I have a 2000 silverado 2500HD with a 6.0 liter gas engine that shifts out of overdrive when I exceed 65 mph. The computer gives me an engine code P0300 which means that there is multiple engine misfires. The truck seams to be running fine and I can not detect any misfires. I have changed the plugs, plug wires done a fuel injector cleaning and intake cleaning and cleaned the airfilter. Any ideas on what it could be???

 

Welcome to the forum. Multiple engine misfires, huh? Could be fuel pressure or ignition related. Is it under an extreme load when it does this? How many miles are on the plugs, wires, etc. ? Is it shifting back to 3rd gear or is the torque converter clutch unlocking? It would feel almost the same.

 

Plugs and wires maybe have a couple of hunderd miles on them. As far as the load is concerned it's just highway speeds on flat level ground no major hills. It does not back shift to third so I believe that the torque convertor is unlocking.

 

The problem is very common and is most likely caused by one of the following:

1. Contaminated Fuel

2. A corroded ECM ground terminal.
The wire terminal (G103) attaches either to the front or to the rear of the right side cylinder head, depending on the model year of the Full Size Pickup. If the ECM ground terminal has been found to be corroded remove the not or bolt securing the ground terminal and remove the rust with a Scotchbrite bad (or similar) and reinstall.

3. Check and replace as needed any faulty parts of the ignition system (cap and rotor).

4. Faulty Catalytic converter.

5. Crank shaft sensor may need to be shimmed.

 

check the plugs & wires most aftermarket crap. delco or GM. they may be backing off plugs will even happen with factory wires.

 

I'll give that a try XZ1100F1 because I do live in a nice road salt state that definitely could happen. As for the cap and rotor this truck does not have that its all coil packs for each cylinder Redbaron so that is not the problem.

 

So far nothing has worked for this truck. I'm getting nervous that it my be tranny related.

 

Have you checked for transmision codes? let me share some info, I used to be a mechanic (not for GM) & I have experienced conditions where a transmision speed sensor will malfunction & set a trouble code & for some reason will also set a false engine missfire code, therefor there is no real problum with missfire. Its only in the transmision & hopefully a speed sensor will fix it. Again I was not a GM Tech so I don't know if this happen on Chevys. I would highly recomend NOT replacing a bunch of parts hoping to fix it. Take it to a daeler & pay the $75 (or wharever it is) diagnoisis charge to find out whats up, chances are they will know right away whats wrong. When I was a Tech I was so familar with our brand of cars I most often knew what was wrong before I ever pulled the car into the shop just buy reading the repair order. A good GM Tech will be this familar also. The sensor (if thats it) will likely be less than $100 & labor should not exceed 1 hour. Should'nt cost more than $300 (not a bad deal in the long run)

 

Brought the truck into the dealer and what they found was the PCM module needed to be reprogrammed and the crankshaft speed sensor needed to be relearned. That's what they wrote on the repair order. So far it has been running fine no back shifting. I am wondering if the Superchip programmer may have had something to do with this any ideas?

 

Brought the truck into the dealer and what they found was the PCM module needed to be reprogrammed and the crankshaft speed sensor needed to be relearned. That's what they wrote on the repair order. So far it has been running fine no back shifting. I am wondering if the Superchip programmer may have had something to do with this any ideas?