Cylinder leak down test findings, What can they tell you ?
Doing the cylinder leak down test, especially in conjunction with a compression test, should allow you to quickly determine the basic condition of any engine.
While a compression test is a dynamic test (engine moving), a cylinder leakdown test (C.L.T.) is a static test (engine at rest). The compression test measures how much pressure the engine can produce while cranking; in contrast to the C.L.T., which measures how much pressure is lost in the engine.
In a C.L.T. the engine is placed on TDC of the cylinder in question and using a similar type of connector as the compression test, we fill the cylinder with pressure.
The tester then measures the volume of air needed to maintain a predetermined pressure in the cylinder.
This reading is expressed in a percentage.
Good cylinder leakdown readings should be below 5-8%.
The great thing about C.L.T. is that it deals with how well the cylinder is sealing and nothing else. The readings are not affected by carbon deposits, cam timing, or even engine cranking speed.
Another great feature of the C.L.T. is the fact that you can hear where the air is leaking out of the cylinder. When a cylinder has high percentage of leakage, first check the oil filler cap. Do you hear a hissing sound? If so, you may have pressure leaking by the rings.
Is there air escaping out the exhaust? Is it escaping out the intake system? Then a burned valve may be the problem. If two adjoining cylinders have similar low readings and you hear leakage out the other cylinder, then a failed head gasket may be the problem.
Being able to pinpoint the exact source of the compression loss will tell you where the problem is; and not just that you have one. This knowledge will greatly assist you in the next step… the repair.
This is my favorite engine test to do because it can point you in the right direction with very little cost. A great place to start !
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.
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 leak down 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:
- Tailpipe – exhaust valve/seat, cylinder head gasket, cylinder head
- Throttle body, intake, or air cleaner – intake valve/seat, cylinder head gasket, cylinder head
- Radiator – cylinder head gasket, cylinder head
- PCV, Breather, or other crankcase opening – piston rings, cylinder wall, piston, cylinder head gasket, cylinder head
- Outside – spark plug, cylinder head gasket, cylinder head, DI fuel injector
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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