Coolant reservoir / expansion tank cap. Porsche Boxster 986 / Boxster 987 / Cayman 987C / 996 / 997

Replacement Coolant reservoir / expansion tank cap.
This coolant expansion tank cap is a direct replacement for Porsche 996 (1998–2005) and also fits Boxster (986/987), Cayman, and 997 models, upgrading earlier versions with improved sealing and pressure control.

Fits:
Porsche 996 1998 - 2005
Porsche 997.1 2005 - 2008
Porsche 986 Boxster 1997 - 2004
Porsche 987.1 Boxster 2005 - 2008
Porsche 987C.1 Cayman 2005 - 2008

Diagram ref no 5

This cap serves as the pressure seal for the coolant expansion tank, keeping the cooling system at the correct pressure. It regulates venting to the overflow reservoir, preventing blowback, leaks, and overpressure while helping avoid coolant loss, boiling, and overheating.

It replaces earlier versions: 99610644700, 99610644701, 99610644702, 99610644703

Why replacement matters:
• An old or worn cap may fail to hold pressure, causing coolant to boil, vaporize, or overflow
• A bad cap can allow air to enter the cooling system, leading to overheating, hot spots, or poor cooling performance
• A leaking or weak cap can lead to gradual coolant loss, requiring refills and risking engine damage

Installing an OE-Match version restores proper pressure control, reliable sealing, and helps maintain engine temperature stability.


Related reference numbers
Related, superseded, cross reference or alternative numbers for comparison.

99610644704

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