Warm Misfire '91 190e 2.3
 

From a cold start, it runs just fine for a couple minutes, after that, as if someone threw a switch, it goes into another mode and I get (what seems to be) a misfire. That is, it stutters when I try to rev it up in park, while in this "warm mode".

Additionally, it idles steady in park at around 1800 RPM, if I don't drive it and just let it warm up from cold. This differs from the up/down idle (between 0 and 1000 RPM) in park or neutral that I get after the car has been driven and fully warmed up.

I dunno, it's strange.. This is about as accurate of a description of the problem that I can give. Hopefully someone who has had a similar problem can help as I've got to get this thing smogged soon.

First, I would check rubber. If any of the hoses in the idle air bypass circuit are 'leaky' (hardened with age or cracked), the idle air flow will not be properly metered, and the mixture will suffer. The high idle is usually a compensation to try to keep the engine running. There is also a microswitch activated by throttle return which may need adjustment. Also, the rotary idle air valve could be gummed up and need cleaning. It is under the air cleaner, between intake runners, and is in the same path as all the rubber plumbing in question.

Steve

Thanks Steve! I'll take a look at that and see if it helps.