I have a 1990 Grand Prix with a 3.1 auto and I am stumped about why it is running like a POS. It surges at 5-15 MPH, idles rises to 1000-2000 RPM when in Park or Neutral, and in drive the RPM's stay at about 1600 and when I come to a stop (at about 3-5 mph) the RPM's drop to around 500 and it barely idles. I have replaced the IAC valve, TPS sensor, MAP sensor, one of three coils(ignition modules), fan motor, coolent temp sensor, fan relays, and the radiator. I also found out that cylinders 2&5 have only 75lbs of compression and the rest have 120lbs. I was told that it could be that the computer is making the motor idle high because it is overheating. Trust me, it's NOT. Also I'm not sure if this has anything to do with my problem, but the 'low fuel' light and the 'check gauges' light are always on. Some times the 'service engine soon' light pops on also. I bridged the A and B terminals on the DLC and found out that the trouble codes stored were 'idle speed error' and 'Electronic Spark Timing'. The car also will not stay running when the motor is cold. I hate driving with 2 feet.
I'm sick of guessing and replacing parts out of the blue, it gets quite expensive.
Any suggestions/help would be greatly appreciated.
Thanks in advance,
Mike