Bouncing Tachometer?
 

Recently my tachometer has started dropping & bouning from 500 rpm to 1100 rpm when I come to a stoplight after driving a short distance? The only thing I have done to the car is add an EVO High Flow Air kit & SuperSprint Exhaust mufflers. Sounds like a sensor is going out? Has anyone had this problem? I have had the EVO HF Air kit for almost a year now. Is this the MAF meter that is going bad?

Many things could point to this. Bad gas, dirty air or fuel filter, O2 Sensor etc. Any codes at all or just rough idle? How is performance through the gears?

I only use Shell Premium gas and hardly drive the car. Under normal driving conditions the car drive perfect. No code lights have come on at all. I would think it something with the idle control sensor. I need to learn the system of sensors inline before the throttle body...

NORMALLY, if there are any electronic issues, the code pops up. Of course, that is not fool proof. If your car sits quite a bit, gas line issues are common and will often NOT trip a code. Fuel filter, sediment etc. I had that erratic idle on a 99 Box that I solved with Chevron fuel injector cleaner (took two tankfuls to resolve). IN that case, the car ran fine other than the idle issue.

Here is CA, we have horrible gas, no matter what. Don't know what your situation is.

Well I just got off the phone and they told me based on the description of the symptom that it's some senor at the flywheel that tracks engine speed? They recomend a Diagnostic Test to pin point the problem. This kind of rules out the MAF sensor as the car has not poor performance under standand driving conditions? Only at idle does the tach bounce up and down almost to the point of stalling.

Car: 2000 986S (Boxster S)
Mileage: 26,000 miles

and the results.....
 

did you get the results back from the diagnostic that was completed? what was the issue?

i have a 99. mine was doing the same thing...i put through two tanks of injector cleaner, and then had my 60k mile service done...runs like a charm now...no idle bouncing.