I get a lot of mail from people who replaced their vehicles computer, or more properly the Powertrain Control Module (PCM), because they have a drivability problem and still have the same problem they had before they replaced it. Actually the PCM should be the last thing to be replaced when addressing a drivability problem. 80% of all driveability problems are due to a problem with the engine control systems or the engine itself.
Some of the things that will cause drivability problems are Coolant Temperature Sensors, MAP Sensors, Mass Air Flow Sensors and EGR valves. Low coolant levels can cause a drivability problem as well as bad distributor caps, ignition wires and spark plugs. Low compression in one or more cylinders will cause problems as well.
Before you go and replace the PCM, or condemn the new one you put in as bad, there are some very basic things you need to check first. I made a little list here that you could print out and use as a guide when you have a drivability problem. Print it out, do the checks and fill in your results. Then you can check them against the vehicle specifications to see where the problem may lie. Most of the time this will save you a few hundred dollars for a new PCM that you won't be able to return for a refund.
1. Inspect all the engine and vehicle grounds as well as the PCM power and ground connections. Use a voltmeter and check the voltage drop from the POSITVE (+) battery post to the ground connection.
Result: __________volts
2. Check the battery voltage with a voltmeter. At rest it should be between 12.4 to 12.7 Volts.
Result: __________volts
3. Leave the voltmeter connected and crank the engine. Battery voltage should stay above 9.5 volts.
Result: __________volts
4. With the engine running you should have between 13.2 to 14.7 volts. If you have a load tester current should be within 10 amps of the alternators rated output.
Result: __________volts Result: __________amps
5. With your voltmeter set to Kilovolts measure the distributor primary (coil wire) and secondary (spark plug wire) voltages at idle and 2,500 rpm.
Idle: __________volts Result: __________ volts
2,500 rpm: __________volts Result: __________ volts
6. Check the base and advance ignition timing at idle and 2,500 rpm.
Idle: __________degrees Result: __________ degrees
2,500 rpm: __________ degrees Result: __________ degrees
7. With a vacuum gauge measure engine vacuum at the intake manifold at idle and 2,500 rpm.
Idle: __________ inches
2,500 rpm: __________ inches
8. With a fuel pressure guage measure fuel pressure at idle and under load.
Idle: __________ psi
2,500 rpm: __________ psi
9. With your voltmeter measure the O2 sensor switching at idle and 2,500 rpm. It should range up and down betwee0.1 and 0.9 volts centering on 0/5 volts.
Idle: YES / NO
2,500 rpm: YES / NO
10. The Technical Service Bulletins for revised or updated PCM software or PROMs (GM) and other known (there are always lots of them) drivability problems.
