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by Tannon Weber
If you have a compression problem in a cylinder in an OHV or OHC engine, a leakdown test is a reasonably straightforward way to figure out the fault in that cylinder. To perform a leakdown test, you need a pressurized air source (like an air compressor), an air hose, and a fitting for that hose that threads in where the spark plug normally goes.
A cylinder is essentially a pressure chamber, whose walls are comprised of the cylinder wall, the piston, the cylinder head, the valves, the cylinder head gasket, the piston rings, and the spark plug. Some vehicles also have a direct-injection fuel injector, and obviously conventional diesel engines don't have spark plugs; some have glowplugs. Anyway, as these components are the entirety of the pressure vessel, the failure of a given component will usually have ramifications that are observable elsewhere in the vehicle.
Pressurizing a cylinder with compressed air should cause air to leak through a failed component, making noise or causing a pressure change somewhere else on the vehicle. Listening or feeling for these air leaks can tell the technician more about the failure. Pressure doesn't necessarily have to be particularly high, usually 40psi is enough to get a result, as a fault should allow a fairly low amount of pressure to leak. Obviously, shut off the compressor motor/pump in order to hear what's going on with the engine. A list of common failures is as follows:
Intake Valve or Intake Valve Seat - air will escape past the valve and into the intake manifold. Sound should be audible in the throttle plate or air cleaner assembly. Be sure to have the PCV valve disconnected to not have a false-positive.
Exhaust Valve or Exhaust Valve Seat - pressure will leak into the exhaust manifold and ultimately out the tailpipe, and will probably be more audible than felt at the exhaust pipe.
Piston Rings - crankcase is pressurized, leading to air escaping through the PCV valve or breather, or possibly any other passage to the outside like the dipstick tube. If the PCV valve is left hooked up to the intake it could cause a false positive.
Cylinder Head - a cracked head might leak if it's really, really badly damaged. Depending on the head design, it could leak into the water passage leading to bubbling audible in the radiator, into the oil passage leading to noise at the PCV or another crankcase opening, to an intake or exhaust port leading to a sound similar to a failed valve/seat, or to the outside. This is a less likely failure.
Cylinder Head Gasket - as the gasket touches the cylinder, adjacent cylinder(s), water passages, oil passages, and the outside of the engine, this one could potentially manifest in many different ways as well. Typically a badly failed cylinder head gasket will be evident when the head is removed, as there will be evidence of combustion in the gasket or on the mounting surfaces.
Cylinder Wall - a badly scarred or gouged cylinder wall could let pressure past the piston rings. Should sound the same as a bad ring. The only way to tell would be to remove the head and physically inspect.
Piston - again, will manifest the same way as a cylinder wall or piston ring. Visual inspection would probably tell.
Spark Plug - probably difficult to tell on a leakdown test unless pressure leaks past the fitting.
Fuel Injector on direct injection engines - should leak past the injector to the outside, or as a remote possibility into the injector and into the fuel rail, theoretically.
To look at it another way, in order of likelihood:
Remember, start with simple, more likely failures before assuming something catastrophic. Gaskets, valves/seats, and rings are all good places to start depending on the symptoms. Cracked heads, cracked pistons, and bad DI fuel injectors are all pretty small possibilities.
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