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The answer is leaking intake plenum gaskets. I am assuming you are getting lean codes also. When replacing the gaskets you also need to replace the bolts that hold the plenum down.

Vehicle Description: 2000 Ford Explorer 4.0L,

Customer Concern: The engine is stumbling and stalling and has a rough idle when the engine is cold. Trouble codes P0171 and P0174.

Tests/Procedures: 1. Monitor the Short Term Fuel Trim (SFT) reading, during cold engine operation after closed loop is achieved, to verify the engine is running lean. Nurse the engine fuel around the intake gaskets to see if SFT readings can be driven down. Normal SFT readings will indicate single digit numbers and have a tendency to stay close to 0% bouncing from a negative to a positive value.

2. If a leak is present, the SFT value will increase to positive double digit values on both banks at the same time. When the trim number is high, attempt to isolate the leak by nursing added fuel around the intake gasket surfaces and lines of the intake manifold. At any point, the SFT value severely decreases potentially to a negative reading the leak has been found.

Potential Causes: Leaking intake manifold gaskets

** Confirmed Fix **
Intake gasket replacement

Tech Tips: This engine typically leaks during cold engine operation only. Once the engine has reached normal operating temperature, the gaskets swell and the leak is gone. Ford program 00M12 was released to address the leaking gasket issue with a service kit that included a timing chain tensioner for the driver side head and increase warranty coverage for up to 6 years or 72,000 miles.

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