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Discussion Starter #1
I have a relatively new (to me) Jeep Grand Cherokee with the 4.0 engine.

I'm getting a sporadic check engine light (although it's getting to be more common) and what seems to be a misfire while idling. I don't seem to have any misfire while above 1,100 RPM or so.

If I remember right, the code is P0301 (I don't have it written down), but what the code says is Cylinder 1 misfire.

What could be the cause?

Here's what I've ruled out:

Dist. Rotor (New)
Dist. Cap (New)
Plug wires (New and swapped wires between Cylinder 1 and 3)
Spark Plugs (New, swapped plugs between 1 and 4)
Fuel Injector (swapped between 1 and 2)
Burnt Valve (Cylinders 1, 2, and 3 all pressure check to 170 PSI)
Lack of fuel (Pressure tested the check valve on the rail and came back at 47 PSI, which is within the +/- 5 of 49.2)
Fuel injector connector which plugs into the top of the injector (checked via voltmeter, and also there is a huge difference when revving the engine with it in vs it out... no big difference at idle though)
Crankshaft position sensor (Replaced)

Here's what's 99% ruled out:
Leak in the intake manifold affecting one cylinder. Sprayed carb cleaner all around with no effect.
(PCM) Computer (again, see above, the fuel injector connector is registering on the voltmeter)

I could use some help here. I haven't shaved in three days!!! :confused:
 

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Discussion Starter #2
Turns out I got what quite possibly is the worst tank of gas in the history of mankind... No idea why only one cylinder misfired though... but I ran all of the gas through with injector / fuel cleaner and filled it back up with 91 and the problems seem to be going away.
 
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