A compression test was performed on an engine and lower than specified compression was measured in one cylinder. What would be the likely cause?

A compression test was performed on an engine and lower than specified compression was measured in one cylinder. What would be the likely cause?

• A. Worn piston rings
• B. A worn or burnt valve
• C. A faulty fuel injector
• D. Leaking liner O-rings

Answer: (B)

A compression test measures how well a cylinder can build pressure during the compression stroke, which depends on the seals inside the combustion chamber. If the reading in one cylinder is lower than specification, it points to a leakage path that prevents the chamber from trapping the air-fuel charge. A worn or burnt valve is the most likely cause for a single-cylinder drop because the valve may not seal fully when it closes. If the intake or exhaust valve isn’t sealing, air can escape past the valve during compression, preventing the cylinder from reaching normal pressure. A valve that’s been overheated or is seating badly leaves a persistent leak, which shows up as a low compression reading specifically in that cylinder. Piston rings, by contrast, tend to affect compression more broadly. Worn rings often cause blow-by and lower compression that can show up across multiple cylinders, plus symptoms like increased oil consumption and visible crankcase pressure. A faulty fuel injector affects how the engine runs and fuel delivery but doesn’t directly reduce the cylinder’s static compression pressure. Leaking liner O-rings could cause leakage, but they’re less commonly the single-cylinder cause for a low compression test and usually accompany other signs like coolant or oil leaks. If you want to differentiate between a valve and rings, you can perform a wet compression test. Adding a small amount of oil into the cylinder and repeating the test—if the pressure rises significantly, rings are the culprit; if it doesn’t, the valve likely isn’t sealing properly.

A compression test measures how well a cylinder can build pressure during the compression stroke, which depends on the seals inside the combustion chamber. If the reading in one cylinder is lower than specification, it points to a leakage path that prevents the chamber from trapping the air-fuel charge.

A worn or burnt valve is the most likely cause for a single-cylinder drop because the valve may not seal fully when it closes. If the intake or exhaust valve isn’t sealing, air can escape past the valve during compression, preventing the cylinder from reaching normal pressure. A valve that’s been overheated or is seating badly leaves a persistent leak, which shows up as a low compression reading specifically in that cylinder.

Piston rings, by contrast, tend to affect compression more broadly. Worn rings often cause blow-by and lower compression that can show up across multiple cylinders, plus symptoms like increased oil consumption and visible crankcase pressure. A faulty fuel injector affects how the engine runs and fuel delivery but doesn’t directly reduce the cylinder’s static compression pressure. Leaking liner O-rings could cause leakage, but they’re less commonly the single-cylinder cause for a low compression test and usually accompany other signs like coolant or oil leaks.

If you want to differentiate between a valve and rings, you can perform a wet compression test. Adding a small amount of oil into the cylinder and repeating the test—if the pressure rises significantly, rings are the culprit; if it doesn’t, the valve likely isn’t sealing properly.