Weird idle
 

Drove the car this morning and it acted normal, then drove it again from about 9:30 to 10:00 and the idle got progressively odd. It was normal at the start of the trip but by the end idle would drop to 1.1k when the clutch was depressed then surge to 1.9 then drop back to 1.1.

I put the car in neutral and the idle continued to do this.

I first noticed it being odd at about the third stop sign of the trip and by the time I was later on in my driving I noticed that engine braking in third didn't have the same affect as usual.

No CEL and I have yet to drive the car again since the idle was odd, it's like the throttle is stuck barely open.

2000 boxster S, just had a dealer oil change a week ago today, driven only 4 days since then.

(Edit: just remembered, the stop I first noticed it on was a hard stop, this one stop sign has no warning and is around a blind curve, I'd say I had to use about 50% of the brakes power while normally I never use more then 10 to 15 when driving.)

Just started the car again, it's acting normal.... Odd.

Sounds like you have a vacuum leak going to the brake booster. Check your connection that connects to the front left of the motor on the intake runner. Follow that line to the front as it goes under the car. You might hear it leaking.

Alright, I find it odd that this cleared up on the next drive but I am glad it did.

So if that's where the leak is would that explain why the vehicle only did this after an instance of heavy braking.

I had a time before that a stop sign really snuck up onto me and I got to feel the cars antilock brakes. Didn't have the issue then. (I'm usually a very light braker and prepare for stops well ahead so I found it very impressive to see just how well Porsche brakes are after all this time.)

One of the connections might have came loose enough to open up. A simple bump in the road might have caused it to reconnect enough to were you don't have a vacuum leak right now. I suspect it's the connection under the car. Your car will have a choppy idle. Kind of sounds like it's a V8 with a big cam. You say you lost braking ability too, and that why I suspect that line.

Braking was normal. It was engine braking that was odd. When I would let off of the gas in third, the car would not decelerate as quickly as usual.

It's odd as the idle was normal after restarting the car. Once shut off and left to sit for a while and restarted idle was normal.

Hm, 2000 Boxster. Did you ever clean the throttle body and MAF?

I cleaned the throttle body and replaced the MAF last winter chasing a CEL, it ended up being caused by a vaccum leak near the recently replaced AOS instead though.

The idle was acting like a vaccum leak issue for sure, but it was rather well behaved and smooth, just gradually increasing until 2 then drop and increase again.

No CEL this time and the car drove normal on the next drive.

Kinda puzzling.

I wonder if the battery was disconnected for any reason when it was at the dealer just recently and maybe that has something to do with the car relearning to idle or something.

Huh, mine has done that at least twice before. Once at a stoplight, and once on a cold start in the garage. Each time, restarting the engine cleared it right up. A third time I recall it happening, it was more severe and restarting didn't help. That time it turned out to be bad gas and refilling the tank cleared it up immediately.