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Roadside Jerry
Roadside Jerry, GMC Mechanic
Category: GM
Satisfied Customers: 2948
Experience:  Have worked on GM vehicles for over 40 years
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GM Chevy Silverado 1500: I have a 2009 Chevy Silverado 1500

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I have a 2009 Chevy Silverado 1500 4.8L that keeps popping code P0300. I have replaced plugs and wires, # XXXXX fuel injector, and # XXXXX coil pack, but nothing has helped. I got the ideas about the #8 cylinder from a mechanic friend that had said #8 was misfiring most.
The next step would be to do a compression test. A slightly low reading , can cause enough of a miss to set a code in the computer.It appears that you have done all you can as far as the ignition and fuel systems. If it passes the compression test one other thing that can cause a miss is a vacuum leak. If you can have a freeze frame reading of when the miss occurs,this would help . If the freeze frame show the miss is at low speed,and on a cold engine,it can be an intake gasket leak.
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Customer: replied 5 years ago.

It runs fine at highway speeds, but at idle or very low speeds it runs rough and I get the stabilitrac off light. The stabilitrac light goes back off once I get back to highway speed.

The stabiltrac is programmed to shut down when the check engine light is on or the computer sensesca faultvin the engine performance . The tests I described will show you what the problem is.. Either item I mentioned a vacuum leak, or a valve leaking will cause a miss that is felt at lower speeds and just about disappears at higher engine rpms.