1. Leaky vacuum hose connections (check the tightness of the fit on the fittings).
2. Clogged air filter (see «Air filter - filter element replacement»).
3. Insufficient fuel pressure in the engine power system (see «Checking the technical condition of the power system»).
4. Damaged cylinder head gasket (see «Cylinder head - removal and installation» or «1.6i engine cylinder head - removal and installation»).
5. Wear on the timing belt and/or idler pulley (see «Timing Belt - Checking Condition» or «1.6i engine timing belt - checking condition and adjusting tension»).
6. Camshaft lobes wear (see «Camshafts - removal and installation» or «Engine camshaft 1.6i - removal and installation»).