The Wikipedia entry is hard to grok:

Valve float is an adverse condition which can occur at high engine speeds when the poppet valves in an internal combustion engine valvetrain do not properly follow the closure phase of the cam lobe profile. This reduces engine efficiency and performance and potentially increases engine emissions. There is also a significant risk of severe engine damage due to valve spring damage and/or pistons contacting the valves.

Can someone translate this into something more understandable?

Diagrams, videos and animations are a welcome bonus.

  • Nobody has said a thing about harmonics ... I may add something more elaborate later. Commented Dec 30, 2015 at 18:50

3 Answers 3


Valve float means the valve doesn't fully close at the proper time, because the return mechanism (usually a coil spring) isn't strong enough to close it. This usually happens at high RPM.

It can damage an interference engine if the valves make contact with the top of the piston. This can bend the valve stem, or chip the edge of the valve.

In addition, while in valve float, crossflow can result in backfires and/or afterburning, and possibly unburnt fuel getting to the catalytic converter.

  • 2
    Welcome to the mechanics stack exchange site! Cheers. Commented Dec 30, 2015 at 16:40

Most valves are opened by the cam lobes, and closed by springs. However, it takes time for the spring to 'unwind' and push the valve back. If the engine is going fast enough, this time is longer than the time it takes for the cam lobe to rotate from 'fully open' to 'fully closed', so the valve doesn't follow the cam - a gap appears momentarily.

This means that the valve stays open slightly longer than it should do - as it simply cannot close fast enough. It also means that the valve is 'loose' for a moment (i.e. not pressed up against the lobe by the spring), allowing it to rotate and move around more freely than it should - increasing wear.

There is a youtube video here - beware, flashing images.


Valve float is bad because the valve gets a running start at the valve seat instead of being gently lowered on to the valve seat by the cam lobe. It is this sudden crashing into the valve seat that stresses the valve and leads to damage.

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