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Rpm Flucuation At Idle


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My '07 Shelby GT (manual Transmission) with 54K just developed a problem.

At idle with the clutch all the way down the idle is fluctuating about 50-100 RPM from

"normal" After about 5 seconds the idle goes up for about a second, drops and then

repeats after another 5 seconds. Has anyone run into anything like this?

Any ideas what might be going on? The car was running fine last Wednesday.

Clarence

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My '07 Shelby GT (manual Transmission) with 54K just developed a problem.

At idle with the clutch all the way down the idle is fluctuating about 50-100 RPM from

"normal" After about 5 seconds the idle goes up for about a second, drops and then

repeats after another 5 seconds. Has anyone run into anything like this?

Any ideas what might be going on? The car was running fine last Wednesday.

Clarence

Could it be the MAP sensor , try disconnecting the battery for a minute , sometimes that helps unscramble the computers brain & reset everything.just a thought

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Is it naturally aspirated or forced induction?

 

Either way, it's sounding like false (unmetered) air is getting past the MAF sensor.

 

Don't forget to start with the basics, making sure everything else (besides computer control inputs/outputs) is good. Igniton system, vacuum leaks, etc.

 

Throw a vacuum gauge on the intake manifold port to see what it is telling you. That's a real good place to start.

 

 

Phill

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Try cleaning the throttle body. Brenspeed has a video of how to do it. They state this is the most common problem with poor idle. Simple to do and probably would be good to do with your mileage. Good luck. Harley

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I second cleaning the throttle body. It will cause rpm fluctuations at idle when dirty and if it hasn't been done yet you're long overdue. Cheapest and easiest place to start.

 

I agree, I third it.

 

I've fixed *many* EFI cars by cleaning the throttle body/blades. When the coke up it messes with the minimum idle setting (air bleed) and causes all kinds of havoc.

 

But in all honesty, it's always been stalling when coming to a stop that I've cured by cleaning a TB, not idle surging.

 

But X 2, like has been said, way overdue and it's something that needs to be done periodically anyway.

 

 

Phill

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Ok. Found the video.

What about needed gaskets putting things back together?

Clarence

 

You don't need to remove the throttle body to clean it. It even say so at 1:30 of the video.

 

Spray some carb cleaner on a shop rag and be sure to get the *edges* of each throttle blade as well as the throat where the blades rest. If you can avoid spraying a bunch of cleaner (puddle) into the TB, try not to. But it's okay if you do.

 

 

Phill

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  • 4 weeks later...

Greetings all,

Sorry for the delay. We had a close friend pass away earlier this month.

I cleaned the throttle body as everyone suggested and it seems to have

solved the problem. The idle is nice and steady as it should be.

And yes after 7 years and 54K miles the inside of the throttle body was

quite dirty. Thanks everyone for the assist on this.

Clarence

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Glad you fixed it. I can't stress enough on cleaning the throttle bodies on the 3V's. 2 most common things I see when Ford dealers send cars to my shop are 3V throttle body issues, and GT500 supercharger pulley installs gone wrong....

 

Alex

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