Q: How do I lean down the fuel

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The truck is overfueling and fouling plugs. Runs good under full throttle, but sputters when up to speed.

My car has 200000 miles.
My car has a manual transmission.

This may be related to many different things such as faulty or leaking fuel injectors causing the cylinders to be loaded with more fuel than what they can burn efficiently. This may also be related to a faulty mass air flow sensor or faulty O2 sensor. As you may know, the O2 sensor reads exhaust gases to determine how much oxygen is present. Based upon this, the computer makes adjustments to fueling of the motor. The mass air flow sensor reads the air intake as it is mixed with fuel prior to entering the motor. This information is relayed to the computer also which is then used to determine whether to add or subtract fuel from the intake. When either of these are not working properly, this may cause an over fueling situation as you have experienced. I would recommend having an expert from YourMechanic come to your location to diagnose your O2 sensors and mass air flow sensor.

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