A compression test tells you a cylinder is down. A leakdown test tells you why. By holding a known pressure in the cylinder and reading how fast it escapes, you can separate a burnt valve from a broken ring from a failed head gasket, and you can do it by listening to where the air comes out.

Proform 66839 Dual-Gauge Cylinder Leakdown Tester Kit Features:

• Diagnoses the condition of head gaskets, piston rings and valves.
• Left gauge reads incoming air pressure, right gauge reads individual cylinder pressure.
• The difference between the two readings is the leakdown percentage.
• 14 mm spark plug adapter included.
• Sold as a kit.