RPM surge while coasting

[MROCGUY]

it's random. if it's going to do it, it's going to do it after letting off of the gas after pulling into a parking lot. it will surge to close to 2K then back down.
is that common or do i need to be concerned?

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[luca1290]

Definitely not common, can be a vacuum leak. Check for cracked hoses and the intake manifold.

 

[JetGray_Mach1]

I would also clean your Throttle body and MAF sensor. Vacuum leaks and dirty sensors can cause some strange behavior.

 

[Pistol_91]

Does it do it on start up? Does it do it when you rev the engine up? Is this the only time it does it? I'd check for vacuum leaks then after that maybe pull the spark plugs and make sure they're ok.

 

[Need4SpeedMotors]

Most likely going to be some sort of vacuum leak. Having the car smoke checked would be the easiest and fastest way to find it.

 

[MROCGUY]

Does it do it on start up? Does it do it when you rev the engine up? Is this the only time it does it? I'd check for vacuum leaks then after that maybe pull the spark plugs and make sure they're ok.

plugs were replaced a few months ago...it only happens when coasting, foot off the pedal and not all of the time

 

[MROCGUY]

Most likely going to be some sort of vacuum leak. Having the car smoke checked would be the easiest and fastest way to find it.

i saw that smoke machine in action in some youtube videos...i will look into that.
is there any other way that i can check? i have Forscan and SCT datalogging software.

 

[Pistol_91]

You can feel around in the engine bay with the engine running and see if there's a port or hose somewhere sucking air in. You'll know when you find it. The engine idle with change more than likely

 

[luca1290]

is there any other way that i can check? i have Forscan and SCT datalogging software.

You should see the fuel trims jumping because it's unmetered air, so the injection strategy compensates - as much as it can before maxing out and tripping a code - what the lambda sensors are reporting lean condition (because it will be lean, the car is injecting too few fuel from the MAF reading only).

Or at least, that's how I always understood it works. Maybe someone with better knowledge can chime in.

 

[slow5.000]

Did you ever find out the cause

 

[MROCGUY]

Did you ever find out the cause

nope..it's weird. i only see it when pulling into a driveway at a gas station, strip mall, that kind of driveway.

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