A sputtering idle or hard-start morning often traces back to one part: the lift pump. Swap in Mercury’s 803529T04 Fuel Pump Assembly and send a clean, steady stream to every EFI injector on your 9.9-30 hp four-stroke. This is the factory diaphragm pump that bolts to the block on small BigFoot, SeaPro, and Jet models—no adapters, no guesswork.

Why the genuine pump makes a difference

• Calibrated diaphragm chamber delivers up to 4 psi at crank speed, keeping the vapor separator full during long troll sessions.

• Twin ¼-in. brass barbs mate to Mercury’s molded fuel lines; stainless spring clips and new O-ring are in the box.

• Lightweight 0.44-lb alloy body resists corrosion yet tucks easily behind the air silencer—3.6 ″ L × 3.0 ″ W × 3.9 ″ H.

• Pre-set check valves maintain prime after lay-up, so cold starts fire on the first twist.

Install is a 15-minute job: two bolts out, hoses off, new pump on, prime the bulb, and you’re back to a silky idle and crisp throttle response. Genuine Mercury parts keep you fishing, not troubleshooting.