Hey guys,

I've been chasing down a misfire and am reaching out for help. It started out as two specific cylinders (4 & 6, if I remember correctly). I threw in a new ignition coil pack and now it is throwing a "random cylinder misfire code." The jeep runs smoothly, it just doesn't have the power it used to. (We all know it didn't have much power to begin with.) To add to the problem, the Jeep will start with no problem and on the first try if it has been sitting in the garage for a while. Every time I take it out, drive it around, turn it off (go grab something to eat/grab a part for another project/whatever) it won't start right up again. When I turn the key all of the accessories come on, I can hear the fuel pump running, but not even a click. Usually after an hour or so it will turn over again.

-I thought that it might have been a crankshaft sensor, but swapping a new one in made no difference.
-Sparkplugs are relatively new
-Plug wires are new
-Fuel pump is new

08 manual.

Any suggestions would be appreciated.

 

Anything that causes the crank to slow down when the computer expects to see it speed up from ignition, will cause a misfire code. It doesn't necessarily have to be from lack of ignition of the fuel in a cylinder.

Misfire is very difficult to diagnose. It has a long list of possible causes.

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]Possible Causes[/TD]
[/TR]
[TR]
[TD]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.

 

Thank you! I'll start working through the list tomorrow!