What Are the Causes of a Sticky Hydraulic Valve Lifter?
Tapping noises coming from an engine may be an indication of sticky hydraulic valve lifters. Engine valve-train components include the camshaft, hydraulic lifters, rocker arm assemblies, valves and valve springs. Overhead valve engines also have push rods that ride on top of the lifters. Defects in any one of these components may cause tapping noises, but sticking hydraulic lifters will cause definite tapping or knocking noise and will affect engine performance.
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Lifter Function
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Hydraulic valve lifters ride on the camshaft lobes and transfer the rotating motion of the camshaft to the push rods. Push rods move the rocker arms up, opening each valve in the correct order. Unlike solid lifters, hydraulic lifters are able to maintain zero lash in the valve-train, providing for quieter engine operation and smoother engine performance. Each hydraulic lifter has a metering orifice and a check valve that controls oil flow through the lifter. Changing oil levels in each lifter provides a means of adjusting to variations in engine operation.
Varnish and Sludge
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As engines go through thousands of heating cycles, varnish and sludge will build up on internal moving parts. Varnish and sludge can coat the internal parts of hydraulic lifters limiting the ability of the lifter to accommodate the operational demands placed on the engine. If the metering valve is stuck in the open or closed position, the lifters will not be able to maintain zero lash in the valve train. Proper valve lash ensures valves open fully and seat completely. Hydraulic lifters provide a means to keep valve-train components in continuous contact.
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Dirty Oil
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The most significant cause of sticking hydraulic lifters is dirty and contaminated engine oil. Particles in dirty oil can keep the metering valve in the lifter from seating properly or from opening fully. Particles in engine oil that clog the lifter's check valve limiting the check valve's ability to control the flow of oil through the lifter. In overhead valve engines, dirty oil, varnish and sludge may cause the lifter to stick in the block potentially damaging camshaft lobes or bending push rods.
Engine Overheating
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When engines are allowed to overheat, internal moving parts will expand and fail to function correctly. If the engine already has varnish and sludge deposits, excessive heat may cause internal components such as hydraulic valve lifters to stick in either the open or closed position. Additionally, excessive heat in an overhead valve engine can cause valve lifters to seize in the engine block. The most effective method of preventing stuck valve lifters is changing engine oil at the intervals recommended by your car's manufacturer.
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References
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