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Moses, I've read through the 4.0L Jeep Six: Cylinder #1 Misfire Trouble Code discussion (and a couple of others related to it) and clearly you know what you are talking about.  For this issue, my jeep went into a mechanic (I'm good with horses, not vehicles) and the following has been done.  Full tune up including new sparks and wires, new bank of injectors, new coil.  None of that fixed the problem.  As far as symptoms go, my Jeep is a 1999 Wrangler TJ with 180,000 miles and cranks relatively easily and will idle a bit rough.  There is a miss you can feel that is intermittent and has never been "bad" although seems to be getting worse lately.  No burning oil or smoking and when spark for cylinder one is pulled it is dry, so no oil backing in. The only other symptom I feel is that my Jeep seems a bit "under powered" at times.  What would you suggest as the next thing to check for my mechanic?  I'm hoping to stop buying parts for trial and error--the old retired and fixed income stuff has a hold on me. :)

Ace

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4 hours ago, Ace Ford said:

Moses, I've read through the 4.0L Jeep Six: Cylinder #1 Misfire Trouble Code discussion (and a couple of others related to it) and clearly you know what you are talking about.  For this issue, my jeep went into a mechanic (I'm good with horses, not vehicles) and the following has been done.  Full tune up including new sparks and wires, new bank of injectors, new coil.  None of that fixed the problem. 

This would be a major tune-up, so nothing lost.  Even the injectors are not a "waste" at this mileage although I take the time to clean and flow test injectors these days before arbitrarily tossing them.  It's done now...

As far as symptoms go, my Jeep is a 1999 Wrangler TJ with 180,000 miles and cranks relatively easily and will idle a bit rough.  There is a miss you can feel that is intermittent and has never been "bad" although seems to be getting worse lately.  No burning oil or smoking and when spark for cylinder one is pulled it is dry, so no oil backing in. The only other symptom I feel is that my Jeep seems a bit "under powered" at times.  What would you suggest as the next thing to check for my mechanic?  I'm hoping to stop buying parts for trial and error--the old retired and fixed income stuff has a hold on me. :)

You describe the symptoms thoroughly.  At the mileage, I would run a cylinder leakdown test to see how the cylinders seal.  A cranking compression test is general and helpful if there is a big difference between cylinders, but a leak down test is much more conclusive.  You want to rule out cylinder imbalance or a cylinder that is simply not pulling its load.  

If cranking pressure is low and you have a misfire, it could be valves with carbon or a valve that is not seating properly.  Here, again, the leak down test (done properly) would pinpoint loss of seal at the rings, valves, head gasket seepage, etc.

Start here, as overall engine condition precedes any other tuning measures.  I would suggest that a fuel trim test is next if your shop has a scan tool.  Fuel trim can be helpful in diagnosing issues like the oxygen sensor being faulty.  In fact, the oxygen sensor would be on my radar screen along with the possibility of a clogged catalytic converter. 

The throttle position sensor is a perishable item that could be off in voltage but not yet throwing a code.  A scan tool will indicate the TPS voltage and any roughness or faults.  At this mileage, the TPS and also the idle air control valve would be two items to consider.  Neither is expensive, I replaced them both recently on our '99 4.0L Cherokee (essentially your engine) with quality parts at Amazon.  This is the second TPS and first IAC valve in 191K miles.  I also did the O2 sensor to eliminate a stumble/stall when the cold engine came off open loop and went into closed loop.

Is the P301 code still appearing after all the work that was done?  This is a vague and nebulous code on these 4.0L engines.

Ace

Ace Ford...I've replied within your question (above) using red to highlight.  Let me know what you discover after following through.

Moses

 

 

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  • Moses Ludel changed the title to Jeep 4.0L P0301 Misfire Code

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