So you have a good battery but no one has mentioned testing the charging system to verify it is recharging the battery after starting the engine. You should see an indication of a problem in the form of a low reading on the "Volts" gauge on the dash or the battery light is on, but there's a more common problem with GM generators starting with the redesigned systems in 1987 models. They often develop bad diodes or a defective internal voltage regulator. Both of those are caused by the huge voltage spikes these generators develop due to their design. Those spikes can also interfere with computer sensor signals and cause weird hard-to-diagnose engine running problems. Very often your Volts gauge will read normally when the generator has one bad diode.
When one diode fails out of the six, you will lose exactly two thirds of the generator's capacity. A typical generator is designed to develop up to 90 amps. With one bad diode, all you will be able to get on a load test is 30 amps. That's not enough to run the electric fuel pump, fuel injection and ignition systems, many computers, head lights, and heater fan. The battery has to make up the difference and will slowly run down over many days and miles. Typically system voltage will still be within the acceptable range of 13.75 to 14.75 volts with the engine running, but a professional load test will show "ripple" to be very high and the generator will only develop one third of its rated capacity for current.
It is real common to go through four to six replacement generators in the life of GM vehicles, but the way to reduce the number of repeat failures is to replace the battery unless it is less than about two years old. You already did that. As the battery ages, it loses its ability to dampen and absorb those harmful voltage spikes.
Start with a load test on the generator and see how much current it will produce and what the ripple voltage or level is.
Sunday, January 6th, 2013 AT 11:27 PM