Jeep JK P0303 Cyl. 3 Misfire - JKowners.com : Jeep Wrangler JK Forum
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post #1 of 7 Old 04-20-2019, 07:48 AM Thread Starter
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Jeep JK P0303 Cyl. 3 Misfire

Whats up everyone?

Got 110K on the clock, the check engine light popped up and it was running pretty rough (more specifically @ idle). Took it to my mechanic, swapped out all Coil Packs & Plugs....still got the code and its still running rough. the mechanic things its internal but I am truly hoping its not for $$$ sake. He mentioned that he thought my best move would be to get a flat rate diagnostic test from a stealership. Any thoughts and opinions here?

Thanks

Kyle
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post #2 of 7 Old 04-20-2019, 08:10 AM
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You'll find me & our dear, departed brother - @RubiCajun - touting multiple times how easily the plug wires and especially those gray boots can get holes not visible to naked eye but enough to let the cylinder spark jump from it to engine block. I have even had brand new and multiple gray plug boots get pinholes when I was installing them new that kept the idke rough & that code thrown.

Easiest way to eliminate this possibility is wait til night , get the Jeep ( I assume it's a 3.8l ) in as dark , light free location as possible. Garage is best. Then , as she idles roughly, you just stand in utter darkness and focus your gaze on the engine bay. If this is occurring you'll quickly see the tell tell blue arc & have IDed your rough idle.







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post #3 of 7 Old 04-20-2019, 02:20 PM Thread Starter
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Quote:
Originally Posted by j3ff3ry_j33p View Post

You'll find me & our dear, departed brother - @RubiCajun - touting multiple times how easily the plug wires and especially those gray boots can get holes not visible to naked eye but enough to let the cylinder spark jump from it to engine block. I have even had brand new and multiple gray plug boots get pinholes when I was installing them new that kept the idke rough & that code thrown.

Easiest way to eliminate this possibility is wait til night , get the Jeep ( I assume it's a 3.8l ) in as dark , light free location as possible. Garage is best. Then , as she idles roughly, you just stand in utter darkness and focus your gaze on the engine bay. If this is occurring you'll quickly see the tell tell blue arc & have IDed your rough idle.






Its actually a 3.6l
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post #4 of 7 Old 04-20-2019, 06:13 PM
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ah.
disregard....


Now we have seen why it is a good idea to identify year of Jeep..


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post #5 of 7 Old 04-20-2019, 07:22 PM
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You need to find another mechanic with some diagnosing skills. Mis fire codes although at times can be a bitch to diagnose but you have to start with the basics. Spark, fuel, compression, sounds like he gave up after spark and he probably still doesn't know if spark is correct. There are just to many questions to ask you about what he did so I'd suggest taking it to another shop.

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post #6 of 7 Old 04-20-2019, 08:34 PM
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When a misfire code comes up, people immediately think it means the cylinder didn't fire. That's not always the case, though. Anything that affects the speed of the crankshaft can cause a misfire DTC.

Below is a list of things to check before parts are thrown at it, according to the factory service manual.

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.

Possible Causes

ASD OUTPUT CIRCUIT
INJECTOR CONTROL CIRCUIT
IGNITION WIRE
SPARK PLUG
IGNITION COIL
FUEL PUMP INLET STRAINER PLUGGED
RESTRICTED FUEL SUPPLY LINE
FUEL PUMP MODULE
FUEL PRESSURE LEAK DOWN
FUEL INJECTOR
ENGINE MECHANICAL
POWERTRAIN CONTROL MODULE (PCM)

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.
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post #7 of 7 Old 04-21-2019, 11:19 AM Thread Starter
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WOW!! Thanks for the responses guys. The dealer has a diagnosis flat fee that I am gonna try to get down to the bottom of it. Ill be sure to bring this in when i go next week
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