Type 25 Thermostat Housing Top - 1986-92 - Waterboxer Engines

This replacement thermostat housing (cover) is designed to fit late-model 1900cc and 2100cc petrol Waterboxer engines installed in VW Type 25 / T3 buses from 1986 to 1992. It is the correct housing used on later cooling system layouts and provides a secure mounting for the thermostat while sealing it against the engine.

On Waterboxer engines, the thermostat housing plays a critical role in coolant flow regulation. The thermostat sits inside this housing and controls when coolant is allowed to circulate through the radiator. When the engine is cold, the thermostat remains closed to aid rapid warm-up. As operating temperature is reached, it opens and allows full coolant circulation. A sound housing is essential to keep the thermostat correctly positioned and fully sealed.

Over time, original housings can suffer from corrosion, pitting, warping, or damaged sealing faces. Any of these issues can cause coolant leaks, incorrect thermostat operation, or difficulty bleeding the cooling system. This replacement restores correct fit and function, making it ideal for repairs, cooling system overhauls, or engine rebuilds.

What this part does and how it works

The thermostat housing bolts to the engine and clamps the thermostat in place. An O-ring seal sits between the housing and the thermostat, creating a watertight seal. Coolant flow is then directed according to thermostat position, maintaining stable engine temperature and protecting against overheating.

Fitment

• Vehicle: VW Type 25 / T3

• Years: 1986–1992

• Engines: 1900cc & 2100cc petrol Waterboxer

• Application: Thermostat housing / cover (late type)

Engine code compatibility

• DG

• DJ

• MV

• SP

• SR

• SS

Important fitting note

• O-ring seal is NOT included

Related parts (sold separately)

• Thermostat including O-ring seal: 025121113F

• O-ring seal only: N90136802

Replacing the O-ring whenever the housing is removed is strongly recommended to ensure a proper seal and prevent future coolant leaks.

Fitting notes

• Clean all mating surfaces thoroughly before installation

• Check the thermostat for correct operation while access is available

• Torque housing fasteners evenly to avoid distortion

• Refill with the correct coolant and bleed the system fully after fitting

A vital cooling system component that ensures correct thermostat operation, leak-free sealing, and reliable temperature control on late Waterboxer Type 25 / T3 buses.