Hi guys. Any additional symptoms or observations would be helpful, but here's something to consider that only pertains to GM vehicles with higher mileage. It has to do with mismatched flow rates of the injectors. Chrysler buys their injectors from Bosch in matched sets and problems with them are just about unheard of. They last well beyond the life of the car. GM grabs a handful of injectors out of a huge bin and throws them in an engine with no regard to matching their flow rates. As the engine ages and the injectors begin to show a little wear, some will not flow as much fuel as the rest resulting in one or two cylinders running lean. The unburned oxygen is detected, but the computer doesn't realize that it's being caused by only a few cylinders, so it asks for more fuel from all of the injectors, even those that are flowing too much fuel already. This can be a hard one to figure out because there isn't exactly a defect that can be easily diagnosed and there won't be a stored diagnostic fault code. Now that it has become more widely known, there is a company that specializes in rebuilding injectors and GM injectors are their most popular product. They are flow-matched and sold only in matched sets. Many owners claim their engines never ran so smoothly since the car was new.
Besides the lean misfire, (which should be detected by the Engine Computer on '96 and newer cars), the computer will be constantly trying to add fuel and that can result in an excessively rich condition. The computer might be able to add enough fuel so the lean cylinders have the correct mixture but the rest will be too rich. Oxygen sensors don't detect unburned fuel, just unburned oxygen. Part of the strategy in adjusting the mixture is to run it too lean a couple of times per second and to run it rich other times. The computer expects the oxygen sensors to detect a specific amount of oxygen from all of the cylinders. To achieve that, it will likely not add enough fuel over the long term to keep the lean cylinders happy. A clue to finding this is by watching the long and short-term fuel trim numbers on a scanner that displays live data. If all of the numbers are positive, the computer is requesting more fuel beyond the pre-programmed values from the factory. At the same time, the system will be listed as "lean" for longer periods of time than it is "rich". It should switch a couple of times per second if all the injectors are flowing equally.
Since the Check Engine light is not on, there is likely no stored diagnostic fault code related to a misfire. You might consider other causes of a rough running engine. GM has more than their share of worn engine mounts. That can allow metal parts to touch and transmit normal engine vibration into the passenger compartment. Bent metal exhaust hangers can do that too. They always have some type of rubber insulator between the two parts of the assembly to isolate vibration.
Also consider low fuel pressure. Chrysler pumps will fail to start up when they're worn from high mileage, but once they start running, they never quit while you're driving. GM pumps are exactly the opposite. They will almost always start up but will stop working unexpectedly and let you sit on the side of the highway. The system is very picky about precise fuel pressure. Most cars will run fine with slightly low fuel pressure but on some GM engines, 2 pounds low can cause misfires from fuel failing to spray from an injector. The low pressure can develop after the pump gets warm.
Another cause that is strictly a GM problem is the generator, and it's a big one. They went from the second best generator design to the world's worst pile starting with the '87 models. Due to their design, they develop huge voltage spikes that can destroy the internal diodes and voltage regulator, and they can interfere with computer sensor signals. It is real common to go through four to six generators in the life of the car. The way to reduce the number of repeat failures is to replace the perfectly good battery at the same time the generator is replaced, (or before it goes bad). As they age, even though they start the engine just fine, they lose their ability to dampen and absorb those voltage spikes. When one of the six internal diodes fails, the generator's output capacity is cut to one third of normal. That can be barely enough to run the fuel pump, head lights, and all of the very many computers with nothing left over to recharge the battery. System voltage will go low and computers can not handle low voltage.
Those voltage spikes cause current spikes which radiate out magnetically from the wiring into sensor wiring. Beside being confused when running on low voltage, those spikes show up as glitches from sensors and will confuse computers even more. Even when system voltage is not low, those spikes can affect computers. A simple test to see if the generator is responsible for a rough running engine is to unplug the small plug on the back. If the engine smooths out, suspect the generator and battery.
Tuesday, January 3rd, 2012 AT 7:59 AM
