motor, warm or cold it jumps up to around 1500 rpm then

adjusts down to around 850 rpm. While this isn't so bad, the more annoying problem occurs when slowing down in traffic. The

motor seems to have a cruise-control mode holding it around 1500 rpm with my foot off the gas. (As in slowing to a stop

sign) It continues to idle high until either stopped or almost stopped then comes down to 850. This problem comes and goes

with no apparent reasons. There are no codes showing on the dash. The problem is worse when the AC is on and makes the

truck difficult to stop.

This High idle problem does not happen when the truck is in park and you hold the motor around 1500. As soon as you release

the gas, the idle comes back to 850. I have checked the vacumn at the brake booster and have 21 inches at idle and the

motor runs great. I have also checked the linkage and has smooth operation.

Any ideas please.

Mr.scotty
Enthusiast
Mr.scotty profile image

Jun 16, 2011, 2:35 PM

Post #2 of 4 (3499 views)

Re: 2000 GMC Sierra 5.7 Has fast idle while braking	Sign In

Sounds like a IAC problem or a dirty throttle body.
I guess a vacuum leak is possible too.
Try cleaning the throttle body real good with a old tooth brush carb cleaner and a good shop towl, let it air dry out real good for about an hour before starting it.
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homes4you
New User

Jun 23, 2011, 1:29 PM

Post #3 of 4 (3469 views)

Re: 2000 GMC Sierra 5.7 Has fast idle while braking	Sign In

Thanks for the Ideas. Today, the weather is cool enough to get out and work on the truck. I started with removing and cleaning the IAC. It was not too dirty nor was the throttle body dirty. I replaced the IAC with a new one and the driveablility problems are solved. It does still idle up a bit when I first start the motor (warm or cold), but I can live with that.

Thanks for your help.

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jun 26, 2011, 6:43 AM

Post #4 of 4 (3448 views)

Re: 2000 GMC Sierra 5.7 Has fast idle while braking	Sign In

Guess: Brake booster may still be in the picture and erratic??

T

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