ASE A8 Engine Performance Practice Test

Cylinder compression test

20. The test results above were recorded during a cylinder compression test. Which of the following is MOST likely causing these test results?

  • A. Burned exhaust valves.
  • B. Weak piston rings.
  • C. A faulty head gasket.
  • D. Incorrect valve timing.
Cylinder compression test


Answer A is wrong. To check for a burned exhaust valve, inject an ounce or two of engine oil into the cylinder and test again. If the pressure increases, the oil has improved the seal between the piston rings and the cylinder. If it doesn't, a valve is most likely the problem.

Answer B is wrong. The compression pressure will increase after injecting oil.

Answer C is correct. When there are low compression pressures in adjacent cylinders, the head gasket leaks compression between the two or a crack in the deck or cylinder head. It is usually the head gasket.

Answer D is wrong. Improper valve timing would not result in this condition. It would affect more cylinders.