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How Do I Perform a Leak-Down Test on My BluePrint Engine?

When Should I Perform a Leak-Down Test?

A leak-down test is useful after basic external checks have been completed, particularly when troubleshooting:

  • One or more cylinders with low compression
  • A persistent cylinder misfire
  • Suspected intake or exhaust valve leakage
  • Suspected piston ring or cylinder sealing problems
  • Excessive crankcase pressure or blow-by
  • Oil consumption
  • A concern that cannot be isolated with a compression test

If the concern is excessive crankcase pressure, also verify that the PCV system and crankcase ventilation are properly installed and functioning before assuming there is an internal engine problem.

What Do I Need to Perform a Leak-Down Test?

You will need:

  • Cylinder leak-down tester
  • Regulated compressed-air supply
  • Correct spark-plug-hole adapter
  • Basic hand tools
  • Breaker bar or appropriate tool for manually rotating the engine

Always follow the operating instructions supplied with your specific leak-down tester.

How Do I Perform a Leak-Down Test?

1. Disable the Engine

The engine must not start during the test.

Disable the ignition and fuel systems as appropriate for the vehicle.

2. Remove the Spark Plugs

Remove the spark plugs so the engine can be rotated manually and each cylinder can be tested.

Keep the spark plugs organized by cylinder. Inspecting each plug may also provide useful information about that cylinder.

3. Position the Cylinder at Top Dead Center

Position the cylinder being tested at top dead center (TDC) on the compression stroke.

At this position, both the intake and exhaust valves should be closed.

Correct piston position is important. If the piston is not properly positioned at TDC, compressed air can rotate the engine or escape through an open valve and produce a misleading test result.

4. Connect the Leak-Down Tester

Install the leak-down tester adapter into the spark-plug hole.

Connect the tester and regulated compressed-air supply according to the tester manufacturer's instructions.

Caution: Compressed air can cause the crankshaft to rotate if the engine is not securely positioned at TDC.

5. Record the Result

Record the leak-down reading for the cylinder.

Repeat the same procedure on every cylinder using consistent test conditions.

Comparing all cylinders is often more useful than evaluating a single cylinder by itself.

How Do I Determine Where the Cylinder Is Leaking?

While the cylinder is pressurized, listen for escaping air.

The location of the escaping air can help determine which area needs additional diagnosis.

Air From the Throttle Body or Carburetor

Air escaping through the intake system can indicate leakage past an intake valve.

Verify that the cylinder is correctly positioned at TDC on the compression stroke before diagnosing an intake-valve sealing problem.

Air From the Exhaust

Air escaping through the exhaust system can indicate leakage past an exhaust valve.

Again, verify correct piston position before determining that there is a valve-sealing concern.

Air From the Crankcase, Breather, or PCV Opening

Some air entering the crankcase can occur during a leak-down test.

However, a cylinder showing noticeably more crankcase leakage than the other cylinders may require additional inspection of the:

  • Piston rings
  • Pistons
  • Cylinder walls
  • Overall cylinder sealing

Do not condemn the piston rings solely because air can be heard in the crankcase.

Compare the result with the other cylinders and consider the complete diagnostic picture.

Air or Bubbles in the Cooling System

Air entering the cooling system during a leak-down test may indicate a sealing concern between the combustion chamber and cooling system.

Further diagnosis should be performed before determining the exact cause.

Should I Perform a Compression Test Too?

Yes.

Compression and leak-down testing provide different information and can be used together when diagnosing an engine concern.

A compression test measures the pressure produced by the cylinder while the engine is cranking.

A leak-down test uses compressed air to help determine where cylinder pressure may be escaping.

For example:

  • Low compression + air from the intake: Investigate intake-valve sealing.
  • Low compression + air from the exhaust: Investigate exhaust-valve sealing.
  • Low compression + excessive crankcase leakage: Investigate piston, ring, and cylinder sealing.
  • Low compression + air entering the cooling system: Investigate the sealing between the combustion chamber and cooling system.

These tests should be used to determine the next diagnostic step, rather than automatically determining that a component needs to be replaced.

Why Should I Test Every Cylinder?

Testing every cylinder provides a comparison.

Record the results for the complete engine rather than only testing the suspected cylinder.

Leak-down readings can be affected by:

  • Engine temperature
  • Leak-down tester design
  • Regulated test pressure
  • Piston position
  • Test procedure

Perform each cylinder test under the same conditions whenever possible.

A cylinder that produces a significantly different result than the others should be investigated further.

Does a Poor Leak-Down Result Mean My Engine Is Damaged?

Not necessarily.

Before determining that there is an internal engine concern, verify:

  • The piston is at TDC on the compression stroke
  • Both valves are closed
  • The tester is properly calibrated
  • The spark-plug-hole adapter is sealing
  • Air-hose connections are not leaking
  • Test pressure remains consistent
  • The result can be repeated
  • The other cylinders have been tested for comparison

If the cylinder continues to produce an abnormal result, record where the escaping air can be heard.

That information can help determine the next diagnostic step.

What Information Should I Record?

When performing a leak-down test, record:

  • Engine part number
  • Engine serial number
  • Cylinder number
  • Leak-down result for each cylinder
  • Test pressure used
  • Location where escaping air can be heard
  • Compression readings, if available
  • Engine temperature during testing
  • Relevant engine symptoms

Complete test results make troubleshooting easier and can help prevent unnecessary diagnostic work.