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Weston York

Weston ... 6/20/2022

2003 Lincoln Navigator Base 8 Cyl 5.40L


Besides coils/plugs and vac leak, what causes misfire and "too lean" codes??

For months I've had random misfire problems and engine too lean codes. I changed coils and plugs and it was better for a day before engine light came back on. Rough idle, low power, mpg loss, light will flash when trying to accelerate at higher speeds. Changed PCV valve and hoses after mechanic found small vac leak. STILL same symptoms persist. Mechanic supposedly checked MAF sensor and other easily accessible potential suspects. Idk what to do. Is it wiring to coils? Fuel injectors? Fuel filter? 2 mechanics so far have tried and failed to find solution.
2003 Navigator 2WD 160,000m 5.4L

1 Answer


Jimm 6/20/2022

There are a variety of misfire reasons and conditions which may contribute.

A fault code P0300 (random misfire) may mean that one or more of the following has happened:
Faulty spark plugs or wires
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector(s)
Burned exhaust valve
Faulty catalytic converter(s)
Stuck/blocked/leaking EGR valve / passages
Faulty camshaft position sensor
Defective computer

Be sure to take the time to thoroughly test and diagnose before replacing any parts.


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