My 2003 yukon its 5.3L it has a misfire in cylinder 6 only, u can feel it only on idle, i replaced plus, wire, coils, and they did a compression test for that cylinder it as good and no one can figure out the problem.

Fuel injector or wire to it. Check the wire with a test light or noid as the car is running. The light should flash. Wires good. Use an ohm meter and check the lead in the injector. Should be 11 to 15 ohms. After that is a compression test on that cylinder. No compression is a blown head gasket. If all that checks good, Then I would be looking at the intake manifold gasket. They can blow and pump coolant in a cylinder. Year make and model helps. Some models have know problems.

The code 300 is a Random/Multiple Cylinder Misfire....a 302 is Cylinder # 2 misfire detected. ..and the same for 304 and 306..being 304 is cylinder 4 misfire and 306 is cylinder 6 misfire...All these at the same time normally is a faulty coil pack..,and or faulty coils...spark plugs.,plug wires.....It should be running horribly or barely ....However it could be internal losing compression.....It could just need a good tune up.....Disconnect battery before changing coils,wires..,or plugs in order to clear the fault codes.....

Try a new plug wire on cyl 3 if that doesn't fix it you could have a bad valve or compression problem on cyl 3 Try doing a compression test also If it has injectors for each cylinder you could have a bad injector for cylinder number 3

Check engine compression and compare. then test running engine compression most engines have about 130-170 cranking compression and about 90-110 running compression. OR bring that cylinder to its Top Dead Center compression stroke apply air to the cylinder and check to see where the air is going open throttle plate bad intake valve, from exhaust pipe bad exhaust valve, from oil cap piston rings.

its faulty coil.

Get the compression test done on each coil.these will let you know if its faulty coil or not.

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It can be issue with spark plugs or faulty spark plugs. Also get the compression test done for cylinders. If any of cylinder shows low compression then required, then that particular cylinder will cause misfire.

Also get obd codes scanned, the error code will let you know which cylinder is misfiring or all cylinders are misfiring.

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For your trouble shooting I suggest you to refer the help oinks below. It has step by step details for many engine misfire issues.

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Click the link below:-----

Ford Explorer Cylinder Misfire?

http://whoisbyme.blogspot.in/2012/06/ford-explorer-cylinder-misfire.html

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Engine Misfire codes on Chevrolet Malibu?

http://schematicsdiagram.blogspot.com/2011/12/engine-misfire-codes-on-chevrolet.html

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Cylinder 6 Misfire?

http://howtobyme.blogspot.com/2011/12/cylinder-6-misfire.html

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Misfire on cylinder 4?

http://schematicsdiagram.blogspot.com/2011/12/misfire-on-cylinder-4.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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Ford Explorer 5.0L: Engine Miss?

http://whoisbyme.blogspot.in/2012/06/ford-explorer-50l-engine-miss.html

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Oil on spark plugs and car misfires?

http://technoanswers.blogspot.in/2012/02/oil-on-spark-plugs-and-car-misfires.html

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Can cylinder misfire Damage the Catalytic converter?

http://schematicsdiagram.blogspot.in/2012/02/can-cylinder-misfire-damage-catalytic.html

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Car misfires and error code P0306 and P0303 is shown?

http://technoanswers.blogspot.com/2012/01/car-misfires-and-error-code-p0306-and.html

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Chevrolet K1500 No spark, random misfire

http://repairhelpcenter.blogspot.com/2012/11/chevrolet-k1500-no-sparkrandom-misfire.html

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2004 Ford F150: Engine Misfire and Lean Code?

http://whatisbyme.blogspot.in/2012/05/2004-ford-f150-engine-misfire-and-lean.html

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1995 Toyota LS: Misfire at idle or low RPM

http://whatisbyme.blogspot.in/2012/04/1995-toyota-ls-misfire-at-idle-or-low.html

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Toyota Corolla: Random Misfire?

http://technoanswers.blogspot.in/2012/04/toyota-corolla-random-misfire.html

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2006 Ford Freestar: Misfire on Cylinder Number 2?

http://whatisbyme.blogspot.in/2012/06/2006-ford-freestar-misfire-on-cylinder.html

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This will help.

Thanks.

you replaced check the electrics but forgot the obvious
bad injector , connections and fuel pressure

Hi. The most common cause that I have found for an " only idle misfire" is low compression. The engine needs at least 100 psi of "dry " compression to idle smoothly in most cases and if any cylinders are higher or lower by more than 20 percent of any other cylinder, you will also have an idle misfire condition felt. Should you perform a compression test, make sure to disable the igntion system completely, not just move the ignition wires out of the way as you will still have spark and the fuel injectors will still inject fuel in nearby proximity to possible ignition source. Always perform two tests, "dry" and "wet" compression. "Dry" is warm engine with spark plugs removed. "Wet" is after performing "dry" test add a few drops of light oil into cylinders and perform compression test again to see if the number rise. If they rise, you have worn piston rings or cylinder walls. If they do not rise, you likely have worn valve seats, misadjusted valves, or head gasket issues, possibly cracks in the cylinder head. If you need to know how to compression test with a tester in detail, message me back and I can help. I'm assuming you may already know due to the fact you state you replaced all the parts likely by yourself so you probably have strong technical merit.

I would start with the inlet manifold gasket, these are know to break down causing vacuum leaks which can cause this issue, also check around engine for other vacuum leaks

Run a compression check , possible causes leaky valve seal, bad rings , bad headgasket

Did you ever find a solution to this? I am having the same issue on 2,4,6. I have replaced the coils. checked the intake for vacuum leaks. I am lost here let me know if you find anything and I will do the same.