Bouncing Idle Problem
 

After some spirited driving I noticed an odd idle problem. The idle will rev to 2000 rpm then fall back down to 800 then rev back to 2000 and over and over. When I start the car from cold it will idle at 2000 for a couple of minutes and then start the bouncing again. So far I have tried disconnecting the battery and cleaning the idle control valve. There is no engine code to read and it is a 97 boxster. Some suggestions would be greatly appreciated. Thanks.

vacuum leak?

I was thinking the same thing as Insite, the description you give sounds like a vacuum or intake leak. The engine revs, then it’s compensated by the computer and falls back to idle rpm and the process starts all over again. Get some carb cleaner (with the red spray tube for precise targeting) and spray around all the places aft of the air flow sensor where parts come together. Squirt around the boots and if that doesn't find the culprit check to see that all your vacuum tubes are all connected. Also, look for a crack in the plastic intake plenum.

Also try
 

in addition to trying what the other guys mentioned, it's possible that the MAF sensor is starting to go. I saw this on my Pathfinder. Here's another symptom, when driving the car, it runs ok but when you slow to a stop the idle hunts for the right RPM. I also never had the check engine go on. With the engine running you can tap the outside of the MAF and if the idle changes on tap it's the sensor (loose wire).

Good luck - post what the resolution ends up being.

-Steve

Thanks
 

Thanks for the suggestions, I will get my hands dirty tomorrow.

Another vote for a vacuum leak. My Honduh DD had the exact same problem with the idle which I traced back to a leak in the intake.