Lean condition after startup...causing miss/rough idle
 

88 944 turbo - the car was having rough idle issues when cold and after reading clarks garage rough idle troubleshooting guide I thought I traced it to the dme temp sensor. I replaced the sensor and the idle when the car initially starts seems to be fixed. It will go directly to 14.7 on my WB02. Then after 30-45 seconds the afrs climb to 15 then 16 then 17 and eventually past the scale. This is when the car starts to miss. It runs beautifully when the afrs are below 16.5. The thing I don't understand is why is the car running great and then degrading. I thought it might be the 02 sensor an have been thinking of changing it but wanted to ask you all first. From what I have read a bad o2 se so will result in a rich condition where fuel economy degrades and a strong gas smell from the exhaust, I have neither of those. The car is just turning lean after the first min of running.

When the car is warm it's still there if I start warm but if I drive and then stop at a light or something the afrs and idle are rock solid. Don't know what to think. Thanks.

chipped? unhook the O2 sensor that talks to the DME and see if the issue goes away.

Agreed. My NA behaved the same until I changed the O2 sensor. I speculated that on mine the heater on the sensor had gone and the signal was way off when it was warming up.