What is the difference between a compression test and a leak-down test?
Short answer: A compression test measures how much pressure each cylinder builds while cranking, while a leak-down test pressurizes a cylinder with shop air and measures how fast it leaks, and shows where the air escapes.
A compression test is the quick screening test. You disable fuel and ignition, install a gauge in each spark plug hole, crank the engine a set number of revolutions and record the peak reading. What matters most is consistency between cylinders; one cylinder well below the others needs a closer look. Check service information for specifications.
A leak-down test answers why. With the piston at top dead center on the compression stroke, you feed regulated air into the cylinder and read the percentage of leakage. Then you listen: air hissing from the intake suggests an intake valve, from the tailpipe an exhaust valve, from the oil fill the rings, and bubbles in the coolant point to a head gasket or cracked head.
Use both together for an accurate engine health picture. Hold the crankshaft securely during leak-down, because air pressure can spin the engine if the piston is not at top dead center. Keep hands clear and wear eye protection.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.