Which factor is the most likely cause of low compression in a single cylinder during testing?

Which factor is the most likely cause of low compression in a single cylinder during testing?

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

Answer: (C)

During a compression test you’re measuring how well a cylinder can trap and compress air. A single cylinder showing low pressure usually points to something that’s preventing a tight seal or adding something that isn’t compressible. A faulty fuel injector can cause a low reading because if it leaks or sticks open, fuel can flood that cylinder. The liquid fuel isn’t compressible like air, and its presence reduces the effective amount of air being compressed, sometimes even creating a hydraulic-like situation that the gauge reads as low pressure. This is why a leaking or stuck injector is a plausible cause for a noticeably lower compression reading in that one cylinder. In other common scenarios, worn piston rings or a burnt valve cause low compression by leaking air past the seal, but they don’t introduce liquid into the cylinder during the test in the same direct way.

During a compression test you’re measuring how well a cylinder can trap and compress air. A single cylinder showing low pressure usually points to something that’s preventing a tight seal or adding something that isn’t compressible. A faulty fuel injector can cause a low reading because if it leaks or sticks open, fuel can flood that cylinder. The liquid fuel isn’t compressible like air, and its presence reduces the effective amount of air being compressed, sometimes even creating a hydraulic-like situation that the gauge reads as low pressure. This is why a leaking or stuck injector is a plausible cause for a noticeably lower compression reading in that one cylinder. In other common scenarios, worn piston rings or a burnt valve cause low compression by leaking air past the seal, but they don’t introduce liquid into the cylinder during the test in the same direct way.