Doing a cylinder leak down test can tell you more about your engine than you think.
Also, a cylinder leak down test can pinpoint issues in specific areas that other tests can not.
A Cylinder Leak down or cylinder leakage test is actually similar to a compression test.
- So, a compression test measures how well your engine’s cylinders are sealing.
- But, a cylinder leak down test takes it to the next level and measures cylinder pressure loss.
Although a compression test won’t give you a detailed picture of your engine’s health, it should be done first.
A compression test is a quick and easy way to show if a catastrophic problem exists, and on which cylinder(s). So, you may have found bad cylinders but the only remaining question is why.
This Is Where The Cylinder Leak Down Test Take Over
Here we can narrow down where the problem may actually exist:
- A burnt or bent intake valve
- A burnt or bent exhaust valve
- Broken piston ring?
- Blown head gasket?
- A cracked cylinder wall
- A cracked cylinder head
Doing A Quick Cylinder Leak Down Test If You Don’t Own A Tester
A cylinder leak down test takes more time to perform; but can give you a more accurate and detailed picture of the engine’s overall health:
- A cylinder leak down test requires the removal of all the spark plugs.
- A threaded coupling is screwed into a spark plug hole.
- Compressed air (80 to 90 psi) is then fed into the cylinder.
- The crankshaft is then turned so that each piston is at top dead centre.
- Most people start with cylinder number one and follow the engine’s firing order.
The neat thing about a cylinder leak down test (as opposed to a compression test) is that; it’s faster and easier to figure out where the pressure is going.
Listening Where The Air Is Escaping From Can Isolate The Problem:
- Intake valve : Air whistling out of the intake, carburetor or throttle body indicates a 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 leak : Air bubbles in engine coolant seen at the radiator filler cap could mean air escaping into the coolant past the head gasket.
- Cracked cylinder or 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.
A cylinder leak down test can also be used in conjunction with; a compression test to diagnose other kinds of problems:
A cylinder that has poor compression but minimal leakage; usually has a valve train problem such as a worn cam lobe; broken valve spring, collapsed lifter, bent push rod, etc.
If all the cylinders have low compression but show minimal leakage, the most likely cause is incorrect valve timing. The timing belt or chain may be off a notch or two.
If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery (bad injector) or ignition problem (fouled spark plug or bad plug wire).
Doing A Cylinder Leak Down Test With A Proper Tester
There are tons of different testers out there that all do basically the same thing. Sometimes you can rent the tool from your local auto parts store. If you purchase a new one they all come with instructions. Doing so will give you more accurate results.
Both tools are designed to measure cylinder pressure in order to highlight and diagnose engine problems. A compression tester is a stand-alone tool that relies on the engine’s compression to build pressure in the cylinder. Finally, a cylinder leak down tester relies on an external supply of compressed air.
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