Hey all, My engine drives fine but at idle it "stumbles". The rpms fluctuate from about 900ish to 600ish and the car vibrates. i put in a cold air intake hoping to help it but it only got worse. Anyone know why it might be doing this?

 

Is your check engine light on?

 

My car does this too sometimes but no CEL so I just let it be..
Are u experiencing any other problems?

 

clean maf // how are coils?

 

front o2 sensor? if not the the one mentioned above

 

Theres no check engine light and no other problems at the moment. i just got the car a few months back and i took it to the dealer when it was still under warrenty but they couldnt find anything wrong. I need to get my hands on a scan tool to check the o2 sensor readings

 

clean the maf and check plugs first those are to easy things to eliminate

 

Do a search on this forum under "hunting idle" or "erratic idle" and you will find many threads. Of all the possible causes, issues that only surface at idle (no other issues with top end performance, decreasing gas mileage, hard starting, etc) most often are traced to:
* a defective neutral switch
* bad motor mounts
* contaminated throttle body

 

Bad motor mounts causes a hunting idle? Amazing!

 

yup motor mounts and neutral switch on the tranny are 2 of the more frequent causes.

could also be a vacuum leak. which aftermarket intake did you install?

 

Actually when i bought the car i noticed it needed new motor mounts so they are new. As far as the intake goes, i bought an off brand intake (not a good idea) which restircted the air flow to much and caused the car to stall out at stops. So i took the filter out of the housing and bought pipe and made a custom intake. I think i shoulda just spent an extra hundred on an intake made for the 8 but anyhow thats what i did. Drove it a little bit and noticed the the stumble was worse so i installed the stock air box in for now. heres some pics of the custom intake.

 

How no one has suggested getting a compression check is beyond me.

Bad motor mounts will not make the cars idle fluctuate like that. the problems existed before the CAI.

So it's either ignition or compression.

 

Could be compression too, usually there are a few other symptoms that can help point to that cause, as noted previously "(no other issues with top end performance, decreasing gas mileage, hard starting, etc)".

 

theres no top end performance issues and it doesnt start hard, i dont know about gas mileage cuz i just got the car about 2 months ago but i havnt noticed a change in mileage since then. Also another thing to note which i didnt notice before is it stumbles the worst when its gets to full operating temp.