Slight stumble at idle

[GranburyChuck]

Okay, I already searched and couldn't find what I thought might help so I am throwing this out.

I have a 2000 C2 tiptronic with 108k miles on it. Car runs great except for a very minor stumble while idling, in gear, at a stoplight. Idle is about 630 RPM, I think. Car will be idling fine then a slight stumble, doesnt drop more than maybe 50 to 100 rpm then picks right back up. Just the kind of thing that concerns me while waiting at a light. Don't want the car to stall right there you know. I have replaced the fuel filter at about 80k miles along with the spark plug and connectors. I don't have a CEL and have not had any codes come up for this type of problem. I had some codes for the O2 sensors but I replaced those about 8000 miles ago. I've had this problem since I bought this car with 70k miles. I cleaned the throttle body although it wasn't very dirty. I could see some oily goo further down the intake tract that I could barely get to (must be more further down). Assume it is not related to the air\oil separator since it has been a very long time since I cleaned the throttle body (~20k miles) and no total failure. I did notice some slight cracking of the rubber boot on the intake manifold (after throttle body) but not sure it is through the rubber to the inside (thus leading to an air leak after the MAF). Is there a way to check that. I know the old way was to spray carb cleaner all around joints or suspected leaks and listen for change in the rpms. Anything else I should look for? I do have a Durametric scanner. Something I can do there? Since the unit didn't come with a manual I haven't used it for much other than clearing codes.

[phillipj]

One easy remedy may be to clean your idle valve on the throttle body. It's on the left lower side of the throttle body. You just have to remove the electrical connector and two screws (torx if I remember correctly)

It gets filled with carbon deposits. If you tap it gently and carbon falls out you might have found your culprit. You can clean it with carb cleaner.

Get it as clean as you can and reassemble. Should take you 30 minutes in and out. Worked wonders when I had some idle problems. It fixed my problem completely and didn't cost a dime... :D

Hope this helps.

[minny]

Put it on the PST2 or Durametric to make sure the Lambda sensors are working fine. My car had the same symptoms and I did what you did with no improvement. There was no CEL and a OBD Scan showed no faults. The Durametric, however picked up 1 Lambda sensor wasn't working at full tilt. Cleared the fault but it returned so I renewed both. Took 15 minutes. Reset the ECU to make sure the fuel trims were starting from scratch. Idle now rock steady.

[Loren]

Sounds like a classic dirty throttle body to me.

[GranburyChuck]

I did not remove the throttle body last time I only cleaned it while it was on the car. Do I need to remove it?

[Loren]

Depends on how dirty it is - you do need to clean around and behind the butterfly.

[GranburyChuck]

That's what I did last time. Even cleaned down into the intake runners a little. Guess I can look at it again. If I take the throttle body off I need a gasket correct?

[Loren]

That's what I did last time. Even cleaned down into the intake runners a little. Guess I can look at it again. If I take the throttle body off I need a gasket correct?

On an egas car there is no gasket - just a sealing ring.

Look for damage to the ring or other air leaks.