My 2002 Boxster occasionally has a hunting idle that only occurs when the car is fully warmed up. Usually 20-30 minutes of good driving, then the car idle will bounce slightly between 600-800 RPM (see video). Sometimes if I give the engine a bit of gas it will go away but not always. This does not happen on cold starts. Even when the RPM drops after the initial start up it ends up falling to 800 RPM which is normal. And the idle is super smooth. What could cause this? If I remove the oil cap the engine has a very rough idle, which I was told is normal. I recently replaced the MAF and then this issue began.

I attached a video for reference

 

This sounds like a dirty throttle body. Disassemble and clean the throttle body.

 

If you didn't install a Bosch MAF get one.

 

The times my 2002 Boxster manifested a hunting idle was 1) Bad AOS; 2) Bad VarioCam solenoid/actuator. With the bad AOS the idle hunted pretty much from a cold start. With the VarioCam problem once warmed up.

Before you condenm the AOS or the VarioCam hardware or even the MAF try an E-Gas calibration.

With the ignition off and your foot off the gas pedal through this entire procedure turn the ignition to "ON". Leave on for at least 60 seconds. Then turn the ignition off for at least 10 seconds. The next time you start the engine the calibration will be completed.

While the ignition is on you will probably hear some mechanical activity. This is normal.

Removing the oil filler tube cap while the engine is running can make the engine run a bit rough. Normal. If you leave the cap off a moment or 3 the engine controller will adapt and the engine will run better but maybe not as good as it was before you removed the cap.

When you put the cap back on the engine can react but smooth out as the engine controller adapts.

Attempting to remove the cap with the engine idling and finding it difficult to impossible is a good sign of a problem with the AOS.

BTW, with my 2002 I found the oil filler tube cap developed a leak -- the o-ring just wore out/shrunk -- and this was a problem at idle. Above idle the leak was so small as to be insignificant.

With the engine idling see if you move the cap around even try to pull the cap up -- even though it is screwed down -- the engine reacts. If it does thats a sign the cap is leaking.

Believe it or not I wore out two oil filler tube caps in 317K miles of driving. The caps in the case of my car lasted around 150K miles.

 

My boxster 987 s 3.2 1 has a hunting issue, the idle I've read can be a simple vacuum leak, but mine surges 500revs or so all the time when warmed up, driving is fine but once you want to maintain a constant speed it just revs up and for no apparent reason, I've tested for vacuum leaks with no luck, I'm not going to chuck money at it, but if a simple fix I'd happily chuck my spanner at it