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Q: need help with code#4 misfire&throttle positioner switch codes and poor running on 2002 Nissan Xterra

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I have replaced spark plugs,wires,cap,rotor,fuel&air filters.My friend purchased vehicle last week.It had #4 misfire code & knock sensor code.I replaced knock sensor and gaskets found no cracked vac lines but she still runs poorly @idle.I added Lucas fuel treatment and dry gas with 10 gallons 93 octane.I did a cylinder compression test on 4 it was 125 lbs.Is there any problems with wiring harness or injector problems?

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Engine size?? Also 125 psi aint a bunch but should fire the cylinder. Other comp. readings? Smooth out with slight throttle?
3.3 i replaced the throttle postion switch also i didnt check comp on the other cyl it does smooth out with throttle do i need to change injector?
Compression readings for other cylinders would be very helpful. Or at least 2 & 6. The other two, front and back, of the one you checked.
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code be $4 ign or inj circuit is bad. rec your seek help from your mech as to not waste time and money
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