During a compression test on a marine diesel cylinder, which sequence is performed?

• A. Disable fuel and ignition, remove glow plugs, crank through several revolutions, record maximum compression
• B. Remove glow plugs, crank before disabling fuel, record minimum compression
• C. Disable ignition only, crank until gauge reads, ignore glow plugs
• D. Disable fuel and ignition, remove glow plugs, crank through several revolutions, record maximum compression

Answer: (D)

To get an accurate compression reading, you want to measure the cylinder’s inert gas pressure during the compression stroke without any combustion interfering. That means turning off both the fuel supply and the ignition system so the engine can’t fire, and removing the glow plugs so they don’t affect the compression path or cause extra heating. With the compression gauge connected in the glow-plug hole, crank the engine through several revolutions until the gauge shows a stable peak pressure, and record that maximum compression.

This maximum reading is the meaningful metric for comparing against specs and evaluating cylinder health. Leaving fuel or ignition active could cause the cylinder to fire or spray fuel, skewing the reading, and leaving glow plugs in place can introduce heat or extra resistance that distorts the result.