One misfiring cylinder can have good spark and injector command yet still fail to produce torque. Compare mechanical test patterns, locate escaping air, and choose the next confirmation—not the next part to replace.
Investigate a simplified four-cylinder gasoline engine. Run relative compression, compare dry and wet compression, and perform a simulated leak-down check on a selected cylinder. Use the location of escaping air together with the pressure evidence to identify what further testing is justified.
Illustrative training values—not OEM pass/fail limits. The engine, gauge readings, leakage percentages, and relative-current trace are invented for pattern recognition. In a real workshop, follow OEM directions for engine temperature, cranking speed, disabled fuel/ignition, gauge connection, crankshaft positioning and leakage thresholds. Applying shop air can rotate the crankshaft unexpectedly; secure the engine and follow approved safety procedures. A single compression or leak-down number is not a diagnosis.
KNOWN CASE Select a cylinder and compare the tests.
Use the same cylinder for a dry/wet comparison. The mechanical gauge and relative-compression pattern provide different kinds of evidence.
For the simulated leak-down test, 100 psi is supplied to a properly positioned cylinder. “Remaining” is the illustrative downstream pressure; loss is not a universal engine-health threshold.
Choose a listening point and test the selected cylinder.
A leak path can transmit sound to nearby places. Treat these as clues, then verify with an approved test before removing an engine component.
Compare evidence, then choose a confirming step.
A relative-compression test compares cylinders while the engine cranks; it does not directly measure compression in psi or identify the leak path. Current polarity, peak shape, and which cylinder aligns with a peak depend on test setup and synchronization.
Bar lengths use a teaching display scale; pressure (psi) and loss (%) are different measurements. More than one fault can give a similar pressure pattern.
Scenario: Cylinder 3 has low dry compression. It rises after a wet test, and most escaping air is heard at the crankcase during leak-down. What does this combination support?
Choose an answer.
Hands-on bridge On a training engine, compare OEM-approved dry/wet compression procedures, relative compression synchronization, and leak-down listening points. Record measured evidence separately from your hypothesis.