Cause
Carbon-seized manifold tuning valve
The valve altering the intake manifold's geometry, seized with carbon so it cannot change position. The engine still runs well across half its rev range — whichever half matches the position it is stuck in — so the complaint is a flat spot rather than a fault, and it is usually blamed on fuelling.
Long intake paths favour low-speed torque and short ones high-speed breathing. Stuck long, the engine pulls strongly from idle and runs out of breath at the top; stuck short, it feels hollow low down and comes alive late. That asymmetry is the diagnostic signature and no fuelling fault produces it.
Carbon accumulates on the shaft and in its bore from crankcase and recirculation deposits, and it builds for the life of the engine. Check the effort needed to move the valve by hand with the actuator disconnected — a seized shaft with a healthy actuator and a free shaft with a failed actuator give the same code and want different repairs.
Typically a moderate job.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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