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2003 GMC Sierra 2500HD Misfire

My son’s truck started misfiring about a month ago. We’ve had it to three different mechanics, including a GMC dealership, and no one has been able to diagnose the problem. The mechanics suggested changing the Mass Airflow Sensor and Filter. After changing it out, the truck ran pretty good for less than a day and went back to the constant misfires. He has also changed the spark plugs, wires, coils, knock sensor, and fuel filter with no change to the misfires. He changed the Throttle position sensor and it again ran good for a day then reverted back to running bad. Any ideas what it could be?

Well first off, tell us what engine is in this truck, how many miles on it and if the check engine light is on. And if the CEL IS on, what codes (the P0123 codes) does it show?

and- when replacing parts are you clearing out the DTC’s?

You need to find a much better mechanic

This truck isn’t particularly exotic or hard to diagnose

Did anybody bother hooking up a vacuum gauge at idle or perform a compression test?

Has anybody hooked up a noid lite yet?

Based on all the parts that have been thrown at the truck so far, I’d say whoever you’ve been going to needs to go back to remedial mechanic school

Exactly why were the knock sensors replaced . . . it’s a fairly involved job on this engine, as it involves removing the intake manifold. Seems kind of odd to replace those sensors on a whim

There is no separate throttle position sensor on a 2003 2500HD . . . this truck has an electronic throttle body, which means that somebody likely replaced the entire electronic throttle body assembly

I’m almost 100% certain this truck has the 6.0 liter gasoline V8

By the way, is there a particular cylinder that is misfiring at an extremely high rate?

Even if there was just a P0300 random misfire code, a good mechanic can soon discover that even with such a vague code, there might be a specific cylinder to zero in on

One clue is that it seems to run better after work is done on it, but that only lasts a short time, then back to misfiring. One reason for that could be the battery is disconnected for the work. This resets all the computers and they go into a learning mode. It may be that whatever the engine computer is learning is the wrong thing, b/c applying that learning is causing the engine to misfire. The sort of stuff that it learns is the optimum idle rpm for various conditions (in gear, w/headlights on, etc), how much gas to inject to get optimum air/fuel ratio, etc. Perhaps when it starts up after a battery-off condition it uses a richer mixture, and as it learns it leans the mixture out. You may have an air leak somewhere that when it does this, it makes it too lean. Usually this would cause a lean code to be stored in computer memory. is there one? Ask your shop to provide the fuel trims, another clue to rich/lean problems. Misfiring can be caused by battery/alternator problems too. Before the first start of the day the battery should measure about 12.6 volts, the 13.5-15.5 immediately after starting the car. Compression problems can cause misfires too, if nothing above pans out.