Cylinder-leakage tests reveal clues about compression loss

Behold, the engine in your service bay with low compression. Just how do you find where the leakage is? Follow along.


How does a cylinder leakage test differ from a conventional compression test? Instead of checking how well each cylinder pumps air, a cylinder-leakage test checks each cylinder's ability to hold air.

The sequence for cylinder leakage is testing is virtually the same for all engines. The engine should be warmed up first because a cold engine allows more blow-by gases than a hot one.

• Remove the air cleaner and prop the throttle open. This makes it easier for you to hear if air is leaking into the intake manifold.
• Remove the oil-filler cap, pull the PCV valve out of its grommet (if equipped) and let it hang off to the side. Relocating the valve prevents air that may leak into the crankcase from traveling back into the intake manifold, which could mislead you into thinking the engine has a bad intake valve.
• Using extreme care, release the pressure from the radiator by slightly loosening the cap, then remove it and top off the radiator with coolant. Removing the cap lets you see if any bubbles rise during the leakage test, a sign that air is leaking into the cooling system.
• Remove the spark plug from the cylinder to be tested. It's up to you to pull as many plugs as you want to.
• Disable the ignition system by disconnecting and grounding the coil wire. On engines with distributorless ignition, disconnect the ignition coil primary supply to the coil pack.
• Turn the engine over (manually or with the starter and ignition disabled) until the piston in the cylinder to be tested is at top dead center on its compression stroke. The piston must be at TDC for three reasons: a) so the valves are closed, b) to position the rings at the most worn part of the cylinder, and c) to prevent the piston from being shoved back down by the compressed air.
• There are a number of ways to find TDC, but the easiest way that works for all cylinders and engines is to use the engine's ability to pump air. Thread the adaptor hose into the cylinder you're going to test and connect a whistle to the end of the adapter. If you don't have a whistle handy, hold your thumb over the adapter's end. Turn the engine over (bumping the starter with a remote switch works nicely) just until you hear the whistle start to blow or air come out of the end of the adapter. Slowly turn the engine to the point where the whistle (or air) just stops. This is TDC. You may have to try this trick a few times to get the hang of it, but it saves a lot of time compared to figuring out TDC by degrees.

• Connect the leakage tester to your shop's air supply. Follow the equipment maker's recommendations for maximum inlet air pressure. Then, zero the gauge with the adjustment knob.

• Connect the tester's outlet hose to the adapter and read the leakage on the gauge. If the crankshaft turns when you feed compressed air into the cylinder, reposition it back to TDC and prevent the crank from turning by holding it with a wrench. Leaving the other spark plugs in place will lessen the odds of this happening.


Every cylinder, regardless of the size and type of engine, leaks a certain amount of air because no cylinder seals perfectly. There is always some seepage past the rings because of bore distortion and the ring end gaps. Regardless, a cylinder that's in poor mechanical condition will always leak a lot more air than a cylinder in good condition.

The amount of leakage, measured in percent, gives you a pretty good indication of the engine's overall condition. As a rule, most engines in good mechanical condition will show less than 20-percent leakage, though some engines can read as high as 25 percent to 30 percent and still perform satisfactorily. In most cases, more than 20 percent leakage means trouble.

Cylinder-leakage testing really proves itself when you pinpoint the leak source:
• Air escaping out the tailpipe indicates a leaky exhaust valve.
• Air escaping into the intake manifold tells you there's a leaky intake valve.
• Air escaping into the radiator indicates a compression leak into the cooling system through a bad head gasket or a crack in the head or cylinder.
• Air escaping into the crankcase indicates blow-by due to worn rings, a worn or damaged cylinder, or a cracked or burned piston (for further diagnosis, a "wet" leakage test is needed).


With a dry leakage test, you can tell whether the blow-by problem is due to the rings or taper wear in the cylinder by turning the crank to reposition the piston halfway down the cylinder. Cylinder wear is always greatest at the top, so moving the piston down from TDC should reduce the amount of leakage if taper is the problem. If the rate of leakage decreases, you've identified the problem and further diagnosis isn't needed. But if the rate of leakage remains unchanged, it tells you that taper wear is negligible and that the problem is either rings or possibly piston damage. That's where the wet test comes in.

Squirt a little oil into the cylinder through the spark plug hole to temporarily seal the rings. Turn the engine over several times so the oil will spread around the rings. Make sure the piston is back at TDC and repeat the leakage test. If the rate of leakage is now lower, the rings are causing the blow-by problem. No change? Then you've confirmed piston damage.


In the event that a leakage test doesn't show a problem, the fault may lie with the valve train. After all, a perfectly tight cylinder with a worn cam lobe, bent pushrod or damaged rocker arm will run just as bad as an engine with a burned valve. Unfortunately, there's no easy way to check other than yank off the valve cover and take a peek.

Add cylinder-leakage testing to your diagnostic strategies. You’ll quickly learn it makes cylinder diagnosis a more clockwork than guesswork procedure.