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Discussion Starter #1
Folks, very close to done on my troubling rebuild of my son?s 08 Wrangler X 3.8L engine. One remaining P0300 code which I?ve isolated to the #5 cylinder. Compression check good at 160psi, wire tests with the same 8 olds as the rest, plug was black yet worked fine when swapped with #4. Spark is strong when wire pulled from idling engine. All this points to a fuel injector. A visual inspection showed injector plugged in properly. I suppose a noid check would verify impulse circuit health. Quite difficult to access the wire plug with this engine. Any recommendations or thoughts?Is there something i?ve missed?
 

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You are getting a P0300 code (multiple cylinder misfire), but narrowed it down to a single cylinder?

You can't really say "All this points to a fuel injector."

Anything that affects the speed of the crankshaft can cause a misfire code. The list of things to check is long.

When a misfire is detected for a particular cylinder, the PCM will shut down that cylinder's fuel injector circuit.

Here is some info regarding any of the six misfire codes:

Possible Causes:

ASD RELAY OUTPUT CIRCUIT

INJECTOR CONTROL CIRCUIT

COIL 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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Discussion Starter #3
Thank you for your detailed list Ronjenx i?ve Got a lot to check tomorrow. Given that this engine was out of the vehicle and completely disassembled there are many possibilities, especially a loose connection. Listening to the exhaust tailpipe at idle it did sound sputtery like more than one cylinder was Misfirepp. The engine reved to near 4K rpm during a test drive today yet as one could imagin performance suffered. At one point today, it returned a P0117. Idle after about 10 miles on the road was smoother yet still stumbling.
 

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I had the multiple misfire code recently, after replacing an injector & installing new plug leads. Turned out, that despite running them carefully & checking twice, (or was it 3 times?), I still managed to cross over 1 & 3...

So, I'd say the first thing to do is recheck, again, that the leads are right. Just in case. It's a lot less work than pulling an injector!
 
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