Parts for your 1998 Ford Falcon-Radiator cap

1998 Ford Falcon radiator cap: not fitted — here’s what it actually uses

Technical sources for the AU-series Falcon — including the Ford AU Falcon Workshop Manual (Cooling System, 1998), Gregory’s Service and Repair Manual No. 273 (Ford Falcon AU 1998–2002), and Ford service training materials of the era — confirm there is no radiator neck or traditional radiator cap on a 1998 Ford Falcon. Instead, the cooling system is a closed, pressurised setup that uses a pressure cap on the plastic expansion/degas bottle (coolant reservoir) under the bonnet. The radiator itself is sealed.

This design moves the pressure and vacuum control away from the radiator and onto the reservoir. The pressure cap on the degas bottle maintains system pressure (typically around 135–140 kPa), helps raise the coolant’s boiling point, and allows coolant to move to and from the bottle as temperatures change. It also helps purge air from the system, improving reliability and reducing hot spots in the alloy head during everyday driving across Aussie and Kiwi conditions.

So, if someone’s hunting for a “radiator cap” for a 1998 Falcon, the bit they actually need is the expansion tank pressure cap. Fitting a cap to the radiator isn’t applicable because there’s no filler neck on the radiator to begin with — and many replacement radiators for AU models remain sealed in the same way.

For servicing, attention should shift to the coolant reservoir cap and bottle:

• Use the correct pressure-rated cap for AU Falcons (commonly around 20 psi/138 kPa). A cap that’s too low can cause boil-over, too high can stress hoses and the bottle.
• Inspect the cap’s rubber seals for cracking, flattening, or swelling. Replace at the first sign of wear.
• Only open the cap when the engine is stone cold. Wrap a rag around it and release pressure slowly.
• Top up with the correct Ford-approved coolant mix — avoid plain water and don’t mix incompatible coolant types.
• If the bottle is stained, brittle, or cracked, replace it along with the cap.

Typical signs of a failing cap on an AU Falcon include coolant loss without obvious leaks, a collapsed upper hose after cool-down, fluctuating temperature readings, or a sweet coolant smell under the bonnet. Addressing the cap and bottle promptly keeps the cooling system healthy and your Falcon happy on long kilometres.

Popular questions

Does a 1998 Ford Falcon have a radiator cap?
No. The AU-series Falcon uses a pressure cap on the expansion/degas bottle, and the radiator itself is sealed. That’s by design and confirmed in AU-series workshop manuals.

What pressure cap does a 1998 Falcon need?
Most AU Falcons specify a cap around 135–140 kPa (about 20 psi). Always match the rating shown on the original cap, the bottle label, or the workshop manual to prevent cooling issues.

Where is the cap located and how do you open it safely?
It’s on the plastic coolant reservoir (degas bottle) near the radiator. Only open it when the engine is completely cold, use a rag, turn slowly to the first stop to bleed any residual pressure, then remove.