My 2004 911c4s has a rough idle. Whole car will vibrate at times. I've changed plugs and ignition coil, new fuel filter, new air filter, and have cleaned injectors and throttle body. A slight increase in acceleration smoothes the idle right out. At speed there's no problem. Idle and vibration are magnified somewhat when air conditioner is turned on. Also had the engine mounts checked out. No problem found. Found a crack in the oil filler tube which is a vacuum leak. Fixed that. Car has 71k miles and is in great shape. Any thoughts?

 

You sure you get the filler tube connected with no leaks?

Had an oil filler tube cap leak on my Boxster. With engine idling see if just trying to move the cap on the tube affects idle. Don't get all gorilla with the cap or you could be replacing the tube again if you crack it. A coworker heard a hiss from my cap when I opened the trunk lid. With a 911 and its engine right there you might not be able to hear it that well, if of course it is leaking. But the engine should react if you move the cap around, or without turning it try to lift it a bit or push down on it to get a better seal.

Then there is the old favorite: AOS. If it is the AOS it is probably too early in the failure mode to give it the old remove the oil filler tube cap test when the engine is idling. But try it after you confirm the cap is not leaking.

If you are feeling frisky remove the throttle body and check the TB for oil. Check behind the TB where the AOS hose connects to the manifold. If you find it oily there...

 

Clean your MAF.

 

Since posting my rough idle problem, I've had a few things done. I had the oil filler tube replaced,the injectors cleaned, the throttle body cleaned, and a broken transmission mount replaced. The engine mounts are fine. There has been no improvement in the rough idle, but the car certainly has a difference in overall performance. I'm thinking this is a vibration and not a rough idle. It may be a vacuum leak but I have no way or idea of how to check. The car runs fine, but it vibrates. Looking for answers, help, and suggestions before going to Carmax to give it up.

 

Does it go away when you turn the AC on?

 

No, it may even be worse with the air conditioner running. It also seems to worsen the longer the car runs especially in stop and go traffic. Like I say, as long as I'm not at a stop and idling all is good. Just a slight increase in rpm's smoothes it right out.

 

I would check your maf as well. That little guy can cause all sorts of shenanigans. Visually check electrical connections and some maf cleaner see if that helps.

 

Thanks for helping. Could the MAF be a problem even if my CEL is not on?

 

Yes.

While I'm not convinced the MAF is the problem -- right now I have no opinion on what is the problem -- I offer the following...

A quick and dirty test is to disconnect the MAF at the wiring harness and with an OBD2 code reader clear the codes even though there aren't any. This resets all the learned fuel maps to their defaults.

Then just drive the car normally. The CEL may come on along with ABS or even PSM as the MAF is not present and these systems rely upon info supplied by the MAF. But you want to see if the symptoms that were present before with the MAF connected are still present with it disconnected.

If they symptoms are gone then reconnect the MAF, do the clear codes thing again, and road test the car. IF the symptoms come back the problem is the MAF.

If the symptoms do not go away with the MAF disconnected the problem lies elsewhere.

While I don't suspect anything horrible is wrong my advice is to always be alert to any deterioration of the engine's condition during these road tests and if the engine starts acting up shut off the engine ASAP and get the car flat bedded to a shop.

 

I had rough idle idle and it was throwing two codes that all pointed to the throttle body. Cleaned the Tb and maf sensor for good measure then did the simple e gas reset. Have you tried an e gas reset?

 

I've cleaned the throttle body and injectors. Will either clean MAF or replace it this week and try the reset. Thanks!

 

Autozone has cleaner made esp for the maf. It drys quickly and leaves no residue.

 

Just a suggestion, I'd have a shop check the fuel trim data RKAT in particular to see if it is negative or positive, only then will they know if it is a lean vacuum leak, or rich MAF. Good luck. Hope this issue gets resolved.

 

MAF has been cleaned. No codes or CEL. Looking at e gas reset and RKAT. Will do smoke test. No improvement yet.

 

Thanks for the update. Let us know what you find.