When checking engine compression, what could cause low compression in all cylinders?

• A. A dirty air filter only.
• B. Worn piston rings, blown head gasket, valve seating issues, or a burnt valve.
• C. A faulty alternator affecting compression.
• D. A clogged exhaust.

Answer: (B)

Low compression across all cylinders means the engine isn’t sealing properly in the combustion chambers. When piston rings are worn, gas leaks past into the crankcase, lowering the pressure in every cylinder. A blown head gasket can allow compression to escape between cylinders or into coolant/oil passages, also dropping readings everywhere. Valve seating issues or a burnt valve prevent a proper seal on the compression stroke, so each cylinder loses pressure as the valves don’t close fully.

A dirty air filter doesn't create a loss of cylinder sealing; it just restricts air intake, which affects power but not the compression readings. A faulty alternator is an electrical issue and doesn’t directly affect how well the cylinders seal. A clogged exhaust mostly affects exhaust flow and backpressure rather than the raw compression measured during a compression test.