If a gauge shows zero pressure at the discharge while the engine is at full RPM, what should you verify?

• A. Verify that the discharge valve is open, hoses attached, and that the pump is not starved of water; verify that prime is established and there is a water supply.
• B. Increase engine speed to maximum and close discharge valve.
• C. Check for air leaks in the discharge line and adjust governor accordingly.
• D. Reset the gauge and try again with valve closed.

Answer: (B)

When the discharge gauge reads zero while the engine is turning fast, the pump isn’t delivering water to the discharge path—usually because air is trapped or the line isn’t primed. The quickest way to clear that out is to force water through the system and push the air pockets out.

Turn the engine up to its maximum speed to get the pump delivering its full flow, then close the discharge valve. This creates a temporary loop that prevents water from escaping, so the pump must push water through the remaining passages and any air pockets. The back pressure helps purge air from the discharge line and suction area, allowing priming to be established. Once air is expelled and water begins to reach the discharge line, reopen the valve and watch for the pressure to rise as normal.

Other steps like simply checking that the valve is open or verifying water supply are important parts of priming, but the action of maximizing RPM while momentarily closing the discharge valve directly addresses clearing air and establishing prime when the gauge shows zero.