In car engines, an engine valve that is said to have dropped refers to a valve in the cylinder head of the engine that has usually broken off at the stem that connects the valve to the rod. After the valve breaks off, the valve seat usually rattles around inside the cylinder and can cause significant and expensive damage to the cylinder head of the vehicle.
Fixing a Dropped Valve
If your vehicle experiences a dropped valve, you'll need to have the vehicle repaired as soon as possible. Driving a vehicle with a dropped valve even for a very short period of time can destroy the inside of a cylinder and do other damage to the cylinder head as well. In most cases, the heads of the engine will need to be machined or replaced. In some cases, a dropped valve may result in a total rebuild of the engine if the piston rods are damaged.
You won't be able to tell the extent of damage caused by a dropped valve, until you remove the cylinder heads from the car engine. Once the cylinder heads are removed, it is always a good idea to repair any damage in the cylinder heads and machine or rebuild the engine if necessary. When changing out one valve, it is usually prudent to change the other valves in the other cylinders as well as they should be kept his uniform as possible to insure proper combustion in your car's engine. Unless you are qualified mechanic, a dropped valve is usually not a good do it yourself repair project.