Joeysjeep

So I recently got my jeep back up and running after the head gasket blew. I ended up replacing the drivers side head and the shop said the passenger side was fine. Had 3 cracks in the drivers side head. Now I have a misfire in the #3 cylinder which was the cylinder that the gasket blew on. I have replaced the plugs and the wires and coil pack. But still does it not as much since the coil was replaced but still does. Also the #3 cylinder was down on compression from the others but still in spec cause the coolant cleaned that cylinder wall real well. So my question to you is what all else do I have ignition wise to replace to try and clear up this misfire. Thanks.

ronjenx

You have to be careful with the word "misfire". With the JK, it doesn't always mean the ignition system is failing to fire that cylinder.

The misfire code is thrown when the computer sees a slight slowing of the crankshaft for any reason.

Here is the troubleshooting chart for the misfire codes.

P0300-P0306 CYLINDER MISFIRE
· When Monitored:
Any time the engine is running, and the adaptive numerator has been successfully updated.
· Set Condition:
The threshold to set the fault is application specific; it is tied to the level of misfire that will cause emissions to increase to 1.5 times the standard or in some cases 1%. It is always a two trip fault above the calibrated RPM. It takes 1 soft fail to set a malfunction and two trips to set the MIL. Three good trips to turn off the MIL.
[TABLE="class: MsoNormalTable"]
[TR]
[TD] [/TD]
[/TR]
[TR]
[TD="width: 452"] ASD RELAY OUTPUT CIRCUIT

[/TD]
[/TR]
[TR]
[TD="width: 452"] INJECTOR CONTROL CIRCUIT

[/TD]
[/TR]
[TR]
[TD="width: 452"] COIL CONTROL CIRCUIT

[/TD]
[/TR]
[TR]
[TD="width: 452"] IGNITION WIRE

[/TD]
[/TR]
[TR]
[TD="width: 452"] SPARK PLUG

[/TD]
[/TR]
[TR]
[TD="width: 452"] IGNITION COIL

[/TD]
[/TR]
[TR]
[TD="width: 452"] FUEL PUMP INLET STRAINER PLUGGED

[/TD]
[/TR]
[TR]
[TD="width: 452"] RESTRICTED FUEL SUPPLY LINE

[/TD]
[/TR]
[TR]
[TD="width: 452"] FUEL PUMP MODULE

[/TD]
[/TR]
[TR]
[TD="width: 452"] FUEL PRESSURE LEAK DOWN

[/TD]
[/TR]
[TR]
[TD="width: 452"] FUEL INJECTOR

[/TD]
[/TR]
[TR]
[TD="width: 452"] ENGINE MECHANICAL

[/TD]
[/TR]
[TR]
[TD="width: 452"] POWERTRAIN CONTROL MODULE (PCM)

[/TD]
[/TR]
[/TABLE]

VISUAL INSPECTION
NOTE: Anything that affects the speed of the crankshaft can cause a misfire DTC.
NOTE: When a Misfire is detected for a particular cylinder, the PCM will shut down that cylinder's Injector Control circuit.
Turn the ignition off.
- Visually inspect the engine for any of the following conditions.
- Worn serpentine belt
- Binding Engine-Driven accessories: A/C Compressor, P/S Pump, Water pump.
- Misalignment of the Water pump, P/S Pump and A/C Compressor pulleys
- Corroded PCM power and ground circuits.
- Improper CKP, CMP, MAP, and TP Sensor mounting
- Poor connector/terminal to component connection. i.e., CKP sensor, Fuel Injector, Ignition coil, etc.
- Vacuum leaks
- Restricted Air Induction system or Exhaust system.
- Internal engine component failures.



SPARK PLUG
Turn the ignition off.
Remove the Spark Plug.
Inspect the Spark Plug for the following conditions.
- Cracks
- Carbon Tracking
- Foreign Material
- Gap size out of specifications
- Loose or broken electrode
NOTE: Lightly tap the bottom of the spark plug on a solid surface. The electrode in the spark plug should not move.

ENGINE MECHANICAL
Check for any of the following conditions/mechanical problems.
ENGINE VACUUM - must be at least 13 inches in neutral
ENGINE VALVE TIMING - must be within specifications
ENGINE COMPRESSION - must be within specifications
ENGINE EXHAUST SYSTEM - must be free of any restrictions or leaks.
ENGINE PCV SYSTEM - must flow freely
TORQUE CONVERTER STALL SPEED - must be within specifications
POWER BRAKE BOOSTER - no internal vacuum leaks
FUEL - must be free of contamination
CAM LOBES - must not be worn excessively
CYLINDER LEAKAGE TEST - must be within specifications
VALVE SPRINGS - cannot be weak or broken

POWERTRAIN CONTROL MODULE (PCM)
Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the related Sensor, Coil, Injector, and the Powertrain Control Module (PCM).
Look for any chafed, pierced, pinched, or partially broken wires.
Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact in the related Sensors, Coils, Injectors, and the Powertrain Control Module connectors.
Refer to any Technical Service Bulletins that may apply.

Damien Ristaino

I Know this is an old thread but I don't see the need to start a whole new one. I have been chasing P0303 and was wondering what else I can do.
New wires, new spark plug in cylinder 3, tested coil for cylinder 3 (good string spark) , swapped fuel injectors from the adjacent cylinder to see if the code follows. No change ..code still exists. No other codes, seems to run fine while driving and freeze data shows at idle.
On the 3.8 the 3rd cylinder is on the passenger side middle. I know because the coil is labeled and I followed the spark wire to the plug. That is the cylinder I'm working with. My next is to replace all the injector O rings. The block side O ring seemed like they needed it. I don't have a vacuum leak or it might be P0300. Air filter is good.

What else?

Mr.T

I'd try replacing the #6 plug and wire, it's a wasted-spark ignition and #3 and #6 fire at the same time. The ignition coil pack is the very easy to replace, but I'd avoid most aftermarket ones.

EHarris

Agree with Mr. T. I'd replace it all meaning all new wires, plugs and coil pack. I run Double Platinum Champion plugs, I went with the NKG wires as they are half the price and I have had zero issues. Rock and Quadratec both have the Coil Pack for around $75. Your total investment is $120. If it doesn't solve your issue, I promise you will notice a performance gain by replacing the system. I definitely did. My money is on the coil pack being bad.