Since no smart people have replied yet, I can offer a few tips. First, stop on a slight incline, shift to neutral, and release the brakes. The truck should start to creep downhill on its own. If it doesn't, suspect a sticking front brake caliper or rear parking brake cable. GM doesn't have much trouble with calipers so a rear cable would be more likely. Often they will release if you crawl down in front of one of the rear tires and flex the cables. Be prepared for it to release though. It would be preferable to not get run over or have to go chasing the truck down the hill!
If the brakes do not appear to be sticking, one thing that is often hard to find is a problem caused by the generator. This only pertains to GM generators from 1987 and newer. Because of their design, they are prone to generating voltage spikes that confuse the dozens of computers on the truck. The spikes can also magnetically induce signals into the wires carrying sensor signals to the computers. It is fairly common for GM vehicles to go through four to six generators in their lifetime. One thing that is becoming more widely known is repeat failures can be prevented by replacing the battery when the generator is replaced, even if it is still good.
Based on oxygen sensor readings in the exhaust system, the Engine Computer can adjust fuel metering to the engine by plus or minus only about 10 percent. That is less than you are finding. When the generator is at fault, the Engine Computer can command the wrong amount of fuel into the engine based on what it thinks are values coming from other sensors. If this is what is happening, you should see excessive black smoke from the tail pipe.
It would pay to manually calculate your fuel mileage in case your trip computer is off. That computer calculates fuel mileage based on tire size, vehicle speed, and the number of milliseconds the computer pulses the injectors. If fuel pressure is low, normally there will be hesitation and stumbling problems, but even if the engine runs well, the injectors will have to be held open longer to get enough fuel into the engine. The engine could be receiving much less fuel than the trip computer thinks is going in. That would reduce its fuel mileage calculation well below the actual fuel mileage.
Sunday, March 21st, 2010 AT 4:08 AM