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DTC Misfire P0300-P0308 are all diagnostic codes indicating a misfire.
P0300 - Random/Multiple Cylinder
P0301 - Cylinder 1
if you find a cylinder-specific misfire code such as P0302 (indicating cylinder #2), check the spark plug, plug wire, coil (if it's a DIS or coil-on-plug system), fuel injector and compression to isolate the cause. On the other hand, if you find a random misfire code (P0300), the problem probably is not the ignition system. It's likely a lean fuel mixture caused by a vacuum leak or dirty injectors.
The misfire condition may be caused by one or more engine exhaust valves that are slow to close. Late closure of an exhaust valve may be the result of no valve rotation and associated build up of carbon on the exhaust valve stem.
2.Verify that an engine misfire condition is present. Use of the DRBIII(R) during a road test, or a Co-Pilot data recording, may help to determine engine misfire and misfire counts. If carbon deposit accumulation is severe, then a cylinder leak down test may detect one or more cylinders leaking greater than 15%. Save any misfire DTC Freeze Frame Data that was stored for later misfire correction verification.
3.Verify that the engine misfire condition is not caused by faulty engine mechanical or electrical components.
4.If the engine mechanical and electrical systems are operating properly perform the Repair Procedure
To fix carbon buildup:
4.0L Jeep Six: Cylinder #1 Misfire Trouble Code - Jeep® YJ Wrangler, TJ Wrangler and LJ Wrangler - 4WD Mechanix Magazine 'Tech and Travel' Forums:
Begin with a compression test, or better yet, a cylinder leak down test. Any internal combustion engine needs four things to be "right", I talk about these basics in each of my books. My mantra for basic engine condition:
1) Normal compression or cylinder seal, I prefer a leak down test for this reason.
Once you get past these concerns, any engine problems are either vacuum leaks, "tune" related or engine management (fuel/spark) issues.
Your TJ 4.0L engine uses an ignition distributor for the high tension spark delivery alongside an MPI fuel system. The distributor spark timing is fully controlled by the PCM (computer) and its sensor readings. The PCM also controls fuel flow through the injectors. A check light or 'MIL' with a consistent "#1 Cylinder Misfire" could be relative to either fuel flow or the spark at that cylinder.
Spark wise, this could be a weak spark lead, worn distributor cap and rotor, the spark plug itself, or engine troubles like low compression or a leaky valve. A misfire can also be fuel related. Poor fuel pressure/supply can be the issue. (See the Mopar EFI comments in the threads from the CJ forum: Mopar EFI Conversion starving for fuel.)
The injector for #1 cylinder can be clogged, too. Or, there may be an unrelated device to consider, like the throttle position sensor (TPS) or the oxygen sensor (upstream sensor). Sometimes, these other components will not send their own signal, yet they contribute to something like the #1 misfire. I don't recommend replacing a number of parts randomly, JJ, I'm just calling attention to related trouble spots...
After ruling out the overall engine condition, I'd check the fuel pressure at the injector rail. This should be around 49.2 PSI +/- 2 to 5 PSI. That test can be revealing. The #1 spark wire lead should also be tested for ohms-resistance, and spark to the #1 plug should appear consistent.
Spark plug and other ignition issues should be ruled out. The #1 injector is more likely to be consistently faulty—not likely to be intermittently faulty. This is still a possibility, though, so don't rule out a defective injector.
For additional troubleshooting guidelines, see these two articles at the magazine, covering tune and troubleshooting on Jeep MPI fuel-and-spark management like yours, JJ:
Moses Ludel's 4WD Mechanix Magazine - Jeep Multi-Point Injection: Operation and Troubleshooting
Moses Ludel's 4WD Mechanix Magazine - Jeep TBI & MPI Advanced Troubleshootingd
Let the forum community know what you turn up. I'm certain other forum members will benefit from this exchange, JJ, your questions are thoughtful!
I'm here for more discussion...