3.5 Cylinder Leakage (Leak-Down) and Combustion-Leak Block Testing

3.3 Cylinder Leakage (Leak-Down) and Combustion-Leak Block Testing (ASE T1 Task A.9)

Principles of Differential Cylinder Leakage (Leak-Down) Testing

While a cranking compression test measures dynamic pressure built by a moving piston, a cylinder leakage (leak-down) test under ASE T1 Task A.9 pressurizes a stationary cylinder with regulated shop air (70 to 100 psi) through a precision internal restriction orifice. By measuring the pressure drop across that orifice on a calibrated percentage leakage gauge (0% to 100%)—and listening to where the escaping air exits the engine—a technician can quantify cylinder sealing to the exact percent and pinpoint whether the leak is past the intake valve, exhaust valve, piston rings, cylinder head gasket, or combustion chamber casting.

Step-by-Step Leak-Down Preparation and Calibration Procedure

1. Set Up the Particular Tester: Read the tool instructions rather than assuming that every dual-gauge tester has a percent scale or the same zeroing connection. Some tools calibrate with a sealed outlet; others compare two psi readings after connection. For example, the OTC 5609 instructions require TDC compression, connected test hose, regulator initially backed off, and slowly applied test pressure not exceeding 100 psi. Use the correct reference pressure and conversion chart for that tester.

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Leak-Down Percentage Standards and Escape-Path Isolation

Once the cylinder is pressurized at TDC compression, evaluate both the leakage percentage and the acoustic/visual exit point of the escaping air:

Carbon Deposits and Position-Dependent Leakage

A valve leak can arise from a burned face, a bent stem, carbon, insufficient lash or a damaged seat. Do not hit a valve or rocker while compressed air is applied: opening a valve can rotate the crank or damage the mechanism. Follow the engine's safe inspection procedure, check lash and valve motion, and inspect the seat if the leak persists.

Leakage is affected by piston position, temperature, tester orifice and pressure. Comparing readings taken at different positions does not prove that rings are intact or identify taper by itself. In particular, BDC after the power stroke can be near valve opening and is not a substitute for the specified TDC test. Measure the bore during teardown when dimensional evidence is needed.

Chemical Combustion-Leak ('Block') Testing and Cooling System Diagnostics

When a medium- or heavy-duty gasoline truck suffers from mysterious coolant loss, overheating under heavy load, rock-hard radiator hoses, or white sweet-smelling exhaust steam, a chemical combustion-leak test (block test) determines whether high-pressure combustion gases are entering the cooling system.

Operating the Bromothymol Blue Block Tester

1. Prepare Coolant Level: Safely remove the radiator cap or degas bottle cap on a cold or depressurized system and siphon coolant to 1 to 2 inches below the bottom of the filler neck. Critical Rule: Never allow liquid coolant to be sucked up into the test chamber; alkaline antifreeze contaminates the indicator chemical and invalidates the test.
2. Fill the Indicator Chamber: Pour fresh blue bromothymol blue test fluid up to the fill line of the block tester Sight Tube (on dual-chamber testers, fill both the lower scrubbing chamber and upper indicator chamber).
3. Run Engine & Aspirate Vapors: Start the engine and bring it to operating temperature so the thermostat opens and combustion pressure peaks. Press the tapered rubber cone firmly into the radiator neck or degas tank opening and squeeze/release the hand aspirator bulb for 30 to 60 seconds, bubbling cooling-system airspace vapors through the blue fluid.
4. Chemical Color Interpretation:
   • Fluid Remains Blue: No carbon dioxide (CO2) was detected in the cooling system headspace during the test.
   • Fluid Turns Yellow (or Green at trace levels): On a gasoline, LPG, or CNG engine, combustion gas contains roughly 12%–15% CO2. When CO2 bubbles through the water-based indicator, it forms carbonic acid (H2CO3), lowering the solution's pH and changing the bromothymol blue reagent from blue to yellow.

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Worked Example: Diagnostic Scenario: Overheating Class 7 Dump Truck

Symptom: A Class 7 dump truck with a 7.3L V8 gasoline engine repeatedly pushes coolant out of the surge tank overflow hose when hauling a full load up grades, even after a new OEM thermostat, water pump, and radiator cap were installed.

1. Chemical Block Test: The technician lowers the surge tank coolant level by 2 inches, brings the engine to 200°F, and draws vapors through fresh bromothymol blue fluid for 45 seconds while following the block-tester procedure; unloaded throttle movement is not equivalent to a loaded road test. The fluid turns bright yellow, confirming a combustion-to-coolant leak.
2. Single-Cylinder TDC Pressurization: With all eight spark plugs removed and a cooling system pressure gauge attached to the surge tank at 5 psi, the technician pressurizes each cylinder at TDC compression with 100 psi of shop air for 60 seconds:
   • Cylinders 1 through 5 and 7–8 show 6%–9% leak-down with 0 psi change on the cooling system gauge.
   • Cylinder 6 (on the example engine's verified Bank 2) shows 28% leak-down, and the cooling system pressure gauge climbs from 5 psi to 11 psi within 30 seconds ; never remove a cap or gauge from a hot, pressurized cooling system.
3. Action: Inspect the verified Bank 2 cylinder head, inspect the Cylinder 6 head gasket fire ring and combustion chamber casting for cracks with dye penetrant, check head and block deck flatness with a precision straightedge and feeler gauges, and repair as required.

Safe Setup Before Applying Cylinder Air

First determine whether warming the engine is permitted and useful for the test. Let the cooling system reach a safe temperature and relieve pressure by the prescribed method before removing a cap. Disable starting and fuel/ignition operation, remove the plugs as specified, and use the engine's cylinder map and normal rotation direction to position the target cylinder at TDC compression. Verify that both valves are closed from the valve-train position or approved timing procedure; TDC on the overlap stroke is a different condition.

Install the approved crank holding arrangement and remove loose hand tools. Air can rotate a crank that moves slightly away from dead center. Do not hold a breaker bar by hand as a restraint. Keep fingers, clothing and equipment clear of the belt and fan path. Start with the tester regulator backed off and raise pressure only as its instructions allow. A dual-psi-gauge tester and a percent-scale tester can require different connection and calibration procedures.

Record the reference pressure, cylinder reading, temperature and escape path. Listen without placing an ear near moving or hot equipment. Coolant bubbles, when observed through a safely opened cold system, justify investigating a cylinder-to-coolant breach. They do not distinguish a gasket from a cracked casting. After each cylinder, remove applied pressure before changing its position or disconnecting the hose. Compare readings obtained with the same setup; a universal percentage table is not a substitute for the tool and engine criteria.