Parts for your 2018 Toyota Mark x-Radiator cap

2018 Toyota Mark X radiator cap — what it does and how to look after it

Based on technical references, the 2018 Toyota Mark X (GRX130 series, 4GR-FSE/2GR-FSE) does use a conventional pressurised radiator cap. Toyota’s Repair Manual for the GRX130 platform includes an “Inspect Radiator Cap Sub-Assembly” procedure, and the Toyota Electronic Parts Catalogue lists a “Cap, Radiator (Radiator Cap Sub-Assy)” for 2016–2019 Mark X models. Typical Toyota petrol applications specify a cap stamped around 108 kPa (1.1 bar), but the cap itself is the authority for the correct rating.

On the 2018 Toyota Mark X, the radiator cap is a small but crucial bit of kit. It seals the cooling system so pressure can build as the engine warms up, which lifts the coolant’s boiling point and keeps things stable under the bonnet on hot days or long motorway runs. Inside the cap are two valves: a pressure valve that pops at a set rating to protect hoses and the radiator, and a vacuum valve that lets coolant draw back from the overflow bottle as the engine cools. That “breathe in, breathe out” action stops hoses from collapsing and helps the Mark X maintain the right coolant volume without air sneaking in.

Because the cap is the gatekeeper of system pressure, a tired spring, nicked rubber seal, or corroded seat can cause hard-to-trace dramas — slow overheating, random coolant loss, or a heater that goes cold at idle. It’s why Toyota has a specific test for the cap in the workshop manual, and why any solid service on a Mark X includes checking it.

Practical servicing advice? Treat the radiator cap as a serviceable item. Have it inspected at every coolant service or major service. A quick pressure test to the rating on the cap confirms whether the spring and seals still hold. If the cap is more than five years old, has perished rubber, or fails a test, replacing it is cheap peace of mind. Always match the correct pressure rating and neck type, a cap that’s too low can allow boil-over, while too high can stress the radiator and hoses.

When removing the cap, never do it hot — wait until the engine is cool to the touch, wrap a rag over the cap, and ease it to the first detent before fully removing. Wipe the filler neck clean, lightly wet the new cap’s seal with coolant, and twist it home firmly to the second stop. While you’re there, check the overflow hose for cracks and confirm the expansion bottle level is on the mark with genuine Toyota Super Long Life Coolant (pink). After any cooling work, bleed air per the repair manual. Done right, the Mark X’s cooling system will stay happy through Aussie summers and Kiwi alpine runs alike.

What pressure rating is correct for a 2018 Toyota Mark X radiator cap?

Most GRX130 Mark X petrol models run a cap stamped around 108 kPa (1.1 bar). Always read the stamping on the existing cap and confirm against the owner’s manual or parts catalogue for the exact variant. Using the correct rating maintains the designed boiling margin and hose/radiator protection.

Go genuine or a high-quality equivalent that matches both pressure and cap style. Avoid “universal” caps that don’t lock correctly on the Toyota filler neck.

How often should the radiator cap be replaced on a 2018 Mark X?

There’s no hard interval, but it should be inspected at every service and pressure-tested whenever the coolant is changed. Many shops recommend replacement around 5 years or 80–100,000 km, or immediately if the rubber seal is cracked, the spring feels weak, there’s corrosion on the seat, or it fails a pressure test.

Given the low cost and high consequence of a dud cap, proactive replacement is smart maintenance.

Where is the radiator cap on the 2018 Mark X, and is the overflow cap pressurised?

The pressurised radiator cap sits on the radiator’s filler neck under the bonnet. The translucent expansion/overflow bottle nearby has a simple non-pressurised cap. Only the radiator cap controls system pressure, don’t mix them up.

If visibility is tight, follow the upper radiator hose to the radiator tank — the cap is on or near that neck.