Parts for your 2010 Mazda 6-Radiator cap

2010 Mazda 6 radiator cap — what’s actually fitted

On the 2010 Mazda 6 (GH series), there isn’t a traditional radiator cap on the radiator itself. Instead, Mazda uses a sealed radiator and a pressurised coolant reserve/expansion tank with its own pressure cap. So if someone’s hunting for a “radiator-cap” on the actual radiator, it won’t be there — the job’s handled by the cap on the plastic tank.

This layout is confirmed in Mazda’s technical literature: the Mazda6 GH Workshop Manual (Cooling System, 2009–2012) shows the radiator without a filler neck and identifies a “reserve tank cap” as the pressure control point, with a specified cap rating typically around 110–137 kPa (approx. 1.1–1.4 bar, engine and market dependent). The 2010 Mazda6 Owner’s Manual backs this up in the Engine Coolant section, instructing checks and top-ups at the reserve tank and warning to never remove the “coolant reserve tank cap” when hot — there’s no instruction to open a radiator cap because there isn’t one. Major aftermarket catalogues list a pressure cap for the expansion/reserve tank on 2008–2012 Mazda 6, not a cap on the radiator neck.

Why Mazda doesn’t use a radiator cap here:

• Packaging and serviceability: a remote, pressurised tank makes the radiator lower and slimmer under the bonnet and gives a single, easy fill point.
• Better air purging: constant degassing to the tank helps keep air out of the system for stable temps.
• Reliability and emissions: a sealed radiator reduces disturbance and helps maintain consistent operating pressure and temperature.

For owners, the practical upshot is simple: treat the cap on the coolant reserve tank as the system’s pressure cap. If there are signs of a weak cap (coolant pushing into the overflow and not returning, soft hoses collapsing after cool-down, or gradual overheating under load), replace the reserve tank cap with the correct pressure rating shown on the cap or specified for the VIN. Always open it only when the engine is cold, and use Mazda FL22 (or equivalent long-life coolant) mixed to the right ratio. If unsure about the exact cap rating for the specific engine (2.5 petrol, 3.7 V6, or diesel where applicable), check the label on the fitted cap or the workshop manual for that variant.

• Where is the radiator cap on a 2010 Mazda 6?
  It doesn’t have one on the radiator. The pressure cap sits on the coolant reserve/expansion tank. That’s the only cap you open for filling, bleeding, or testing system pressure.
• What pressure rating is the correct cap?
  Mazda workshop specs for the GH series list roughly 110–137 kPa (about 1.1–1.4 bar), varying by engine and market. Check the number stamped on the existing cap or confirm by VIN before ordering.
• How should coolant be topped up?
  Top up only via the reserve/expansion tank when the engine is cold. Fill to the “FULL” mark, run the engine with the heater on HOT to purge air, let it cool, and re-check the level. Do not open the cap when hot.