There's two common things to consider before you get too involved in the diagnosis. First, since GM redesigned their generators for the '87 model year, these have caused a real lot of trouble, including elusive engine running problems. Due to their design, they produce huge voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. There's two things you can do to identify this. First is to have the charging system professionally load-tested. In particular we want to see full-load charging current and "ripple" voltage. Charging current will either be close to its design value, commonly 90 to 120 amps, or exactly one third of that. If it's one third, and ripple voltage is high, there is a failed diode already.
As an alternative, you can unplug the small connector on the side / rear of the generator to stop it from working. If the misfires clear up, suspect the generator and battery.
These generators have an extremely high failure rate. It's not uncommon to go through four to six in the life of the vehicle. To reduce the number of repeat failures, replace the battery at the same time unless it is less than about two years old. The battery is the main component in damping and absorbing those harmful voltage spikes but it loses its ability to do that as it ages and the lead flakes off the plates.
The second common problem also applies to only GM vehicles. When most other manufacturers buy their injectors, they buy them in flow-matched sets. Chrysler, for example, buys theirs from Bosch, and failures are very rare. GM just grabs a handful out of a big bin with no regard to matching their flow rates. With high mileage, varnish buildup, debris blocking the screens, and wear on the valves causes their uneven flow rates to result in at least one cylinder running a little lean. At first the unburned oxygen is detected by the oxygen sensor, and the Engine Computer responds by commanding a little more fuel, but that is to all the cylinders on that side of the engine. Naturally fuel mileage goes down, but eventually the lean cylinder(s) will start to misfire. You may never feel that but it's bad enough that the computer can detect it and set the appropriate diagnostic fault codes. The fix for that is to install a set of flow-matched rebuilt injectors.
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Friday, March 20th, 2015 AT 12:44 AM
