What does a dirty throttle body do to an engine?
At idle, the throttle plate is almost closed and the engine breathes through a gap of a fraction of a millimetre. A thin layer of carbon on the plate and bore is enough to change that airflow — and the engine management has to compensate until it runs out of range.
Where the dirt comes from
Crankcase ventilation (PCV) returns oil mist and blow-by gases into the intake, upstream of the throttle on most engines. On engines with exhaust gas recirculation, soot joins it. The mixture bakes into a sticky, dark deposit on the throttle plate edge and in the bore. Direct-injection engines are more affected, because no fuel ever washes the intake side.
What it does
The deposit narrows the gap around the closed plate, so less air passes at a given throttle angle. The ECU learns to open the throttle slightly more to hold idle speed — the idle air adaptation. When the deposit grows beyond the adaptation range, symptoms appear: unstable or hunting idle, a dip or stall when the AC compressor engages or when coming to a stop, hesitation at tip-in, and fault codes for idle speed control or throttle/airflow plausibility. A sticky plate can also make low-speed driving jerky.
Cleaning it correctly
Modern throttles are electronic (drive-by-wire): a motor and position sensors control the plate. They should be cleaned with the ignition off and preferably the connector unplugged; the plate must never be forced open by hand against the motor, which can damage the gears or the sensor. Some bores have a factory coating that aggressive solvents or abrasives remove — so the method is a suitable solvent, a soft cloth, and patience.
After cleaning, the ECU still holds the old adaptation and will over-open the throttle: idle may run high. A throttle and idle adaptation reset with a diagnostic tool, followed by the relearn procedure, brings it back to base values.
When it is not the throttle
The same symptoms can come from vacuum leaks, a failing PCV valve, dirty or faulty airflow sensors, ignition or injector problems. Cleaning the throttle is a maintenance job, not a diagnosis. If symptoms return quickly, the source of the oil vapour (often the PCV system) is the real issue.