I have searched and posted on this topic for help and gotten so much different info I am overwhelmed. In a nutshell, my '73 1.7L starts right up when cold and idles steady at about 1600rpms. After several minutes (warm enough for the AAR to close I guess) the idle starts hunting from 1600 to stall if I'm not ready on the gas pedal. Once it gets good and warmed up, the hunting is minimal to nonexistant and the engine runs strong, idling around 900. Acceleration and cruise is great. The idle knob on the ECU has to be turned all the way rich for decent operation. Originally, I could cover the air inlet on the air cleaner with my hand while it was hunting and the idle would smooth right out; big vacuum leak right? I have replaced all the vacuum hoses and now when I cover the inlet the engine will die. Still hunts though. I think the MPS is OK (granted I tested suction with my tongue) and I presume the AAR is OK since it idles great when first started. The butterfly in the throttle body closes squarely and no cracks are visible. I have tried spraying carb cleaner around various components with no change in rpms; would I not see the rpms climb if there were a crack in the plenum? I wanted to see if there may be any other suggestions before I chuck out some $ to the local Porsche/Audi guys for diagnosis.

CHT sensor?? AAR is controled by the CHT sender?

The AAR receives 12V from the ignition switch. Inside is a bimetallic unit that when heated (from the 12V input and from the engine heat) causes the valve to close. It is closed above a certain temperature (I can't recall what that temp it is though).

A faulty CHT sensor would have a multiple effects on the engine's running condition - not just during warmup or idle.

Sounds like a big vac leak to me. Throttle body seal? What about the phenolic spacers between the heads and intake runners - did you replace those?

Also check for a working Cold Start Valve. May also be a bad MPS, amongst many other things.

I just went thru similiar on my 71' 1.7. finally got a vacuum gage and checked the MPS.............decent to 10 in/hg but would not hold for long (couple of seconds). Had a spare from the 73' 1.7 no longer in the car and pulled vacuum on that.........will hold 20in/hg forever with no change.

Check your MPS the right way by pulling vacuum on it and make sure it will hold 20 in/hg for a minute with less than 5 in/hg leakdown in that time. Car runs awesome, no more hunting. You need to absolutely eliminate the parts or you can be trying stuff for days.

Hope this helps,

Jim