Throttle body - clean in place or remove?

When I removed my airbox the other day I noticed my throttle body was pretty black. The car runs well BUT when it is warmed up and @ idle (approx 600-ish) while not "rough" it's not as smooth as I would like. I read that is a common side effect of a dirty TB so I thought I'd clean it. The pelican guide showed to remove it for cleaning, but I've read several threads saying just to clean it in place. I'd rather not remove it as I don't have that O-ring behind it but could also order one. Also noticed the boots joining it to the intake are dirty as well, the PO never washed much under the hood so it's probably 17 years of oil/grime, and I read that can be a sign of a failing AOS. I'm not burning oil at all and everything looks fine - is it normal to have some build up on them after many years? Or should I be concerned and looking at pulling the AOS? The PO had a reusable air filter which I have since removed and went to paper, thought I had was that might not have been helping keep the TB clean.

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If you believe a TB cleaning is called for it is best to remove the unit to clean it. If you clean it in place there's the concern about the cleaner fluid laden with grime running down the intake runner into one or more cylinders. FWIW, I cleaned the TB in my Boxster a couple of times -- when dealing with a failing AOS -- and never replaced the o-ring or any gasket and the TB has remained leak free all these years since. Now maybe the 996 TB is different and requires the TB o-ring or gasket be replaced. If this is the case then you should replace these items. I can tell you that in my experience it was not clear that the TB even wet with oil -- enough that a drop was hanging down from the butterfly valve -- was affecting the engine's behavior. Far and away the problem with the engine was the failing AOS. A bit of rather dry grime on the TB surfaces is not sufficient reason to mess with the TB. If the TB has any surfaces wet with oil this is a sign of excessive oil vapor being routed from the AOS to the intake and this suggests the AOS is perhaps failing. Cleaning the TB is addressing one of the symptoms of this not the cause.

Thanks. The TB is not wet with oil, but it is dirty/grimy. Problem is I don't know how much is normal for a 17 year old car or if it could be a problem. I'm thinking I might end up tackling that AOS either way as there wasn't a receipt for it in the records and it sounds like it's a common, if pain-in-the-butt failure. I'm not burning any white/oil in the exhaust.

Sounds like yours a '99 cabled throttle? Remove the throttle body so you can clean the area behind the throttle plate and inside the T plenum too. You may also want to remove and clean the idle stabilizer. Power it with a 9v battery momentarily to make it turn while drenching it with throttle body cleaner. The oil build up is normal and it's from the AOS, some more than others. Just clean up the build up and keep an eye on it. If it's building up fast, it's time to replace the AOS. If the AOS is 17 yrs old, it's good to replace anyway.

Thanks. Yes it is a cabled throttle. I am thinking I'll probably order an AOS and do it as a preventative repair if nothing else. I don't mind tearing into new cars as it gets me familiar with them Any difference in quality between OEM and aftermarket AOS? (Uro/Bandi?) Thanks for the advice on the Idle stabilizer. I'm thinking of buying a spare TB assembly from a forum member that's parting out a 99 if I can get it for cheap enough. I'll buy a new gasket for that stabilizer too. The 99 seems a little bit of an odd-duck for parts since it has the cable TB.

Only use genuine Porsche AOS unless you want to do it twice to practice moe

That O-ring came out in perfect shape when I removed the TB as part of the starter replacement - I did not need to replace it. It is a compression interface; no friction involved in removing/replacing, so it shouldn't tear.