The engine crankshaft speed at idle speed sometimes decreases, sometimes increases (the engine management system malfunction indicator light may come on, see above)
1. Leaky inlet manifold. Check and replace intake manifold seals and faulty parts (see "Power supply system - check").
2. The forced crankcase ventilation system is faulty. Check emission control systems, replace faulty parts.
3. The absolute pressure and air temperature sensor is faulty. Perform diagnostics of the engine management system (see "Engine management system"). Replace the faulty element.
4. Insufficient pressure in the fuel rail. Check the pressure in the fuel supply system (see "Power supply system - technical condition check"), replace faulty parts.
5. Wear of the timing belt and/or belt tension roller. Check the technical condition of the engine (see "Engine - technical condition check") and replace faulty parts.
Misfires (engine misfires) at idle and/or under load (the engine management system malfunction indicator light may come on, see above)
1. The spark plugs are faulty. Replace spark plugs (see "Spark plugs - replacement").
2. Leaky inlet manifold. Replace intake manifold seals and faulty components (see "Power supply system - technical condition check").
3. The fuel filter is clogged. Replace the fuel module (see "Fuel module - removal and installation").
4. The fuel injectors are faulty. Check and replace faulty fuel injectors (see "Injectors - checking and replacement").
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5. Insufficient compression in the engine cylinders. Check the technical condition of the engine (see "Engine - technical condition check"). Repair the engine by replacing worn and faulty parts.
6. The ignition coil is faulty. Check and replace the ignition coil (see "Ignition Coil - Checking and Replacing").