В сервис-мануале, глава 9-1, прям в столибик перечислены возможные причины:
INCORRECT ENGINE IDLING SPEED
Engine
1.
Cylinder(s) and cylinder head(s)
• Incorrect valve clearance
• Damaged valve train components
2.
Air filter
• Clogged air filter element
Fuel system
1.
Throttle body (-ies)
• Damaged or loose throttle body joint
• Improperly synchronized throttle bodies
• Improper throttle cable free play
• Flooded throttle body
• Faulty air induction system
Electrical system
1.
Battery
• Discharged battery
• Faulty battery
2.
Spark plug(s)
• Incorrect spark plug gap
• Incorrect spark plug heat range
• Fouled spark plug
• Worn or damaged electrode
• Worn or damaged insulator
3.
Ignition coil(s)
• Broken or shorted primary or secondary
coils
• Cracked or broken ignition coil
4.
Ignition system
• Faulty ECU
• Crankshaft position sensor