First the DTC P0304 is a misfire on number 4 cylinder which would be a sparkplug,plug wire,coil or a fuel injector and sense you've replaced three out of the four I would think you have a clogged or faulty injector.As for the DTC P0300 that is a random misfire detected which could co-inside with the bad injector.If it were me I would replace the injector and go from there and see if that fixes the problem.
SOURCE: Rough idle code P0300
check the fuel pressure when u get a po300 code that means there has been a missfire. if your fuel pressure is low it will cause missfires.
SOURCE: I need to find the fuel filter on a 2004 saturn
Check your plugs, check the ign module, test the injectors,take a fuel sample and make sure there isnt water in your fuel, if all these test good perform a compression test. if the compression test fails replace the head. if the compression test is good. check the timing
SOURCE: 2001 GMC Jimmy Idling rough
I know the post is old but this is for the people who have the same problem like me and still search the web for a solution. P0300 random misfire code, rough idle after it warms up and then exhaust smells like eggs. Check the plugs 5 and 6 should have black carbon (rich fuel) plugs 1 to 4 look good. The problem was for me the Fuel Pressure Regulator. You will have to remove the air plenum to get to the fuel injector spider. Not difficult just follow a repair manual. You should notice the intake under neat will have a blackened appearance for intakes 1 to 4 and 5 and 6 will look clean. The regulator is venting the fuel and it is running down into 5 and 6 hence the fouled spark plugs. The regulator is on the back of the spider and easy to replace the biggest problem I had was getting to the fuel lines to disconnect them. Runs great now.
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