An engine receives good results from a compression test and a cylinder leakage test but poor results from a cylinder power balance test. What could be the potential cause of the problem?

An engine receives good results from a compression test and a cylinder leakage test but poor results from a cylinder power balance test. What could be the potential cause of the problem?

• A. Worn piston rings
• B. Leaky head gasket
• C. Defective lifter
• D. Faulty fuel injector

Answer: (C)

When a cylinder shows good compression and no significant leakage, but its power output is weaker than the others under running conditions, the issue is tied to how that cylinder breathes or is energized during operation rather than the seals or rings. A lifter that’s defective or sticking can prevent the valve from opening and closing properly or at the right time for that cylinder. If the valve doesn’t lift fully or returns to seat at the correct moment, the cylinder won’t be able to ingest the proper air–fuel mix or evacuate exhaust efficiently during running, so it won’t contribute full power even though static tests look fine. The compression test can still read normal because it’s a static measurement during cranking, and the leak-down test checks for sealing at rest, not the dynamic valve motion during operation. This is why a faulty lifter is a plausible cause for a single-cylinder power imbalance while other tests remain normal. Worn piston rings or a leaky head gasket would typically show up as low compression or abnormal leakage, and a faulty fuel injector would often show fuel-related symptoms and wouldn’t specifically point to valve timing issues without additional clues.

When a cylinder shows good compression and no significant leakage, but its power output is weaker than the others under running conditions, the issue is tied to how that cylinder breathes or is energized during operation rather than the seals or rings. A lifter that’s defective or sticking can prevent the valve from opening and closing properly or at the right time for that cylinder. If the valve doesn’t lift fully or returns to seat at the correct moment, the cylinder won’t be able to ingest the proper air–fuel mix or evacuate exhaust efficiently during running, so it won’t contribute full power even though static tests look fine. The compression test can still read normal because it’s a static measurement during cranking, and the leak-down test checks for sealing at rest, not the dynamic valve motion during operation. This is why a faulty lifter is a plausible cause for a single-cylinder power imbalance while other tests remain normal. Worn piston rings or a leaky head gasket would typically show up as low compression or abnormal leakage, and a faulty fuel injector would often show fuel-related symptoms and wouldn’t specifically point to valve timing issues without additional clues.