Cylinder Head, Valve Train, and Camshaft Diagnosis and Repair
What the candidate must know
This domain covers the head casting, valves, springs, guides, seats, rocker arms, pushrods, lifters, and the camshaft. Know how to evaluate valve recession, valve stem-to-guide clearance, seat concentricity, spring free length and installed height, and camshaft lobe lift and base circle. Understand valve lash adjustment (mechanical vs. hydraulic lifters), overhead cam timing marks, and the relationship between cam timing and compression/vacuum symptoms.
Traps in questions
- Valve recession (valve sinking into the seat) reduces valve lash on mechanical systems and can hold valves open, causing a misfire and low compression. Questions often blame the lifter instead.
- A burned exhaust valve shows low compression that does not improve with oil, plus a leak-down hiss at the exhaust. Do not confuse it with ring wear.
- Cam lobe wear shows up as reduced valve lift, a dead cylinder that passes compression, and sometimes a misfire that moves with the cam. A wiped lobe can also cause a rhythmic tick that sounds like a lifter.
- Cam timing off by one tooth produces low vacuum, hard starting, and poor power across all cylinders - not a single-cylinder fault. Look for the pattern, not one cylinder.
- Spring installed height matters more than free length; a spring that is short when installed loses seat pressure and causes valve float at high RPM.
- Hydraulic lifter preload must be set with the lifter on the base circle of the cam, not on the ramp.
Worked example
A misfire on #2 with 90 psi compression that does not rise wet. Leak-down shows air at the intake. The intake valve is not sealing - check lash, then valve and seat condition. If lash is correct and the valve is burned or recessed, the head comes off.
Rule of thumb
If a cylinder fails leak-down at the intake or exhaust, the problem is in the head. If it fails into the crankcase, the problem is in the block.