On some engines, a cylinder relief (safety valve) is provided for each cylinder. The valve opens when the cylinder pressure exceeds a safe operating limit. The valve opens or closes a passage leading from the combustion chamber to the outside of the cylinder. The valve face is held against the valve seat by spring pressure. Tension on the spring is varied by an adjusting nut, which is locked when the desired setting is attained. The desired setting varies with the type of engine and may be found in the manufacturers technical manual. Cylinder relief valves should be set at the specified lifting pressure. Continual lifting (popping) of the valves indicates excessive cylinder pressure or malfunction of the valves, either of which should be corrected immediately. Repeated lifting of a relief valve indicates that the engine is being overloaded, the load is being applied improperly, or the engine is too cold. Also, repeated lifting may indicate that the valve spring has become weakened, ignition or fuel injection is occurring too early, the injector is sticking and leaking, too much fuel is being supplied, or, in air injection engines, the spray valve air pressure is too high. When frequent popping occurs, stop the engine and determine and remedy the cause of the trouble. In an emergency, cut off the fuel supply in the affected cylinder. NEVER lock relief valves closed, except in an emergency. When you must take emergency measures, be sure to repair or replace the valves, as necessary, as soon as possible. When excessive fuel is the cause of frequent safety valve lifting, the trouble may be due to the improper functioning of a high-pressure injection pump, a leaky nozzle or spray valve, or a loose fuel cam (if adjustable). In some systems, such as the common rail, the fuel pressure may be too high.

A safety valve that is not operating properly should be removed, disassembled, cleaned, and inspected. Check the valve and valve seat for pitting and excessive wear and the vale spring for possible defective conditions. When you remove a safety valve for any reason, you must reset the spring tension. This procedure varies to some extent, depending on the valve construction.

Except in emergencies, it is advisable to shut the engine down when troubles cause safety valve popping. Clogged or partially obstructed exhaust ports may also cause the cylinder safety valve to lift. This condition will not occur often if proper planned maintenance procedures are followed. If it does occur, the resulting increase in cylinder pressure may be enough to cause safety valve popping. Clogged exhaust ports will also cause overheating of the engine, high exhaust temperatures, and sluggish engine operation.

You can prevent clogged cylinder ports by removing carbon deposits at prescribed intervals. Some engine manufacturers make special tools for port cleaning. Round wire brushes of the proper size are satisfactory for this work You must be careful in cleaning cylinder ports to prevent carbon from entering the cylinder-bar the engine to such a position that the piston blocks the port.