 Worn valve guides and/or defective oil seals can often be diagnosed by a puff of blue smoke from the exhaust pipe on releasing the accelerator pedal after allowing the engine to overrun, when there is high manifold vacuum.Such a condition occurs when changing gear.In multi-valve engines, the conventional two-valves-per-cylinder setup is complemented by a minimum of an extra intake valve (three-valve cylinder head) or, more commonly, with an extra intake and an extra exhaust valve (four-valve cylinder head), the latter meaning higher RPM are, theoretically, achievable.