I am getting error code P0300, Random Multiple Misfires. I have replaced various parts in the engine, including the ICM, three coils, six fuel injectors, and both the upper and lower O-rings. I also replaced the lower intake manifold gasket, the entire upper manifold, all six spark plugs and wires, and the fuel pressure regulator. I am still getting an error code P0300. I ran a fuel pressure test and the leak down shows pressure loss.
It sounds as though you have done an almost complete tune-up of the vehicle. However, there may be a few components left to check.
A clogged fuel filter can restrict fuel volume causing a lean condition, and set random multiple misfires as all cylinders are lean. I believe your year model does come with a serviceable fuel filter; your vehicle lies on the border of model change where serviceable fuel filters were replaced with a filter in the fuel pump assembly. Handling volatile liquids can be very dangerous, so I would suggest having a trained mechanic, like the ones at YourMechanic, assist with a fuel filter replacement.
Also, a certified mechanic will be able to verify fuel pressure loss or a low fuel pressure situation with a comprehensive engine misfire inspection, and lead your diagnosis from there.
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