Cooling

Your engine is powered by a rapid series of controlled explosions. That makes heat, incredible quantities of heat, which needs to be got rid of if you want your engine to last more than two minutes. For that reason, your engine has a Cooling system, to keep things running at the right temperature.

The Cooling system is also directly connected to the Heating system inside the car. Learn more about the heating system.

Overview

Heat is a major factor in the running of the engine. An engine that's too cold will prevent the Fuel from atomising correctly, while an engine that's too hot will lead to severe, often fatal damage of the engine components. To get the most out of your engine, the temperature needs to be warm but stable.

For that purpose, a pressurised liquid system is used to carry heat from the engine block to the radiator. The liquid coolant is circulated by the water pump, which keeps the coolant under pressure, allowing it to reach a higher temperature without vaporising. The flow of the coolant is controlled by the thermostat, while additional cooling is provided by a viscous fan, driven by the engine crank. An expansion tank is fitted to give the hot coolant space to expand into.

Although the system is sealed, vents and pressure valves will allow the coolant level to drop over time, so it will need to be topped up on occasion. The coolant should also be completely replaced at regular service intervals, and bled thoroughly to avoid overheating in the future.

Temperature of the system at any time is shown on the temperature gauge.

Components

Radiator

The radiator is the core of your cooling. While there were a variety of radiators fitted to the E30, they are all of a standard construction featuring top and bottom tanks, bridged by core pipes covered in cooling fins. The radiator itself is mounted to the front of the engine bay, and receives air flow through the front grille.

Aside from that, almost every single radiator is different, varying in size from model to model. Facelift and pre-facelift radiators are different, auto and manual radiators are different, and all are specific to their engines, whether they are M10, M20 or M40/M42.

Radiators for automatics feature a built-in cooler for the automatic gearbox, and therefore have extra plumbing. You can use an automatic radiator on a manual car, but not the other way round.

Aircon-equipped cars also have different, larger radiators. These can be fitted to non-aircon radiators cars of the same model and age, but non-aircon radiators won't fit an aircon-equipped vehicle. The difference is the mounting "boss" on one side of the radiator, which holds the temperature switch for the aircon electric fan.

Shroud

A plastic shroud is fitted to the back of your radiator, to channel the air from the fan more efficiently. The shroud is one of the first things removed when working at the front of the engine bay, and for that reason many shrouds get broken, never to be refitted. This has a significant impact on the efficiency of the cooling system. If your shroud is broken or missing, source a replacement immediately.

Fan

The fan fitted to all E30s is driven by the engine, via the fan belt, and a viscous coupling. This viscous coupling allows the fan to just freewheel most of the time - the forward motion of the car is enough to cool the radiator. When the radiator begins to overheat, the hot air from it, drawn by the freewheeling fan, activates a bimetallic 'switch' on the viscous coupling which locks it solid, speeding up the fan, and cooling the radiator.

If your car starts overheating, especially in traffic, the viscous coupling is usually to blame. For that, we have a basic test:

To test the viscous coupling, you need a cold engine. Start the engine, and then gently try to stop the fan blades turning with a loosely rolled up newspaper. When the engine is cold you should be able to easily stop the fan. If it shreds the newspaper when run from cold your viscous coupling is broken, and needs replacing.

Thermostat

The thermostat

Thermostats can break, and often do, leading to overcooling. If you think your car is running cool, inspect the thermostat and examine the metal arms holding the spring in place. If one of these has snapped, it will allow water to run through with no restriction. Therefore, the thermostat will need to be replaced.

Expansion Tank

Tank Cap

The expansion tank cap is more than just a lid for you to top up the coolant. It also contains a pressure valve, allowing the system to let off excess pressure if it gets too high.

In 1997 BMW issued a safety recall on the tank cap. Because of the high pressure, the heater matrix pipe would blow a valve and spray the passengers legs with hot coolant. For that reason, BMW introduced a lower-pressure cap, rated at 1.4 bar. If there's a chance that your car is still running its original tank cap, take your VIN to BMW and claim your new, free tank cap.

That is, unless you have a 318iS. These cars use a 2.0 bar cap as standard.

Common Problems

Overheating

Overheating can swiftly lead to engine death, so it's important to

Overcooling

Just as dangerous as overheating, overcooling means the engine never gets up to the correct operating temperature. That leads to rich running, poor performance and increased engine wear.

The cause of this problem lies exclusively with the thermostat.

Leaks

Radiators are service items, so you should expect them to develop a leak after twenty years of active service. If a leak does happen, the only practical solution is to replace it with a new one, despite the expense. In the event of a leak while driving, external fixes such as glues or two-pack seals can be used to botch a repair, but this is purely a temporary fix. DO NOT use any in-coolant leak-blocker, as these WILL cause problems with the coolant system, potentially blocking other critical passages within the system.