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P0303 ignition misfire on cylinder #3.
1.Fuel injectors, related wiring, sensors, computer issues
2. Running out of gas, or poor fuel quality
3. Evaporative emissions system (EVAP) concerns: fuel vapors leaking into engine
4. Incorrect Fuel Pressure
5. EGR system concerns: leaking EGR valve or restricted ports
6.Base engine concerns: low compression, valve train problems and timing issues
7. Ignition system concerns including, but not limited to:
Faulty spark plugs
Faulty coil or related wiring
Ignition module or related wiring issues
Ignition related sensor faults or wiring issues
You will have diagnose each element until you find root cause.
And it could be more than one element faulting out.
If you have a coil pack (3 coils and no distributor), one of your coils may be bad. Especially if the two cylinders not getting spark come from the same coil. Each coil has 2 plug wires, serving 2 cylinders.
I would first be sure that you have spark to that cyllinder. Pull the wire off and stick an extra spark plug in the wire. Lay it on the engine block somewhere and atart the car to see if it's sparking. If it is, you have a valve or cam problem. If it isn't check the plug, the wire, distributer, coil pack etc.
Sounds like carbon fouling. Easy fix is run an aggressive fuel injector cleaner as recommended by your local parts store through the gas tank. Also ensure that spark plug wire is firmly attached. If you took the plug out-you know it is a pain to get to it! A easy way to see is to start motor and idle, pull #6 wire off at distributor when it should be attached to plug and see if the idle changes. If it doesn't, the wire is not plugged on the spark plug or the rotor/distributor may be faulty.
Another more difficult task may be the removal and replacement of the fuel injectors them selves (or cleaning by a specialty shop). Good Luck!
Check the firing order to be sure that they were correctly installed. If they were, then remove plugs and recheck gap. Sometimes they get smashed shut during installation. Next thing I would check would be compression, O, how did it run before the plugs and wires were changed, If it was already running poorly, then compression is likely. You don't mention model or year, but if you have these, cap and rotor can also be the culprits, as can a weak coil. Last but certainly not least, listen to your injectors (again assuming it's injected, no model or year,) whith a stethoscope, and be sure that they are all firing. If you haven't found the problem after all of these suggestions, please get back to me and we'll find more things to look at! Best of luck!
I have changed plugs using the platinum plugs and the problem don't go away. These plugs can make a car run worse in some cases. I have found it best to use the OEM or a step above on 4 cylin and 6 cylin engines. The Plats work fine in 8 cylin engines. You could also have a bad plug wire. The plug wire could be jumping spark. At night, open the hood, start the car with lights off. Look around the plug wires for sparks jumping. If a wire is bad, the spark is leaving the wire and grounding itself to the nearest metal causing rough engine idle and misfire.