CNG Engine Mechanical Diagnosis and Repair
What you must know
Dedicated CNG engines often run higher compression and see different wear patterns than gasoline units: valve seat recession is a known concern, so hardened seats and correct valve lash are important. Know how to diagnose mechanical faults with compression testing (wet/dry), cylinder leak-down, vacuum gauge, and valve lash inspection. Because gas is dry and doesn't wash cylinder walls with liquid fuel, ring/cylinder wear patterns can differ; also, gas can be harder on ignition and valve seats. Know torque specs and the need to relieve fuel pressure and disable the ignition before mechanical work.
Traps
The exam will test whether you can separate a mechanical misfire from an ignition or fuel misfire. A cylinder that fails both dry and wet compression, or shows high leak-down past the intake or exhaust, is mechanical. Another trap: adjusting valve lash without verifying the engine is at the correct temperature and cylinder position. Also remember to disable the fuel system before cranking a compression test - a gaseous injector firing into a disconnected intake is a hazard.
Rule of thumb
If a misfire doesn't follow an injector or coil swap and compression is low, it's mechanical. Confirm with leak-down and listen at the intake, exhaust, and crankcase for where the air escapes.
Worked example
Cylinder 4 misfires; swapping coil and injector changes nothing. Dry compression is 90 psi vs. 150 psi on others; wet test adds only 5 psi. Leak-down shows air from the oil fill. Diagnosis: ring/piston sealing - engine mechanical repair.