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2004 Chevrolet Monte Carlo - stumped by P0300 code

engine light came on then started flashing. put on my diagnostic machine and came up with evap vent solenoid. Changed that but still says P0300 engine misfire detected. Changed the EGR valve, reset the computer , started car and engine light came on again. Ran diagnostics and the P0300 engine misfire detected. put new wires and plugs in about 5 months ago. cant figure out what is going on. Car starts, just vibrates.

A worn/stretched timing chain can cause random misfires.

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Tester

Is p0300 the only code now? i.e. the evap problem has been fixed?

p0300 means the engine is misfiring, and not just on a single cylinder, but on multiple, probably all the cylinders. That sort of thing is caused by something common to all the cylinders. Engine air filter clog, exhaust system clog, incorrect fuel pressure, something like that. Ask your shop to do a fuel trim test, should provide a starting point.