I have an1987 350 iroc tpi and recently got a second hand ecm and eprom out of a 1988 same car and ecm +prom P4 12227165 eprom27C128.the car ran a bit sluggish so I took it all out put my 87 ecm back in with the 1988 eprom, what a differance in power good gear shifts and a louder engine note,I wonder if the 88 eprom had been modified?.The only problem I have is that the A/C fan feeps running all the time.my car had the stock fan switch in that cut in at about 220 or 30 and the drivers side fan was being used, since I put the jet switch in the fan cuts in at 200 the system has changed fans now the A/C fan runs.I have deleted the A/C and cold start injector a year ago.so as I was saying since my ecm eprom change my fan runs full time.Any ideas Guys would be help full.

 

Yes, since you messed with the fan switches the ECM dosen't know what to do. Stock fan set is controled by the ECM.

 

For 87, if you removed the fan pressure switch (deleted a/c and all related lines) the blue wire needs to be grounded to prevent the ECM from seeing a fan request input. If the plug is still there and wiring in contact, a jumper in the plug will work just fine.
The fan pressure switch is/was located on the aluminum tube that is/was mounted on the pass side frame rail.

 

Thank's Deadbird I will try that, it sounds like a good plan. God above wasn't much help at all all though he has all the power.