'81 308 GTS 3k rpm

The cold idle on a US 308i is (intentionally) set kind of high.

Are you saying that this is a "new" or different behavior/problem? -- e.g., it goes to 3K RPM at warm restarts too?

If it is high when "cold", does it go back to a "normal" idle when the water temp reaches an indicated ~160 deg F?

 

Check the floor mat ! (is it pushing on the gas pedal ?)

 

If it stays high even after warm up, a major vaccum line might have gone loose or broke. Same thing happen to mine I found the the vaccum hose going to the brake booster came loose it was sucking too much air, I reconnect and problem solve

 

I wanted to reduce the high RPM during cold start up. Reduced RPM from 1300 - 1400 to about 650 - 750 RPM. Make sure all you other vacume connections are good. Here is what worked for me. I re routed the vacume hose from the cold start air valve, bypassed the sensor (market with red X) direct to Plenum input. Keep the small hose you will remove for future owner.. Hope this helps..
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The "extra" air added into the intake during cold start up on your 308i comes from two sources -- the cold start air valve and the AAV. At lower temps, the AAV opens a bit more so the colder weather could explain the RPM increase.
Over 3K RPM does seem a bit much -- the usual strategy is to either disable the cold start air valve (although I can't follow AWulff's rework suggestion since this would cause the cold start air valve to always be open), but this creates an over-rich cold-running condition because the injection system is adding extra fuel anticipating that the extra air is being added. The better solution IMO is to add an orifice restriction in the air line that passes thru the cold start air valve so to reduce the cold start RPM to the 1.8K~2.2K range (which will still run over-rich when cold but not as much) -- e.g., plug the cold start valve air line with a short section of wooden dowel and then drill a small hole in the center of the plug -- enlarge the hole until the desired RPM is reached when cold.

Not quite sure what you're asking here, but the way the system should work is:

Cold
thermoswitch in coolant tank = open
which leaves the electrovalve de-energized
which allows the vacuum from the intake plenum to reach the cold start air valve mechanism
which opens the cold start air valve

Warm (when the coolant temp gets to 150~160 deg F)
thermoswitch in coolant tank = closed
which energizes the electrovalve
which blocks the vacuum from the the intake plenum from reaching the cold start air valve mechanism
which closes the cold start air valve

If you still have a high idle when warm (and it isn't the carpet ), just unplug the small vacuum line from the cold start air valve (and block the line with your finger so extra air doesn't enter the intake) and see if this drops the idle speed.

 

Yes and No, the idle screw will adjust for metered air, any vacuum leak will be unmetered air and will cause a lean condition. Once you have plugged any and all vacuum leaks then you can adjust the idle with the screws. the WSM has a very good 'guide' on how to adjust the idle correctly.

most people have disconnected the bypass for the cold start, there are two in actuality, one is as described by 91tr and the other is part of CIS system and works similar, and is in line with the AAV. confused yet?

 

No -- the idle air bypass screw would affect both cold-running and warm-running -- i.e., closing the idle air bypass screw to lower the cold-running idle RPM would also lower the warm-running RPM.

 

Ferrari did that for emission reasons, at least according to R&T back in 82 who mentioned that cold fast idle thing in a test article. They were concerned about it too but were assured by Ferrari that it was perfectly normal.

 

I was pretty close. I was guessing R&T and guessing 82 also.