intimidtr3

I have a 95 Sonoma with the 2.2 5 spd.I have given it a full tune up and replaced the O2 sensor.Sometimes it runs fine and sometimes it runs rough and pings in the exhaust like little backfires.I took it to parts store and they hooked it up to obd 2 and it could not link with the ecm.It is intermittent and is not temperature related.I am not wanting to replace parts I don't need to but I am stumped.I consider myself a descent mechanic but this is about to convince me otherwise.Any help would be appreciated.

enginetune

So why wouldn't it connect to a scanner? That is the question. That usually means a 12 volt supply to the computer is missing. There sould be one constant battery voltage and as many as 4 keyed power. And don't forget the ground circuits. It is possible to have one keyed supply missing and still operate but a few items may not communicate. It might not set a code because the system doesn't know it's supposed to be working. Get a diagram of the ecm connectors and trace it to see what is missing. A long shot is a faulty output to the ALDL. that usually translates to a defect inside the ecm. Check the wiring first.