How to Tell If You Have a Bad Lobe on Your Camshaftby Richard Rowe
All cams wear eventually, and most never notice it aside from the fact that the engine doesn't feel quite as perky as it used to. But when a single lobe wears out before the rest because of bad oil, excessive spring pressure or bad valvetrain parts, you could easily end up with a bucking, popping, backfiring mess of an engine. There are several ways to diagnose a wiped lobed, but the most effective is to just pull the valve cover off and check the cam's lift directly.
A flattened cam lobe will often manifest first at low rpm, especially for engines with hydraulic lifters. At idle and low rpm, the valve is barely open even when the cam is brand new; when the lobe flattens out, the valve may not open at all until the lifter pumps up at higher rpm. So a miss from a single, slightly flattened cam lobe will often start to go away as engine rpm rises and the lifter pumps up, but will probably reappear at very high rpm as the cylinder begins to once again starve for airflow. Engines with solid lifters may or may not experience low-rpm or idle problems.
Lifter tap is a very common symptom of flattened cam lobes, and is often the very first and most noticeable one. However, it's possible for an engine to have one or more completely wiped lobes with no lifter tap at all; it depends on the design. Again, the tapping may be worse at idle and low rpm, and quiet down when the lifters pump up with oil.
Backfiring and Popping
This will usually start out at high rpm, and gradually work down to lower rpm as the lobe completely flattens out. Rather than a complete and very violent backfire, this single-cylinder backfire will typically manifest as a popping sound through either the intake or exhaust. If you have a modern car with OBD-II, you'll almost certainly get a consistent single-cylinder misfire code for each cylinder with a bad lobe. Whether it happens through the intake or exhaust depends entirely on which lobe is dead.
All of the above are signs, but the definitive test is measuring valve lift at the spring with a dial indicator. You'll need to take the valve cover off, and install the dial indicator on the rocker studs one at a time. Position it to measure the rise of the valve spring at the retainer, or the top of the rocker arm, depending on your engine design. Be consistent with the placement from valve to valve. Turn the engine over by hand with a wrench on the crank center-bolt, with the spark plugs removed. Record the reading for each cylinder; they should be near-identical -- within 0.01-inch or so -- for each of the intake or exhaust valves relative to the rest. If you get a much lower reading for one intake valve compared to the other intakes, you've found your bad lobe. The same goes for the exhaust lobes and valves. Remember, only compare intake to intake, and exhaust to exhaust; many engines have "split-pattern" cams with different specs for the intake and exhaust lobes.
Special Cases -- Overhead-Cam Engines
If you've got an overhead-cam engine, particularly one with VTEC or a comparable variable-valve-timing system, it may be simpler and more effective to measure the cam lobes directly with the dial indicator. First, because there's no reason not to, and it's probably easier to access the top of the cam lobes than the valves. Secondly, because engines with VTEC-comparable systems won't show a flat secondary lobe at the valve when you're turning the engine over by hand. The secondary lobe only engages at high rpm, so measuring it directly is the the only way to confirm the lobe's status.