Cylender 3 misfire down hills

[Erney1]

Ok, I've done the searches and tried the tab fix of insulation on cyl 3, changed plugs etc. My issue is the dreaded cylender 3 misfire on a 2004 tj 4.0 with coil rail. It's always cyl 3 and only happens on a warm engine while coasting down hill and no skinny pedal input. It goes into a blinking hard mode and if not corrected into the vehicle limp mode. I dont detect any harsh idle, etc. It dose not give the classic turn off warm then hard restart before cooling down, it only happens when coasting down the hill. I've seen this mentioned but never saw a fix. If it goes into limp mode, I shut down, clear the code and restart with no issues.

Thank you
2004 rubicon, automatic

 

[CharlesHS]

Ok, I've done the searches and tried the tab fix of insulation on cyl 3, changed plugs etc. My issue is the dreaded cylender 3 misfire on a 2004 tj 4.0 with coil rail. It's always cyl 3 and only happens on a warm engine while coasting down hill and no skinny pedal input. It goes into a blinking hard mode and if not corrected into the vehicle limp mode. I dont detect any harsh idle, etc. It dose not give the classic turn off warm then hard restart before cooling down, it only happens when coasting down the hill. I've seen this mentioned but never saw a fix. If it goes into limp mode, I shut down, clear the code and restart with no issues.

Thank you
2004 rubicon, automatic

Recently I had a multiple cylinder (not all cylinders) misfire problem that was progressively getting worse with the occasional blinking CEL.
I had a long time Jeep Technician connect a Factory Diagnostic Tool to read cylinder misfires. Ran the engine at 1800 rpms for 200 seconds and there were many multiple cylinder misfires.
Two things cured the problem; first was to replace the sparkplugs with the factory recommended NGKs and second was combustion chamber cleaner. Sprayed out the throttlebody with Mopar Combustion Chamber Cleaner and then sprayed a lot of cleaner into the TB, shut the engine off and let soak for 10 minutes. Upon restarting there was quite a plume of smoke that lasted for almost a minute. Reconnected the tool to read the cylinder misfires; the problem was gone.

 

[Erney1]

Thank you, I will definitely try this. I'm not sure it's the problem as it's only cylender 3 and only idling down a longer hill when warm, but it's worth a try. I've put new plugs (the recommended auto lite xp 985 iridium) in and swapped #3 plug with #1 and still have the cylender 3 misfire on downhill idling.

Any thoughts on why this is isolated to #3 and only at idle down long grade? Im guessing, could be very wrong, since I swapped plug locations and its isolated to cyl 3 it rules out coil pack, plugs, vaccuum leaks? Since not hot start issues then not thinking the heat soak problem? Possibly the injector, but it only happens at idle down long grade? Im stumped. Will try throttle body clean as suggested tomorrow.

 

[CharlesHS]

There are a few reason why the misfire could be isolated to cylinder 3.
Spark could be grounding thru the rubber tube for the coil that slips over the spark plug?
Possibly an intermittent problem with the valves for cylinder 3.
Small intake manifold leak for cylinder 3.