Removal, Installation and Inspection of Thermostat

As the engine temperature rises, the thermostat opens the main cooling circuit. If the thermostat remains closed due to a fault, the engine overheats. A sign of this is the temperature gauge in the red zone. In this case, the coolant remains cold. A faulty thermostat may remain open after the coolant has cooled down. A sign of this is that the engine cannot reach operating temperature or the coolant temperature rises more slowly than before, and in winter, a loss of heater output.

* If the engine overheats after a short distance driven, the cause may also be radiator contamination due to limescale deposits.

Removal

Model 525tds
1 — thermostat housing
2 — bleed bolt
3 — thermostat
4 — O-ring
5 — bolt

Models 520i, 523i, 528i
1 — thermostat housing
2 — thermostat
3 — O-ring
4 — gasket
5 — M6 bolt
6 — M8 bolt

PROCEDURE

1. Remove the fan, refer to Fan and Fan Clutch Removal and Installation.
2. Drain the coolant, refer to Coolant Replacement. Coolant Antifreeze Concentration Check. Visual Inspection of Cooling System.
3. Disconnect the hoses from the thermostat housing (on the front side of the cylinder block). To do this, open the clamps.
4. Models 520i, 523i, 528i: Remove the lifting eyes from the engine. Disconnect the cable duct at the clips and remove it. Refer to the accompanying illustration.

5. Unscrew the thermostat cover.

* The mounting bolts are of different lengths. It is recommended to record their lengths.

6. Note the position of the thermostat and remove it.

INSPECTION
1. Heat the thermostat in a container of water. The thermostat must not touch the walls of the container. Measure the temperature with a suitable thermometer. Refer to the accompanying illustration.

2. At approximately +88°C the thermostat valve begins to open. The start of opening can be seen on the thermostat.
3. Check whether the thermostat expands and closes, and whether the opening start temperature matches the value specified on the thermostat. Replace the thermostat if necessary.

INSTALLATION

1. Clean the sealing surfaces on the housing and cover.
2. Models 520i, 523i, 528i: Insert the thermostat into the cylinder head so that the arrow points upward. Refer to the accompanying illustration.

3. 525tds: Insert the thermostat so that the vent valve points upward. The arrow is located in the groove of the cover. Refer to the accompanying illustration.

4. Lightly lubricate the thermostat O-ring with glycerin and install it, refer to the illustration in the "Removal" subsection.
5. 520i, 523i, 528i: A new paper gasket must always be installed.
6. Install the thermostat housing and evenly tighten the mounting bolts to the torque specified in the Specifications.

* Do not overtighten the bolts. Do not forget to install the lifting eyes on the 6-cylinder engine.

Tighten the lifting eye mounting bolts (M8) to 20 N·m.

7. Secure the cable duct with the clips.
8. Install the fan and fan cover.
9. Fill the system with coolant and bleed the air, refer to Coolant Replacement. Coolant Antifreeze Concentration Check. Visual Inspection of Cooling System.
10. Start and warm up the engine so that the viscous fan clutch engages. Check that the lower part of the radiator warms up and check the thermostat housing for leaks. If leaks are present, tighten the mounting bolts.