Two main causes of low compression are which?

Two main causes of low compression are which?

• A. Piston rings not seating or broken, and valves not seating or burned.
• B. Spark plug fouling and improper spark timing.
• C. Crankshaft bearing wear and piston slap.
• D. Cylinder head warpage and valve guide wear.

Answer: (A)

Low compression happens when the combustion chamber isn’t properly sealed. The two main sealing surfaces are the piston rings and the intake/exhaust valves. If the piston rings aren’t seating or are broken, gas can leak past the rings during compression, lowering the built-up pressure. If the valves aren’t seating correctly or are burned, gas leaks past the valve seats as the piston moves—the pressure can’t be maintained inside the cylinder. Those two issues directly undermine the ability to trap and build compression. Spark plug fouling or improper spark timing affect ignition and combustion efficiency, not the actual trapped compression. Crankshaft bearing wear and piston slap relate to moving clearances and noise rather than a primary loss of cylinder pressure. Cylinder head warpage and valve guide wear can cause leaks, but they’re typically less common as the primary cause compared to rings and valve sealing, and often present with additional symptoms.

Low compression happens when the combustion chamber isn’t properly sealed. The two main sealing surfaces are the piston rings and the intake/exhaust valves. If the piston rings aren’t seating or are broken, gas can leak past the rings during compression, lowering the built-up pressure. If the valves aren’t seating correctly or are burned, gas leaks past the valve seats as the piston moves—the pressure can’t be maintained inside the cylinder. Those two issues directly undermine the ability to trap and build compression.

Spark plug fouling or improper spark timing affect ignition and combustion efficiency, not the actual trapped compression. Crankshaft bearing wear and piston slap relate to moving clearances and noise rather than a primary loss of cylinder pressure. Cylinder head warpage and valve guide wear can cause leaks, but they’re typically less common as the primary cause compared to rings and valve sealing, and often present with additional symptoms.