I have a 91 B4C 5-speed that has had the 305 engine upgraded to a 350. The guy who had it before me replaced the ecm with one from a auto xmission. He said the original ecm would not run the new 350. After the he installed the new computer the (chk eng light) comes on when the computer trys to find the x mission. I was wondering if that has anything to do with an idel problem that happens when I don't let it warm up before driving.If i get in it and take off the first time I stop at a light the engine surges any where from 200 to 500 RPM's. This is very annoying. The idle sensor has been replaced. help thanks

 

Yes it does matter a little bit. There is a flag in the calibrations defining A4 or M5. The M5 cars had a little higher idle I believe. Although you should get this problem taken care of, I would also hook it up to a diagnostic scanner. There could be somthing else wrong. My thoughts are maybe a bad O2 sensor.