Irregular (rough) engine operation can be caused by improperly adjusted or leaky valves. Operate the engine at rpm which makes the malfunction most pronounced. A non-firing or low compression cylinder can be located by momentarily loosening and then retightening fuel lines at the fuel injection pumps one at a time. Continue this until a loosened fuel injection line makes little or no difference in the engine operation. This same test can also indicate faulty fuel injection so further checking of the cylinder is necessary.
The preceding test is merely a quick means of pinpointing the source of cylinder compression loss. Removal of the head and visual inspection of the valves and seats is necessary to check for minor valve defects which do not have much effect on engine operation. This is usually done during general engine reconditioning.
The following procedure provides a more complete check of the sealing ability of the individual valves without removing the cylinder head:
1. Remove the fuel injection nozzle, leaving the precombustion chamber in place.
2. Adapt an air hose to the precombustion chamber. This can be done with either a threaded fitting or by holding a rubber adapter in place.
3. Rotate the crankshaft until the piston in the suspected cylinder is at top center (TC) on the compression stroke. In this position the valves of the cylinder are closed.
4. Force air into the cylinder and then check for escaping air. Air escaping from the exhaust opening indicates exhaust valve leakage. Air escaping from the air cleaner inlet indicates inlet valve leakage. If air escapes from the crankcase breather during this test, the piston, rings and/or liner can be at fault.
On turbocharged engines, it may be necessary to remove inlet and outlet connections on both sides of turbocharger to notice leakage.