Signs and Symptoms of a Bad Lifter Flat Tappetby Richard Rowe
Lifter tappets aren't quite as simple as they seem at first glance. The hydraulic lifter is a sort of self-contained hydraulic system, utilizing a number of tiny components in order to control valve train movement. While replacing a collapsed lifter is simple -- at least compared to fixing the engine if you don't replace it -- don't rush into the job until you know what you're dealing with.
Lifters are the small cylinders that sit in between a camshaft's lobes and the pushrods. With a hydraulic lifter, the bottom of the pushrod sits on top of a plunger, which in turn sits on top of a cylinder full of oil. Oil pressure pushes up on this plunger, stacking up slack, or "lash," in the system. In addition to taking up valve lash, most lifters stay slightly collapsed at low engine rpm, slightly reducing the distance that the valve opens to enhance bottom-end torque and while encouraging a smoother idle.
When the plunger inside of a lifter fails, it'll fail to maintain oil pressure and will remain in a collapsed state at all times. When that happens, the lifter will fail to take up lash in the system, which causes parts inside to hammer against each other during initial contact. This hammering manifests as a very noticeable tapping that increases in volume and frequency with engine rpm. A bad lifter will throw off the affected cylinder's ability to move and burn air and fuel, which will typically lead to spark plug fouling and a consistent miss on that cylinder.
A collapsed lifter puts a great deal of stress on a vehicle's valve train, and the weakest link is usually the pushrod. Collapsed lifters can easily bend pushrods, which will subsequently fall out of the space between the rocker arm and the top of the lifter. Once a push rod falls out of the space, the very least you can expect is a dead cylinder. Worst-case scenarios include broken rocker arms, broken valves, cracked heads, a damaged cam or complete engine destruction depending upon what breaks, how it breaks and when.
Replacing Flat Tappets
Replacing a collapsed lifter isn't quite as simple as you might think. While the shade-tree mechanic might simply pop the intake off and replace the faulty lifter, Ol' Scruffy isn't looking at the big picture. Cams and flat-tappet lifters are a matched set; they're broken in together at the factory to establish matching wear patterns. Putting a new lifter on an old cam is like trying to sharpen a set of ice skates with a waffle iron, and it's only a matter of time before that new lifter takes out your cam. But dropping in a replacement lifter is better than allowing lash to destroy your engine, so consider it as a band-aid fix.
Replacing Roller Tappets
Roller-tappet lifters and cams are the exception here, since far less wear occurs when using a roller tappet. Generally, you can get away with replacing a single collapsed roller lifter without replacing the cam. This isn't true in all cases, but again, better to replace the lifter and risk a cam replacement later than to allow a $15 part to destroy your entire engine.