Codes P0300 and P0650, suddenly missing badly, throttle not working and transmission not shifting right

RUSER

MEMBER
2003 GMC SIERRA
6.0L
V8
2WD
AUTOMATIC
370,704 MILES

Pulled cattle trailer 400 miles last two days, 70 MPH over hill and valley. Started next morning and without trailer it barely had the power to get up a low hill, the throttle and RPMs were not corresponding properly and the transmission wasn't shifting. Could barely get up to 45 briefly.
No blown fuses found, no blowby at all at idle or 2000 RPMs, no smoke, no oil usage, transmission oil level good and looks and smells new. The ground on the rear left side of the motor has the ground cable and the black wire firmly attached. Because I didn't know how when they got changed last we changed the plugs and wires. Only codes on my cheap OBD2 reader P0300 Random/multiple cylinder misfire detected and P0650 MIL control circuit/open.

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

If all you have is a multi cylinder misfire and the P0650 code then we could have an issue with the PCM but we need to chase down the misfire first.

I attached the info for the misfire code. Let's just run through the basics to check and make sure there are no obvious issues.

Then we need to check for a vacuum leak using this guide:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Once we have a better idea of how these systems are operating we will know better what the possible causes of this are.

However, I suspect you will find something with the vacuum leak so I would start with this if you are going to choose one.

Please let us know if you have questions on this. Thanks

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