From E30 Zone Wiki
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.
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.
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, although many replacement non air con radiators will come with the boss fitted and blocked off with a screwed plug.
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.
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 to start with. Get the engine running, 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.
Now repeat this test with an engine that is significantly overheating. i.e. The temperature gauge needle around the three quarters mark. When the newspaper meets the fan, the newspaper should get shredded. If the fan stops with the same resistance as when cold, then your viscous coupling is broken, and needs replacing.
The coupling should be no more locked on a hot engine (one that is at its normal operating temperature) than a cold one. It's only when the radiator can't cope, and the temperature half way across the core has risen to a significantly higher temperature than normal, that the air coming from this area is hot enough to trigger the coupling.
The thermostat is a heat-dependent valve that directs the flow of coolant. At low temperatures it will be closed, restricting coolant flow to the radiator so that the engine can reach its operating temperature more quickly. Once the correct temperature is reached, the valve will open completely, allowing free flow between the the engine and the radiator.
Thermostats are built to operate at a specific temperature, and a number of different thermostats were fitted to the E30. Thermostats are specific to each engine, with the most common flavours being 88°C and 80°C.
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.
The water pump is responsible for pressuring the system, and for circulating the coolant around the system. The pump is powered by a belt from the crankshaft pulley.
Pumps are service items, and should be replaced with every other cambelt change. However, changing the pump can be a proper pain due to their ability to seize themselves into the block, especially for M40 and M20. Unfortunately, the only solution is to soak them in penetrating oil, and to offer gentle persuasion with a wooden mallet and chisel. For M20 engines, the pump locates one end of the cam belt tensioner spring, making it difficult to change the pump when a cam belt is in place.
When things get hot they expand, and your coolant is no different. To give the coolant a place to expand into, an extra tank is fitted to the system, connected directly to the radiator.
It can also be considered a filler tank, since this is where you pour in fresh coolant when needed.
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. The pressure valves in the original caps would jam up as they aged, preventing excess pressure from venting when the engine got too hot. Because of the increase in pressure, the heater valve would blow and spray the passengers legs with hot coolant. For that reason, BMW introduced a new 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.
To link all of the other components together, a network of rubber hoses runs around the engine. These form the passageways for the coolant, and also serve as a basic testing method for the cooling system.
The most important hoses run to the top and bottom of the radiator. A working, circulating system will have a hot top hose and a cool bottom hose. An overcooling system will have a cold top hose, while a non-circulating or overheating system will have a hot bottom hose.
Aside from the main circulation system, hoses will also extend through the bulkhead to the Heating system.
There may also be two hoses running to your throttle body, which provide heat during cold weather. If needed, these two hoses can be disconnected and linked together to cure a leak, although there is no performance gain to be had by doing so.
In a closed liquid system, any trapped air is a big, big problem. Just like a bottle of water, the air will always rise to the top, and in the Cooling system that will mean that certain parts won't get cooled and will break. To avoid an expensive repair bill, it's essential to get all the air out of the system, and that's know as Bleeding.
Bleeding is crucial when you've changed the coolant as part of a Service. Following the correct bleed technique is essential.
Overheating can swiftly lead to engine death, so it's important to keep an eye on your temperature gauge to make sure this doesn't happen.
When it does, you need to ascertain why.
First, check the coolant level. If it is extremely low, investigate all joints in the system for leaks.
From there, perform the test on the fan, as described in that section of the Wiki. The viscous coupling is a likely culprit at this point.
Next, open the oil cap on the engine, and look at the oil and the rim of the cap. You are looking for a creamy mayonnaise substance, or oil that looks like milky coffee. If you find it, prepare for the worst. Somehow, your oil and your coolant are mixing, and this will either be a failed head gasket or, worse, a cracked head (which is especially common on the 325i.
If your oil is fine, your next step is to check the water pump. A basic check is to warm up the engine, let it idle, and then squeeze the top radiator hose. BE CAREFUL, this hose will be hot! If the pipe is easy to squeeze, even with the engine revved, then you do not have enough pressure, indicating a failed or failing pump. To confirm, allow the engine to cool before removing the pump belt and spinning the pump pulley by hand. The pump should spin freely, but without any wobbles or play in the pulley. If it turns stiffly, or the pulley wobbles, the pump should be removed for a more thorough check.
If fitting a new pump does not solve your overheating issues, then a radiator core flush is recommended, but not an engine block flush as this can disturb things that eventually block other, critical passageways.
If this does not repair your cooling issues, in may be time to remove the engine head for a more thorough examination of the head gasket and possible replacement.
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 symptom is usually that the temp gauge needle barely leaves the blue area; however, don't confuse this with a low temp gauge reading, which isn't really a problem at all.
The cause of this problem lies exclusively with the thermostat. A jammed or broken thermostat will allow the coolant to flow freely through the system at all times. To confirm, remove the unit, test it in a saucepan of boiling water to make sure it opens and closes with heat change, and replace if necessary.
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.
If your leak is inside the car then check out the heating system.
Temp Gauge Reading Low
Many owners, especially those who have just bought their first E30, get nervous when they see the temp gauge not reaching the middle point, and immediately jump to the conclusion that the car is overcooling. But hold your horses, it's not actually a problem.
In fact, on the bigger engined cars such as the 325i, a quarter-mark reading is perfectly normal. Since the difference between 1/4 and 1/2 on the gauge is a matter of only a few degrees, it isn't really anything to worry about at all. So calm down.
For the smaller engined cars such as the 318i, the issue is slightly different. Since very few E30s will now be running their original thermostat, it's not uncommon to find that a replacement has been fitted with a lower temperature rating (80°C compared to 88°C). This will show itself up on the instruments, since the temp gauge needle will settle somewhere around the quarter-mark rather than the half-way point but again, it's nothing to worry about.
Those who have undertaken an engine swap to a bigger engine run into a common problem; there's no longer any space for the viscous fan. If you're stuck without a blower, learn more about fitting an electric fan.
Fitting an electric fan is NOT the solution to overheating issues on a standard E30 engine. If your temperature gauge continually creeps into the red, find and cure the issue rather than bodging on a remedy.