Is this something I should be concerned with? I've only had the car 3 months so I'm not sure what's normal:

When I'm sitting at a stop light with the car warmed up I can hear & feel a random "burble" or "shimmy" in the idle every 5 or so seconds. It's a bit random & not rhythmic at all. I can't see it on the tach, the needle stays pretty solid. AC on or off it doesn't matter.

No other issues. Durametric doesn't report anything. The idle speed stays solid, varying by 40 RPM every once in a while but that variation happens much less frequently than the shimmy that I feel when driving it.

1999 C2 w/ 38K miles. New plugs & coils.

Is this normal or should the engine idle butter smooth?

 

oh I’m all too familiar with the shimmy. I also have a 99 that shimmied with stable rpms and no codes.

could very well be a vacuum leak.

likely candidates;

1. AOS bellows is torn
2. Oil filler tube is cracked.

I replaced both and shimmy is gone. Runs like a top now.

smoke test could narrow it down quickly.

report back!

 

OK. I'll do some research on doing a smoke test. I doubt it's the filler tube. I think the PPI would have caught that & I recently changed the oil. Could be the AOS or one of the vacuum tubes. I also read that injectors can cause it.

 

Yeah, I was very close to replacing the injectors. My AOS had a ripped bellows and I was certain this was the cause. But it wasn’t. I handled that filler tube a dozen times in my first six months of ownership and didn’t notice any issues until one day it was nearly split in half. I’m certain it had a crack before it got to that point. Looking back, a smoke test would have saved a lot of time.

 

Ummm, so what's the best cigar to use for that?

 

Not sure. Google evap smoke tester. I should have done this test first, but I chose to just replace stuff since the car was 20yrs old with no maintenance history and I enjoy wrenching.

does the car rev easily to 6k or sound/feel labored? Just my experience but after the vacuum was improved the car revs so much easier and the shimmy is gone.

 

It seems like it revs normally. Except for the random shimmy, everything seems fine.

 

I would also check and clean the ICV (Idle Control Valve) on the side of the TB. The ICV has a bad tendency to get gummed up inside. Get it off of the TB, and clean it with TB cleaner. Soak it good and you can use a 9V battery with some leads to cycle it back and forth. Use Q-tips to clean it inside. It is not black inside...it is really silver. So cleaned up it will be all shiny silver inside.

 

If you’re up to it wrap a piece of duct tape around the area most likely to crack in the oil filler tube and see it you notice a difference in the idle.

here’s a pic of my broken tube. Same crack location as many others I’ve seen. I’m certain there was a crack there before it got to this point. And I handled this thing a dozen times and noticed nothing before one day wrenching it just appeared. Weird.

 

Not a bad idea. I can give that a shot.

 

You're a Porsche owner, you were born to answer this question.

 

I’m in a similar situation. 2004 base coupe, 75k miles. Car consistently idles around 650rpm hot or cold. Small -+20ish rpm fluctuations which I understand is normal. However my relative idle is about 100rpm low which I believe is leading to my slight shimmy, passenger seat shaking symptoms.

I’ve tried the ignition reset (key on for a minute then off, then fire up). No change. Tried pulling the oil cap with it running and the idle actually improves.

Interested to see what fixes yours.Out of curiosity how does yours idle?

 

LOL. Sadly I never got into the cigar thing much.

 

Mine idles t 680 with an occasional 720.

 

Sounds like we have a very similar issue.