Beyond getting an overall picture of engine condition, the leakdown test is an excellent way to pinpoint where engine problems are before tearing down the engine. Listening for where the air is escaping by ear can isolate the problem.
Intake Valve: Air whistling out of the intake, carburetor, or throttle body indicates leak at the intake valve.
Exhaust Valve: Air heard hissing out of the tailpipe, turbocharger, or exhaust manifold means an exhaust valve leak.
Piston Rings: Whistling or hissing out of the PCV valve, oil filler cap hole, or dipstick tube means the air is pushing past the rings. Suspect ring or cylinder wall wear.
Head Gasket: Air bubbles in engine coolant seen at the radiator filler cap could mean air escaping into the coolant past the head gasket.
Cracked Cylinder Head: Bubbles in coolant or coolant being pushed up out of the radiator neck can also indicate cracks in the cylinder head or cylinder walls.Tools and Supplies
You will need a compressed air source, a leakdown gauge kit, spark plug socket, basic hand tools, vehicle service manual, and a notepad to record results.
|Step 1 - Remove the spark plugs and rotate the engine to put the cylinder to be tested at top dead center. Tip: insert a long screwdriver or extension into the spark plug hole and turn the engine by hand with a socket on the crankshaft. When the screwdriver stops rising or falling you're at TDC. Don't turn the engine backwards if TDC is missed. Go around again. In a four-cylinder engine, cylinders one and four and two and three are at TDC at the same time.|
|Step 2 - Figure out which spark plug adapter works best and connect the gauge into the hole. Put the vehicle in gear and set the parking brake to prevent the engine from turning when air is compressed into the cylinder. Start with the regulator turned counter-clockwise to zero the incoming pressure. Connect compressed air. Turn the regulator clockwise to pressurize air into the cylinder.|
|Step 3 - Record leakage percentage for the cylinder. Remove the oil dipstick, radiator cap, and oil filler cap. Open the throttle body or remove the air cleaner. Listen and watch. Wherever air is escaping will indicate where the problem is. Disconnect the gauge and move onto the next cylinder.|