Missing the fuel cap may have caused your code, though I would have also expected you to have gotten a code more directly related to a pressure problem in the EVAP, which needs the fuel cap tightly in place and air tight. Those EVAP codes would be from P0440 to P0455
What motor do you have in that car? 2.0L dohc? If your spark plugs and wires are located through the center of the valve cover. The gasket might be bad and leaking oil into the plug holes. This will cause the spark to short out through the cylinder head instead of going through the plug. If this is the case, check the condition of the gasket. If it's hard and brittle it's just old and leaking, but if it's still soft. Then you might still have a blow by problem forcing the oil out with pressure.
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Swap the #4 coil with #3, swap #4 spark plug with #2 and swap #4 injector with #1. Clear codes. Drive the car until it has a misfire code, again. If you get P0303, it's a bad coil. If you get P0302, it's a bad plug. If you get P0301, it's a bad injector. If you get P0304 again, it's a mechanical engine problem, like a burned valve, bad intake manifold gasket, worn cam, etc. or a wiring harness issue to the coil or injector, maybe even a bad computer.
Code P0304 Indicates a misfire in cylinder 4. The computer will notice these long before they get bad enough for the driver to notice. Check your plugs and plug wires for thightness. I know from expierence that "New" parts does not mean "Good" parts.
If you have access to a scan tool still.... swap the #1 and #4 plugs and if you still get a P0304 reading you know the misfire is likely in the #4 cyl. wiring. (A faulty injector could cause a misfire but it's less likely a culprit than the wiring) If you get P0301 code after you swap the plugs.. then you'll know it's a faulty plug.
I have an 2002 Kia sedona the code p0304 means an 4 cylinder misfire and also the wiring and plugs.I noticed an while back that when I press the gas smoke will come from my tailpipe and now my van is hesitating to go and it stumbles and the check engine light is on,I have the same code p0304.
po301 thru po306 all misfire codes cylinder 1-6 remove plugs and replace them..... set the plug gap first
i suggest u replace the wires also if they never been replaced all these misfire codes can set the others and one is for the oxygen sensor ( 02s or HO2S) CIRCUT MALFUNCTION do u know how to check wireing systems
do the tune up plugs (set gap) .052-.056 change the fuel filter also. clear the codes hows it run
this info is for 98 E-250 4.2Litre
what engine do u have 4.2L or 4.6L i know it is a v-6 by the codes