Gasoline Engines
Fundamentals
Every gasoline no-start or driveability complaint reduces to four basics: air, fuel, spark, compression. Know the four-stroke cycle (intake, compression, power, exhaust), the difference between port injection and direct injection, and how coil-on-plug ignition uses crank and cam signals to sequence spark. Waste-spark systems fire two cylinders at once - one on compression, one on exhaust - so one failed plug or coil can affect two cylinders.
Tests you must interpret
- Cranking compression: throttle held open, all plugs out, good battery. Follow a dry test with a wet test (a few squirts of oil). Pressure rises = ring sealing; pressure stays low = valves or head gasket.
- Cylinder leakage: cylinder at TDC on compression, regulated air applied. Listen at the oil fill (rings), intake (intake valve), exhaust (exhaust valve), and radiator (head gasket). Leakage above roughly 20% is excessive.
- Vacuum gauge: steady low reading suggests a vacuum leak or retarded timing; a fluctuating needle suggests a valve problem.
- Fuel trims: positive long-term trims (adding fuel) point to a vacuum leak or weak pump; negative trims point to leaking injectors or high fuel pressure.
Traps
Questions love a wet test that changes nothing - the answer is valves or head gasket, not rings. Another favorite: a misfire that follows the coil when swapped between cylinders, proving the coil rather than the plug or injector.
Worked example
Specs show 150 psi highest and 110 psi lowest. 110 ÷ 150 = 0.73, or 73% of the highest - below the common 75% rule, so that cylinder fails and needs leak-down or teardown diagnosis.