2.4 Compression & Cylinder Leak-Down Testing

2.4 Compression & Cylinder Leak-Down Testing

Exam Focus: Master the execution and evaluation of dry compression, wet compression, and cylinder leak-down tests. Knowing how to interpret variance formulas, calculate leakage percentages, and trace escaping air to specific internal failure exit points is essential for ASE A1 certification.

Dry Compression Testing Procedure & Standards

A dry compression test measures the maximum pressure generated inside a cylinder during cranking. It tests the static sealing capability of the valve train, head gasket, and piston ring assembly.

Step-by-Step Test Procedure

1. Bring Engine to Temperature: Run engine until it reaches normal operating temperature to ensure proper clearance expansion and oil distribution.
2. Disable Fuel & Ignition: Disconnect the fuel pump relay/injector harness and primary ignition feed to prevent raw fuel washboard or high-voltage arc damage.
3. Remove Plugs: Remove all spark plugs to allow maximum, uninhibited cranking speed (minimum 200 RPM).
4. Block Throttle Open: Prop or hold the throttle plate at Wide-Open Throttle (WOT) to ensure unrestricted air intake.
5. Crank and Record: Screw the compression gauge into cylinder #1. Crank for 4 to 6 compression strokes. Note the pressure reading on the first stroke (evaluates valve seating) and the final peak pressure. Repeat for all cylinders.

Evaluation Criteria & Formulas

• Standard Compression Range: Typically 125 to 185 psi for conventional gasoline engines (varies by compression ratio).
• Maximum Allowable Cylinder Variation: Pressure differences between the highest and lowest reading cylinders must not exceed 10% to 15% (or max 20 psi).
• Minimum Allowable Pressure Formula: $\text{Minimum Allowed Pressure (psi)} = \text{Highest Cylinder Pressure} \times 0.85$
• Example: If the highest cylinder reads 160 psi, the lowest acceptable cylinder is $160 \times 0.85 = 136\text{ psi}$.

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Wet Compression Testing & Diagnostic Logic

When a dry compression test identifies a low cylinder, perform a wet compression test to isolate whether the leakage originates from the piston rings or the valves / head gasket.

Wet Test Procedure

Inject approximately one tablespoon (15 mL / 0.5 oz) of 30W engine oil into the low cylinder through the spark plug hole. Crank the engine a few revolutions to distribute oil around the piston perimeter, reattach the gauge, and crank at WOT.

Interpreting Wet Test Results

• Compression Increases Significantly (by > 15–20+ psi): The added oil temporarily seals the gap between the piston rings and cylinder wall. This confirms worn piston rings, stuck compression rings, or scored cylinder walls.
• Compression Remains Low / Unchanged: Oil cannot seal leaking valve margins or a blown head gasket. This confirms leaking/burnt valves or a ruptured head gasket.

Compression Test Decision Tree:
[ Perform Dry Compression Test ]
                |
     Low Cylinder Pressure Detected?
        /                        YES                   NO
       |                    |
[ Perform Wet Test ]    [ Cylinder OK ]
    /          Pressure Rises  Pressure Unchanged
    |                |
[ Worn Rings ]  [ Burnt Valve / Head Gasket ]

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Cylinder Leak-Down Testing (Differential Pressure)

While a compression test is a dynamic cranking test, a cylinder leak-down test (differential pressure test) is a static test performed with the engine stationary. It measures the percentage of compressed air escaping from a cylinder and pinpoints the exact failure location.

Test Procedure Setup

1. Rotate the engine crankshaft manually to bring the tested cylinder to Top Dead Center (TDC) on its compression stroke. At TDC compression, both the intake and exhaust valves are fully closed.
2. Lock the crankshaft in place (or engage parking brake / manual transmission) to prevent air pressure from pushing the piston down.
3. Connect regulated shop air (typically 90 to 100 psi) to the leak-down tester manifold.
4. Thread the adapter hose into the spark plug hole and apply pressure.

Leakage Percentage Mathematical Formula

The leak-down tester features two gauges: Gauge 1 (Input/Supply Pressure) and Gauge 2 (Cylinder Pressure). $\text{Percentage Leakage} = \left( \frac{\text{Supply Pressure (Gauge 1)} - \text{Cylinder Pressure (Gauge 2)}}{\text{Supply Pressure (Gauge 1)}} \right) \times 100\%$

• Calculation Example: Supply pressure is set to 100 psi. Cylinder pressure gauge reads 75 psi. $\text{Percentage Leakage} = \left( \frac{100 - 75}{100} \right) \times 100\% = 25\%$

Leakage Evaluation Standards

• 0% to 10% Leakage: Superior sealing (new / excellent condition).
• 10% to 20% Leakage: Acceptable sealing (normal operational wear).
• Exceeding 20% to 30% Leakage: Excessive mechanical leakage requiring repair.

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Leak-Down Exit Point Diagnostics (Pinpointing the Fault)

When air leakage exceeds acceptable limits, listen and observe where the air escapes to identify the failing component:

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Technician A / Technician B Case Studies

Technician A states that during a leak-down test, air escaping from the oil dipstick tube indicates worn piston rings or cylinder wall damage. Technician B states that if air bubbles appear in the radiator filler neck during a leak-down test, the engine has a leaking exhaust valve. Who is correct? Technician A is correct; Technician B is incorrect. Air escaping into the crankcase and out the dipstick tube passes by the piston rings. Air emerging as bubbles in the radiator indicates cylinder pressure leaking into the liquid cooling jacket, caused by a blown head gasket or a cracked cylinder head/block—not an exhaust valve.