• The waterpump (centrifugal type) is driven by a belt. It allows the coolant to circulate.
• The thermostat controls the coolant flow into the radiator. Closed when the engine is cold (nothing goes in the radiator, open when the engine gets warm to flow the coolant inside the radiator and cool it down.
• The primary coolant fan is mounted to the front of the waterpump with the fan clutch.
• The fan clutch is a viscous fluid type that control the speed of the fan based on engine compartment.
• A two speed auxiliary cooling fan is mounted in front of the condenser. It is uses when the A/C operates and when coolant temperature exceeds 196F (91C)
• The fan shroud is also an important item not to overlook: the presence of a shroud, with about 1/2″ to 3/4″ clearance between the tips of the fan blades & the shroud opening, and the fan inserted about 1/2 way into the shroud opening is essential for efficient cooling performance.
First: make sure that your radiator is in good condition, check the belts, the hoses and the coolant level. Make sure that there is no coolant leaks. Check the condition of the shroud for cracks & its proper installation.
Symptoms
Overheating with A/C on, or very high temperature only.
1. Check that the auxiliary fan is working properly(Bentley manual page 170-4 from Here).
2. Check the fan clutch.
Car fluid or coolant circuit element just changed:
Your overheating is probably due to air trapped in the coolant circuit. Use the bleed screw to bleed the circuit. Then bleed again, again, again, and again.
Car overheating while driving. Not stop and go:
1. Bleed the cooling circuit.
2. Check the thermostat.
3. Check the water pump.
4. On the 535i, check the bypass hose located and attached just above the upper hose neck on radiator and runs along the inner walls to the Resivior tank. If it is plugged, the car will overheat.
Make sure to check over your upper and lower radiator hoses. Make sure they aren’t worn or leaking. We recommend replacing them if this is the first time touching your cooling system.
More Information
Tests to Run:
How to check the thermostat:
A slow-working thermostat is hard to diagnose.
When the engine is warming up and the temperature needle is near the center, touch the top radiator hose. If hot, the thermostat is working (could still be a slow one). If there is a cold area in the hose or radiator, check the thermostat.
Overheating Quick Trouble Shooting!
0 – Check the coolant level
1 – Check if the thermostat works
2 – Headgasket replacement
3 – Water pump replacement
4 – Check the head for cracks
If you’ve got BMW M50 B25 (525i after 89) engine the order is a bit changed! (these models tend to have a plastic impeller on the water pump from the factory which suddenly walls down from the shat that should drive it … so you actually end up with water not moving around at all in the engine – thus the extremely rapid overheating)
0 – Check the coolant level
1 – Check if the thermostat works
2 – Check the waterpump
3 – Check the fan clutch
4 – Check the cooling fans for the radiator
5 – Replace headgasket
6 – Check the head for cracks
How to Test the Clutch Fan:
1) Make sure that the thermostat is working!
2) When the car is cold, the fan clutch should turn with small resistance and not spin freely.
3) Let the car idling and the hood not closed but down. After 5-10 min, the temperature will rise at the 1/2 mark, check that the fan is blowing a LOT of air inside the engine compartment and increasing in noise level, no need to put your hands, you will feel it. If when the temperature hits the 3/4 mark, the fan is not blowing a lot of air, that means that: either the thermostat is not working or slow, or the fan clutch is bad.