What a leak-down test actually measures
Compression tests are good. Leak-down tests are better. Here's what each one tells you about an engine — and why we run both.
A compression test tells you how much pressure each cylinder can build on the firing stroke. Low numbers across all cylinders might mean bore wear or a skipped timing event; an isolated low cylinder points to valves or rings on that hole alone. It is quick, cheap, and familiar to every technician — and it can miss slow leaks that only show up when the piston is not sealing the bore the same way a power stroke does.
A leak-down test brings the cylinder to a known position (usually top dead centre on the compression stroke), introduces regulated dry air through the spark or injector port, and measures how much pressure escapes. More importantly, you listen: hiss at the intake means intake valve leakage; at the exhaust, exhaust valve; at the oil filler cap, past the rings; bubbles in the coolant, head gasket breach. That directional information is what compression alone rarely provides.
What good readings look like. Compression should sit within the manufacturer band cold and hot, with cylinder-to-cylinder spread inside our pass criteria. Leak-down is expressed as a percentage of input pressure lost; small loss is normal, rising loss tracks with bore and valve condition. We run both because they stress different failure modes: compression catches gross mechanical loss; leak-down catches subtle valve seat issues and ring problems that would pass a lazy compression check.
What we reject at Motorol. Variance outside our published band, leak-down above our reject threshold, visible damage on endoscope, or oil analysis that contradicts mechanical tests — any of these fails the engine. We do not blend results across cylinders to "average out" a bad hole. Failed engines are stripped for parts or recycled, not washed forward with new oil and a hope.
For the full step-by-step with pressures, equipment, and archive rules, read How we test on this site — the workshop page mirrors exactly what happens on the bench before your pallet leaves.