Vehicle engines are precision machinery; the movement of many parts must be synchronised carefully to make the engine perform properly. Intake and exhaust valves are critical components for optimum engine performance. These valves contain mechanisms that must be adjusted correctly to allow for changes in temperature and compensate for material wear. Running a vehicle with incorrectly adjusted intake and exhaust valves can have a detrimental effect on your vehicle.
All of the intake and exhaust valves in an engine must open and close at correct intervals to allow the engine to run smoothly. These valves have a small amount of clearance between the valve itself and the mechanism that activates the valve. This clearance is called "lash." If the valve lash is set incorrectly (too much clearance, or not enough), the engine may respond by running rough at idle, particularly while warming up.
Intake and exhaust valves that are not adjusted to open and close at the proper times degrade an engine's ability to make maximum power. Intake valves control when and for how long fuel is allowed into the combustion chamber, and must be synchronised with the speed of the pistons to allow the maximum amount of mixture into the engine. Exhaust valves perform a similar function, except their purpose is to allow burnt gases to leave the engine. If the valves aren't adjusted correctly, the engine will not burn fuel at maximum efficiency. Power and mileage then dramatically decrease.
The most serious result of incorrect valve lash adjustment is damage to the valves and related components. Setting the clearances loosely causes parts of the valve mechanism to hammer together, damaging valves and creating a knocking or rattling sound. Setting the clearances too tight can prevent valves from completely closing (or not closing for enough time), which may cause extreme heat damage and complete valve failure. Always keep your engine valves adjusted according to the manufacturer's specifications.