How is a compression test used to diagnose overhaul issues and what results indicate problems?

How is a compression test used to diagnose overhaul issues and what results indicate problems?

• A. Only check oil pressure and ignore compression.
• B. Use a combustion gas analyzer to infer compression.
• C. Measure engine temperature to infer compression.
• D. Test each cylinder for compression; low or inconsistent readings suggest rings, valves, or seats wear; follow with leak-down.

Answer: (D)

Directly measuring how well each cylinder seals is the key idea here. A compression test shows the engine’s ability to compress air in each cylinder, which depends on the sealing of piston rings and the valve seats. In overhaul diagnosis, low readings mean the cylinder isn’t sealing well, usually due to worn or damaged piston rings, scored or worn cylinder walls, or leaking intake/exhaust valves and seats, or a compromised head gasket. If the readings vary across cylinders, it points to uneven wear or isolated problems in specific cylinders. After you identify low or inconsistent numbers, a leak-down test is used to pinpoint exactly where the leakage is coming from—whether through the rings, valves, or a gasket—by pressurizing the cylinder at top dead center and listening for air escape paths. This combination is far more informative than just looking at oil pressure, temperature, or using a gas analyzer alone, which don’t directly quantify or locate compression-related issues.

Directly measuring how well each cylinder seals is the key idea here. A compression test shows the engine’s ability to compress air in each cylinder, which depends on the sealing of piston rings and the valve seats. In overhaul diagnosis, low readings mean the cylinder isn’t sealing well, usually due to worn or damaged piston rings, scored or worn cylinder walls, or leaking intake/exhaust valves and seats, or a compromised head gasket. If the readings vary across cylinders, it points to uneven wear or isolated problems in specific cylinders. After you identify low or inconsistent numbers, a leak-down test is used to pinpoint exactly where the leakage is coming from—whether through the rings, valves, or a gasket—by pressurizing the cylinder at top dead center and listening for air escape paths. This combination is far more informative than just looking at oil pressure, temperature, or using a gas analyzer alone, which don’t directly quantify or locate compression-related issues.