General Engine Diagnosis
Verify, then isolate
General engine diagnosis is the discipline of turning a complaint ("it's noisy," "it uses oil," "it overheats") into a measured condition before any part is replaced. Confirm the symptom yourself and gather history: when it happens, hot or cold, idle or under load, and what was recently serviced.
Every engine needs correct air, fuel, compression, and spark at the right time. Mechanically, rely on three tests: compression (dry, then wet), cylinder leak-down, and engine vacuum at idle. A dry compression reading that climbs noticeably when oil is added points to rings and cylinder walls; a reading that stays low points to valves, head gasket, or valve timing. Leak-down localizes it: air from the oil filler means rings or a holed piston; from the intake, an intake valve; from the exhaust, an exhaust valve; bubbles in the radiator, a head gasket or cracked casting. A steady 17-22 in-Hg of vacuum with a small needle wobble is the classic vacuum-leak or lean-misfire signature; a low, steady reading suggests late valve timing or a large leak.
Traps: a misfire code does not prove an ignition fault - swap a coil or injector and watch whether the misfire counter follows the part or stays with the cylinder. Oil consumption is not automatically rings; check the PCV system, guides and seals, and look for external leaks first. Warm the engine fully, hold the throttle open, and disable fuel and ignition before compression testing.
Rule of thumb: leak-down above roughly 20 percent means a real sealing problem; under 10 percent is healthy. A wet-test gain of more than about 10 psi per cylinder points at the rings.