Parts for your 2018 Mazda 3-Radiator cap

2018 Mazda 3 radiator cap — what it does and how to look after it

Based on Mazda technical references (2018 Mazda 3 BN workshop manual cooling system section, Owner’s Manual guidance, and Mazda EPC parts listings), this model does not use a traditional cap on the radiator neck. Instead, the system is sealed at the radiator and uses a pressurised cap on the coolant reserve tank (degas/expansion bottle). So yes, a “radiator cap” is relevant on a 2018 Mazda 3 — it’s simply located on the coolant tank, not the radiator itself.

That pressurised cap is a small but critical bit of kit. It holds the cooling system at a set pressure (typically around 1.1 bar/110 kPa, depending on engine variant), which raises the coolant’s boiling point and keeps temperatures stable under load. It also controls the flow of coolant to and from the reservoir as the engine heats and cools, preventing air pockets and keeping the system topped up properly.

For a 2018 Mazda 3, servicing the radiator cap is simple and well worth it. Only remove it when the engine is completely cold — under the bonnet, a hot cap can release scalding coolant. During routine servicing, check the cap’s rubber seals for cracks or flattening, make sure the spring-loaded valves move freely, and look for any dried coolant crust around the neck that can stop it sealing. If the cap shows wear, replace it with the correct pressure-rated cap specified for the BN-series Mazda 3. Avoid cheap, off-spec caps — the wrong pressure can trigger overheating, hard hoses, or slow leaks.

Common signs the cap’s had its day include:

• Coolant pushing into or overflowing from the reservoir after drives
• Collapsed or rock-hard radiator hoses as it warms and cools
• Gurgling sounds, slow warm-up, or the temp gauge creeping up under load
• Sweet smell or stains around the tank cap area

As a rule of thumb, many workshops will test or replace the cap every 4–6 years or 80–100,000 km, especially if the coolant’s being renewed. On this Mazda, the cap lives on the coolant reserve tank — easy to reach, easy to swap. Match the pressure rating, top up with the correct Mazda-approved coolant, and bleed per workshop procedure if the system’s been opened. Keeping that little cap healthy goes a long way to keeping the Skyactiv engine happy, whether it’s commuting or heading off on a weekend run.

Popular questions

Where is the radiator cap on a 2018 Mazda 3?
It’s on the coolant reserve (expansion) tank, not on the radiator itself. Look for the black warning-labelled cap on the plastic tank in the engine bay. The radiator is sealed from factory.

What pressure rating should the cap be?
Most BN-series 2018 Mazda 3 Skyactiv-G models use a cap around 1.1 bar (about 110 kPa). Always match the rating to the vehicle’s spec label or trusted parts listing. Using the wrong pressure can lead to overheating or hose issues.

How often should the cap be replaced?
If there are no symptoms, many techs recommend testing or replacing it every 4–6 years or 80–100,000 km, or any time the coolant is renewed. Replace sooner if there are leaks, damaged seals, or pressure test failures.