I have a 91 3.1 with 158k.Very rough idle.The problem im having is the ECM will not turn on my fan, even at 250 degrees no fan.
I can run fan by grounding dk green wire at relay.
also ,no code is set when i unplug the DST when i set timing.
My question is, can parts of the ecm fail while having fair drivibility.
no codes are present..

 

Before jumping to the ecm, did you make sure the sensor is functioning properly and the relay is ok? You jumped it around the relay or off the relay for ground? I don't think your ecm would be the culprit if the car is still running well as the idle and fuel mixtures would most likely be off and it would run rough or something if the numbers the computer was getting/giving were off...

 

I'd say the fan issue is the fan switch - I never had one work right, even the expensive ones. Does the car have A/C? If so, try turning on the A/C to see if the fan comes on. If so, then I'd guess the fan switch is bad. You can leave the A/C turned to on, and have the fan run constantly, until you get it resolved (I have to do this on my 1990 RS now for going on 2 years 'cause I got tired of replacing fan switches that didn't work for more than a month).
A/C on makes fan always run.