In a dry compression test, if compression is significantly improved by a wet test, which component is likely worn?

In a dry compression test, if compression is significantly improved by a wet test, which component is likely worn?

• A. Burnt valves
• B. Defective head gasket
• C. Worn valve guides
• D. Worn piston rings

Answer: (D)

The key idea is using a wet compression test to locate where the leak is coming from. When compression improves significantly after adding oil to the cylinder, it means the sealing problem is around the piston rings. The oil coats the rings and the bore, helping seal the small gaps and reduce leakage past the rings, so the reading goes up. That points to worn piston rings as the likely cause. If the issue were a burnt valve or a defective head gasket, the leak path is through the valve seating or the gasket, and the added oil doesn’t effectively seal those paths, so the compression reading wouldn’t improve much. Worn valve guides can affect oil consumption and valve alignment, but they don’t primarily create a seal leak that would be fixed by oil in the cylinder, so they’re less consistent with a large wet-test improvement.

The key idea is using a wet compression test to locate where the leak is coming from. When compression improves significantly after adding oil to the cylinder, it means the sealing problem is around the piston rings. The oil coats the rings and the bore, helping seal the small gaps and reduce leakage past the rings, so the reading goes up. That points to worn piston rings as the likely cause.

If the issue were a burnt valve or a defective head gasket, the leak path is through the valve seating or the gasket, and the added oil doesn’t effectively seal those paths, so the compression reading wouldn’t improve much. Worn valve guides can affect oil consumption and valve alignment, but they don’t primarily create a seal leak that would be fixed by oil in the cylinder, so they’re less consistent with a large wet-test improvement.