Koyo 08-15 Mitsubishi Evolution X Aluminum Filler Neck

Billet Aluminum Coolant Filler Neck for the 2008-2015 Mitsubishi Lancer Evolution X

The Koyo FN2316 is a billet aluminum replacement for the factory plastic coolant filler neck on the 2008 through 2015 Mitsubishi Lancer Evolution X (CZ4A) with the 2.0L turbocharged 4B11T engine. The OEM plastic filler neck is subject to thermal fatigue over time, as the neck is directly exposed to the pressurized coolant circuit and repeated heat cycling from the turbocharged engine.

Cracks form in aged plastic filler necks at the sealing surface and hose interface, producing slow coolant seeps that are often unnoticed until the cooling system loses pressure consistency. The FN2316 eliminates that failure mode entirely with a precision-machined billet aluminum body that resists deformation at elevated coolant temperatures and pressure.

The neck uses a direct-replacement fitment with the factory hose connection and mounting, so the swap requires no modification to the existing cooling system plumbing. One important note: the FN2316 converts the radiator cap interface to a deep-plunger style cap; the OEM cap is not compatible and a replacement deep-plunger cap must be sourced separately before installation.

Key Features

• Billet aluminum construction eliminates the thermal fatigue cracking that affects the OEM plastic filler neck on the 4B11T-powered Evo X
• Direct-replacement fitment; uses factory hose connections and mounting points with no modification required
• Converts to a deep-plunger style radiator cap interface for improved sealing pressure retention
• Compatible with OEM coolant hoses and clamps; no new hoses required
• Precision-machined sealing surfaces maintain consistent cap seal under high operating pressure
• Adds a clean factory-finish aluminum appearance to the Evo X engine bay
• Fits all transmission variants of the 2008-2015 Evo X including manual (5MT) and TC-SST

This Part Fits

Product Specifications

What Is Included

• 1x Koyo FN2316 billet aluminum coolant filler neck
• Radiator cap is NOT included; a deep-plunger style cap compatible with the Evo X cooling system is required and must be purchased separately

Frequently Asked Questions

What radiator cap do I need with the Koyo FN2316 on the Evo X?

The FN2316 converts the filler neck interface to a deep-plunger style radiator cap. The factory OEM Mitsubishi radiator cap uses a different sealing design and will not fit. Koyo sells a compatible deep-plunger style cap; their part number SK-C13 is widely compatible with this style of neck. Confirm the cap pressure rating matches your cooling system specifications (typically 1.1 bar / 16 psi for the stock Evo X cooling system) before installing.

Will the Koyo FN2316 work with the Lancer Ralliart as well as the Evo X?

The FN2316 is listed specifically for the Mitsubishi Lancer Evolution X (CZ4A) with the 4B11T engine. The Lancer Ralliart uses a different 4B11 variant with a different cooling system routing. Verify fitment directly with Koyo or your vendor before ordering for a Ralliart application, as the filler neck geometry may differ between platforms.

Will the factory OEM coolant hoses and clamps work with this aluminum filler neck?

Yes. The FN2316 is designed as a direct replacement that retains the factory hose connection diameter and mounting interface. The OEM hoses and clamps can be reused without modification. Inspect the OEM hoses for cracking or hardening before reinstalling them, and replace them if they show signs of age-related deterioration.

How difficult is it to swap the Koyo FN2316 onto an Evo X?

The swap is straightforward and can typically be completed by an experienced home mechanic. The process involves draining a portion of the coolant, removing the OEM plastic filler neck by unclipping the hose and unbolting the neck from its mount, and installing the FN2316 in its place. The entire job generally takes under an hour. The system must be refilled, bled, and pressure-tested after installation.

Is the OEM plastic filler neck actually prone to failure on the Evo X, or is this a preventive upgrade?

For stock-power street Evo X builds in mild climates, the OEM plastic neck can last the life of the vehicle without issue. However, on modified Evo X builds with elevated coolant temperatures and pressure, or on vehicles in high-heat climates where the plastic experiences repeated extreme thermal cycles, the neck is a documented weak point. Many Evo X owners performing a cooling system refresh opt for this upgrade as a preventive measure, especially when replacing the radiator or performing a major engine service.

Does this upgrade require any coolant flush or system modifications?

No changes to the coolant type or system configuration are required. The FN2316 is a direct replacement. A partial coolant drain and refill is necessary to complete the swap, and the system should be bled of air bubbles after refilling. Refer to the Mitsubishi Evo X service manual for the correct coolant bleed procedure on the 4B11T engine to ensure no air pockets remain in the system after reassembly.

Can this filler neck be used with an upgraded Koyo aluminum radiator on the Evo X?

Yes. The Koyo aluminum racing radiator for the Evo X (V2979) features a billet filler neck incorporated into the radiator itself. If you are using the Koyo V2979 radiator, check whether it already includes a compatible filler neck connection before ordering the FN2316 separately, as the radiator may already integrate the aluminum neck design. On stock radiators or other aftermarket radiator brands, the FN2316 is the correct standalone upgrade.

Does Koyo provide a warranty on the FN2316?

Koyo provides a manufacturer warranty covering defects in materials and workmanship on this filler neck. Under the Magnuson-Moss Warranty Act, installing an aftermarket aluminum filler neck cannot void your vehicle's manufacturer warranty on unrelated systems. The dealer must demonstrate a direct causal link between the aftermarket component and any claimed failure to deny coverage under that act.