As you know, I’ve been a stickler in trying to get my 1988 V12 XJ-S to idle as smooth as possible, and I’ve pretty much pulled it off!
Getting the two throttle plates set perfectly seems to be a big help. But I’ve noticed it sometimes slowly moving away from that smooth idle now and then. So today I tried giving both throttle bodies a spray of throttle body cleaner, and idle is suddenly back to smooth!
My PCV system is working perfectly, and I am getting no oil accumulation in the air filter housings. But I do detect oil vapor in there, which is perfectly normal.
So perhaps a monthly cleaning of the throttle bodies is a maintenance thing?