Parts for your 2018 Ford Mondeo-Radiator cap

2018 Ford Mondeo radiator cap — not fitted, and here’s why

The 2018 Ford Mondeo (Mk5/CD391) does not use a traditional radiator cap on the radiator itself. Instead, the cooling system is a sealed, pressurised setup that fills and vents through a remote coolant expansion (degas) bottle with its own pressure cap. This layout is detailed in Ford’s Workshop Manual (WSM) for the 2018 Mondeo, Section 303-03 Engine Cooling, which specifies filling via the degas bottle and notes a sealed radiator without a service filler neck. The 2018 Mondeo Owner’s Manual also cautions against opening the coolant reservoir cap when hot, reinforcing that the reservoir cap is the system’s only service opening. Motorcraft service procedures for Mondeo/Fusion of this generation describe the same degas-bottle-first process.

Why a radiator cap isn’t used on this model:

• Modern sealed system: A remote degas bottle maintains system pressure and coolant volume without a filler on the radiator.
• Better air separation: The high-mounted bottle allows air to purge more effectively, helping reduce hotspots and cavitation.
• Packaging and safety: With low-profile front ends, removing the radiator filler neck improves packaging. Having a single, shielded cap at the bottle is safer and tidier under the bonnet.
• Consistent servicing: One cap means one service point for checks, pressure control and top-ups.

What owners are really after when searching “radiator cap” for a 2018 Mondeo is the coolant expansion/degas bottle cap. That cap holds system pressure (the rating is printed on the cap, commonly around 140 kPa/20 psi) and is a genuine wear item. As part of routine servicing, the cap should be inspected for brittle plastic, a cracked or flattened rubber seal, weak spring action, or signs of dried coolant crust. If there’s unexplained coolant loss, hoses that collapse as the engine cools, or boiling into the overflow, the cap is a prime suspect.

Servicing tips for the degas bottle cap on a 2018 Mondeo:

1. Only remove the cap when the engine is completely cool — the system is pressurised.
2. Wipe the neck and cap seal clean before refitting to prevent weeps.
3. Screw the cap on until it seats firmly, don’t overtighten.
4. Use the correct Ford-approved coolant for the vehicle’s VIN and climate, and mix ratio as specified in the Owner’s Manual.

Bottom line: there’s no radiator-mounted cap on the 2018 Ford Mondeo. The correct service part is the coolant expansion (degas) bottle cap, and keeping it in good nick helps maintain pressure, raise the boil point, and keep the Mondeo running cool across Aussie and Kiwi conditions.

• Does the 2018 Ford Mondeo have a radiator cap?
  No. It uses a sealed cooling system with a pressure cap on the coolant expansion (degas) bottle, not on the radiator.
• What should be bought instead of a radiator cap for this model?
  The correct part is the coolant expansion/degas bottle cap, matched to the Mondeo’s engine and year.
• When should the expansion tank cap be replaced?
  It should be inspected at every service and replaced if the seal is damaged, the spring is weak, or there are symptoms like coolant loss, hose collapse after cool-down, or overflow after short drives. Many owners choose preventative replacement around 5–7 years.