The carbon can build up on the intake valves and cause misfires.
The carbon lowers the octane level of the gas and also partly burns some of the fuel.
How I do a valve cleaning:
Buy engine de-grease for the outside of the engine
start engine when it is cold.
remove the pcv hose
spray into the throttle body for 30-60 seconds or stop when the engine chokes
turn engine off
wait 30 mins
start engine, white smoke will appear
have oil change since the cleaner will thin the oil
I use Lucas Upper Cylinder Lubricant to keep the valves clean for the type of engine below.
SOURCE: nissan v6 idle
looks like that either you do have an air mass not correct working,or the throttle body is too dirty and you have to clean it.Also check for any cracked, snapped vacuum hose.
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