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I need some help I’m not a mechanic totally but I have been going to school for it and my 03 Silverado 5.3 is misfiring and I don’t know what the problem could be I replaced the spark plugs coils and all the cylinders are getting spark and I had a compression test and the the cylinders have near perfect compression so I don’t think it’d be that. I even replaced the ecu/pcm in it and nothing changed it’s a hard idle and had no power can’t even make it a block really it also dies when I hit the brakes after going anywhere over 5 mph. 

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Had same symptoms on 01 burb. When idle becomes too low or erratic this can cause misfires. Tried a lot of things, but eventually found it was the isc.

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Checked the fuel pressure it’s running 50 psi it passed evap test too

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You Know 50 is borderline,as far as the stumbling goes,I would disconnect the electrical connection on the MAF,test drive, improvement?,and nice to know evap passed,that's a whole nother pia

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Posted (edited)

Did you check/change fuel pressure? Check fuel pressure regulator? Are you getting any codes?

 

Key on engine off, fuel pressure should be 55-62.

 

What plugs did you use and what did you gap them to? .040 is recommended by GM. Originally was .060 and had misfire issues. Check for vacuum leaks at intake

Edited by txab

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Turns out my cats were clogged and was not letting the exhaust leave the engine causing random misfires in all cylinders 

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