Parts for your 1990 Mitsubishi Pajero-Radiator cap

1990 Mitsubishi Pajero Radiator Cap — What it does and how to look after it

Yes, a radiator cap is absolutely used on the 1990 Mitsubishi Pajero. Technical sources including the Mitsubishi workshop manual for late-1980s to early-1990s Pajero models specify a pressurised cooling system with a radiator cap in the 88–108 kPa (around 0.9 bar/13 psi) range. Major AU/NZ parts catalogues from brands like Tridon, Dayco and Gates also list direct-fit caps for 1983–1991 Pajero variants, confirming fitment across common petrol and diesel engines of the era.

On a 1990 Pajero, the radiator cap does more than just close the filler neck. It seals and pressurises the cooling system so coolant can safely run hotter without boiling, which is crucial when towing, crawling off-road, or slogging through a scorching Aussie or Kiwi summer. Inside the cap are two valves: a pressure valve that vents excess pressure to the overflow bottle when the coolant expands, and a vacuum valve that draws coolant back from the bottle as the engine cools. That two-way action maintains the right level in the radiator, protects hoses and the core, and prevents air pockets that can cause hot spots.

When the cap gets tired—weak spring, hardened rubber seal, sticky valves—you’ll often see tell-tales like coolant pushing into the overflow and not returning, overheating at idle or low speed, a collapsed upper hose after cool-down, or crusty deposits around the filler neck. Left alone, a dodgy cap can mimic bigger faults, sending owners chasing water pumps and head gaskets before fixing the simple bit on top.

Good practice for a 1990 Pajero is to check the cap at every service (or about every 10,000–15,000 km) and replace it every 2–3 years, or sooner if any symptoms show. Always match the correct pressure rating (typically 0.9 bar/13 psi for this generation) and the proper cap “reach” for the radiator neck. A quick in-bay cap pressure test is cheap and accurate. Inspect the rubber gasket for nicks or flattening, ensure the spring action is firm, and check the radiator neck for pitting that could stop the cap sealing properly. When removing the cap, only do it stone cold—pop it with a rag for grip, and never crack it when hot. After fitting a new cap, verify coolant level in both the radiator and overflow bottle, and recheck after the next drive cycle.

• Signs it’s time: repeated coolant loss, overflow bottle not returning fluid, hose collapse after cool-down, or overheating at idle.
• What to fit: a quality 0.9 bar cap suited to the Pajero’s filler neck, avoid universal caps that don’t seat correctly.
• Handy tip: if you pressure-test the system, test the cap separately with a cap adaptor for a proper read on its opening pressure.

Q: What pressure rating should the 1990 Pajero radiator cap be?

Most 1990 Pajero models use a 0.9 bar (13 psi) cap, aligning with workshop specs of roughly 88–108 kPa. Always confirm the rating stamped on the old cap or in service information for the exact engine and radiator fitted.

Q: How often should the radiator cap be replaced?

Inspect at every service and replace every 2–3 years, or immediately if there are leaks, weak spring tension, hardened seals, or symptoms like coolant not returning from the bottle or a collapsed top hose after cool-down.

Q: What are the symptoms of a failing radiator cap on a Pajero?

Common clues include overheating at low speeds, coolant being pushed to the overflow and not returning, a sweet smell or white crust around the neck, air in the system, or the upper radiator hose flattening as the engine cools.