Mix goes lean when engine fully warm (CIS)

[bsimonson]

Today my car (US '80 SC with O2-sensor) began to idle poorly when fully warmed up.

It started fine from cold and ran well for the first ~10 minutes. All of a sudden it began to stall when going to idle unless I let it down slowly using the throttle. When I looked at the duty cycle it was at about 97% (I have relocated the port to under the passenger seat).

I parked on the side of the road and had a look for any obvious faults but could not find any. When I restarted after a couple of minutes, the duty cycle was back at 45-50% and the idle was perfect. After another few minutes of driving the duty cycle went back at 97% and the car had a poor idle. If I remove the oil cap when the car idles poorly, the RPM does not change, if I do it when it idles fine the RPM drop noticeably.

I repeated this a few times with the same result.

So to me it seems I have some kind of temperature dependent vacuum leak. I recently tried to verify the AAR and when I ran the fuel system by bridging the fuel pump relay it just left a very slight opening, so I figured it would close fully when seeing a higher voltage from the running engine. But even if it doesn't, how could it make such a drastic change to the mix when the temperature goes up?

Any ideas?

Thanks, Björn

[jason2guy]

i have the same stuff happen when hot. idle goes up and up and away. ive rulled
out the distributor. its the air box, if that thing isnt new its blown. if one day i replace that part its gonna be with one of those stainless pieces. ive been running with no o2 sensor forever and in my opinion they run better