Quicksilver Fuel/Water Separating Filter for Mercury V6 EFI/DFI (1996+) – #35-18458Q4

Mercury and Mariner V6 outboards from the late ’90s (including EFI 2-strokes and Optimax direct injection models) require clean, dry fuel for trouble-free operation. The Quicksilver 18458Q4 is the genuine water-separating filter for these engines, combining filtration and water sensing. It replaces earlier filter setups with a modern spin-on design that includes a water sensor. Installation is straightforward – the filter threads onto the engine’s filter head in place of the old one, and the included O-ring ensures a tight seal. When water is present in the fuel, the float sensor can trigger a dashboard alarm, allowing the operator to act (drain or replace) before water causes damage. By capturing debris and moisture, this filter maintains proper fuel pressure and good injector/carburetor performance. That translates to smooth idling, quick acceleration, and full power when you need it. It’s especially critical for Optimax motors, where fuel cleanliness directly affects the life of the compressor and injectors. Replacing this filter should be part of every annual service (or ~100 hours) for your 2-stroke V6.

• For Optimax Owners: This filter is a vital component of Optimax (direct injection) engine maintenance – those engines run extremely high fuel pressure. Dirty or wet gasoline can lead to costly damage to the fuel rails and injectors. By changing the filter on schedule, you prevent potential issues and keep the Optimax’s outstanding fuel economy and power intact.
• Versatility: The filter is also compatible with some later carbureted 2-stroke V6 Mercurys that were retrofitted with such an external water separator. For those older engines, upgrading to this filter provides the added benefit of a built-in water sensor – something their original filters might lack.
• For Harsh Conditions: If your boat operates in saltwater, tropical climates, or you frequently refuel from varied sources, the risk of water and gunk in fuel is higher. This filter is built for the marine environment – the metal canister is corrosion-resistant, and the internal media can even trap emulsified water. This gives peace of mind during long offshore fishing trips or passages.
• Safety & Spare: It’s recommended to carry a spare 18458Q4 filter on board. If you experience a drop in power or sputtering – which can indicate a filter full of water/debris – you can quickly change it out even at sea. Swapping is simple: turn off the engine, unplug the sensor lead, unscrew the old filter (careful of fuel in the bowl), oil the new O-ring and spin on the new filter, then reconnect the sensor. This minimizes downtime and gets you back to running at full strength.
• Quicksilver Quality: You’re getting a product designed by Mercury for Mercury – that means assured quality. There are Sierra and other aftermarket equivalents, but only Quicksilver guarantees 100% integration with the water alarm system and proper fuel flow characteristics.