There are several common driveability symptoms which can be caused by intake valve deposits; stumble, hesitation and loss of power under load. Stumble and hesitation, especially when the engine is cold, are by far the most common problems caused by excessive intake valve deposits. The porous carbon deposits act like a sponge, absorbing enough fuel vapor to cause these symptoms.
Severe carbon deposits can also cause a loss of power at high engine rpm. When deposits accumulate sufficiently to restrict airflow through the intake valve, the volumetric efficiency of the engine is effected, causing the engine to loose power.
The best way to confirm excessive deposits is to visually inspect the valves using a borescope. If repairs are necessary, equipment is available to clean the valves without removing the cylinder head. Refer to Toyota Technical Service Bulletins for more information on procedures and special service tools.
The fuel delivery system is responsible for maintaining constant fuel pressure across the injectors, in addition to metering the correct amount of fuel into the intake manifold.
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