After replacing the water pump crossover gaskets is when the multiple misfire p0300 started. The car will idle high then low and shakes. Plugs wires intake manifold gasket injectors fuel filter egr idle control sensor temp sensor 4 02s all been replaced
I don't think you really understand what this fault code means, it means that likely all the engine cylinders are misfiring at random no set pattern, not any particular cylinder. I would start with checking for engine vacuum leaks like the intake manifold gasket. that is all I can say since you provided no make, model or year and engine type and size information.
A code P0300 may mean that one or more of the following has happened:
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders ( spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.
Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open
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