Toyota-Lexus-Subaru-Honda Valve Adjust

The engine’s valves allow the engine to breathe in fresh air mixed with gasoline, and breathe out exhaust gasses after combustion. The camshaft opens the valves.

When the valves are closed, the engine requires a certain amount of clearance between camshaft and the next item in the valve train—be it a rocker arm that engages with the valve, or a shim in a bucket that engages directly with the top of the valve.

While the valve is closed, it dissipates heat into the head and on out into the coolant.

As the valve train wears, it can wear in a manner that causes the valve to sink into the head, in which case the valve train loses it’s clearance and is said to be getting tighter.

As a result of this, the valves have less opportunity each cycle to dissipate heat, which causes them to run hotter and accelerates their wear and the tendency to sink further into the head. If left unadjusted, eventually tight valves burn, which results in a severe loss of compression and power in that cylinder. There is no increase of noise or tapping as the valves are getting tighter. As they get tighter, but are at the last stages just prior to burning, there can be some loss of compression detected accompanied by a somewhat rougher idle.

The valve train can also wear in the opposite direction, where small amounts of metal are worn away from the surface of the camshaft, from the end of the valve stem, and either the rocker arm or the valve adjusting shim and bucket. When this is the case, clearances are getting larger, and the valve is said to be loose. If they get loose enough, it shows itself by noisy valves that tap louder than usual. As the valve train wears in this manner, the cam lobe tends to first engage with the rest of the valve train further and further up the ramp, which results in an increasingly harsh engagement and accelerates the valve train wear.

Both of these conditions are best addressed by routine valve adjusts scheduled on the basis of miles driven.