High-quality replacement expansion tank cap designed for Citroën and Peugeot cooling systems. This 1.4 BAR pressure cap maintains the correct system pressure, raising the coolant boiling point and ensuring reliable circulation between the radiator and the overflow reservoir. Frequently searched by product codes, this cap is a practical, cost-effective part for workshops and DIY mechanics who need a quick, reliable fix for pressure- and leak-related cooling problems.

The cap is ideal for routine maintenance or immediate replacement when the cooling system fails to hold pressure, coolant is lost from the reservoir, or the engine shows signs of localized overheating. Installation is straightforward for experienced technicians and competent DIYers: no special tools are required beyond basic workshop equipment, but always follow safe procedures when working on a hot cooling system.

Technical Information

• Manufacturer: Stellantis
• Model: Citroën, Peugeot
• Product Codes: 1304E4, 1306E4, 1306C9
• Additional Numbers: 13040000

Function And Benefits

The expansion tank cap is a spring-loaded valve assembly with a sealing gasket that keeps the cooling system at a constant pressure (1.4 BAR). By maintaining this pressure the cap:

• Raises coolant boiling point to prevent localized boiling and vapor locks.
• Keeps coolant in the pressurized loop while allowing excess fluid to flow to the overflow reservoir when pressure rises.
• Allows vacuum return of coolant from the overflow reservoir as the system cools.

These functions help prevent engine overheating, maintain consistent heater performance, and protect hoses and joints from excessive stress.

How To Replace (Basic Guide)

• Allow The Engine To Cool Completely Before Any Work. Never Remove The Cap From A Hot System.
• Wear Protective Gloves And Eye Protection.
• With The Engine Cold, Unscrew The Cap Anti-Clockwise. Some Caps Require Downward Pressure Before Turning To Release The Valve.
• Inspect The Reservoir Neck, Sealing Surface And Gasket. Clean Any Deposits Or Corrosion.
• Fit The New Cap, Ensuring The Seal Seats Correctly, And Turn Until Fully Seated. Do Not Over-Tighten.
• Refill Coolant If Required, Bleed Air From The System Per Manufacturer Procedure, Run Engine And Recheck For Leaks And Correct Level.

Installation Recommendations

Always follow these precautions to ensure safe and correct installation:

• Perform Replacement When The Engine Is Cold To Avoid Scalding From Pressurised Coolant.
• Inspect The Reservoir Neck And Surrounding Hoses For Damage Or Deformation; Replace Damaged Parts To Ensure Proper Sealing.
• Use Coolant And Bleeding Procedures Recommended By The Vehicle Manufacturer To Avoid Air Pockets.
• Check The Cap’s Pressure Rating (1.4 BAR) Matches System Specification—using a wrong-pressure cap can cause overheating or coolant starvation.

Why This Part Most Commonly Fails

Expansion tank caps typically fail due to age-related wear and material degradation. Common causes include:

• Rubber Gasket Deterioration: The seal hardens or cracks, allowing pressure loss and leaks.
• Spring Fatigue Or Valve Wear: The internal spring and valve can lose their calibrated tension, preventing the cap from holding the correct pressure.
• Chemical Attack Or Deposits: Contaminated or old coolant can leave deposits that interfere with the sealing surface or valve movement.
• Physical Damage: Impact or improper removal can deform the cap or reservoir neck, leading to poor sealing.

If the cap fails to hold 1.4 BAR, the cooling system may boil at lower temperatures, causing overheating, coolant loss, or air ingress into the system.

Note: Customers frequently search for this part by product code. Verify the code printed on your old cap to ensure correct replacement.