The summer is here, and also – overheating problems of the engines. In this entry – basics of cooling for all most popular BMW engines.


About basic things – there is a lot of information on the Internet. So I will put an accent on practical nuances and things, which are not widely described.


One of very important nuances – in warm weather, even in idle, the engine has to use large cooling contour (thermostat/radiator) to not overheat, which means – the cooling agent has to circulate via radiator and all systems, which are included in the cooling (thermostat, radiator, fan coupling, electrical fan), has to function as it’s intended by manufacturer.

In the beginning – shortly about engine temperature indication. Most of BMW vehicles has engine temperature indicator in the instrument cluster. Nuance – since long ago (since instrument clusters are fitted with a microprocessor, it means, since times of E32 or even before) the arrow of the indicator will be in the middle state if the true temperature of the engine is around 90 +/-15 oC, it means: 75 .. 105 oC. The reason for it – to not rise unnecessary anxiety of the user, because the temperature of the engine during driving cycle fluctuates slightly. But at the same time, we have to understand, that even small deviation of the arrow from normal condition actually means – the engine is significantly overheating, and the problem is serious!


If IKE/KOMBI loses connection with DME and doesn’t receive information from DME regarding the temperature of the engine, the indicator will show max possible value. It’s done with purpose – so the driver of the vehicle pays attention to the problem.


Several models (for example, E60), don’t have direct indications visible in KOMBI. Theoretically, temperature values can be found in the hidden menu, but it has to be activated every time by the input of the code – it’s quite uncomfortable. Accordingly, we have to rely on the engine management unit – if the temperature will rise over the norm, the driver will get noticed with a symbol in yellow color. If the temperature continues to grow, KOMBI lights up the indicator in red, and the engine is turned off. These vehicles have many effective tools to avoid overheating – water pump with managing power, manageable thermostat, high power electro-fan. So – if KOMBI lights up the indicator in yellow color, the problem is serious and you have to pay attention to it immediately.


In short about exploiting nuances of each component of the engine cooling system.


1. Fan coupling

Fan couplings were used in older models and engines. There were two types of visually different fan couplings:

a) with U type bimetallic plate;

b) with spiral type bimetallic plate.

A common feature for both versions – both U and spiral type plate are changing their physical characteristics depending on the temperature and they are also temperature’s sensor. These plates have to be clean! Very often, they are clogged with dirt and oil, which makes them incapable to perform. If you see, that the fan coupling is clogged with dirt, clean it obligatory!

One more essential nuance – work-life of the fan couplings is limited! Both bimetallic plate and also viscose material, which is used as torque carrier, are losing their properties. It’s only normal, that the fan coupling has to be replaced time by time.

Second significant nuance – for the coupling fan to turn on, the hot air (in large amounts) has to reach the bimetallic plate, which means, that the radiator has to be with good air permittance! If the radiator is clogged with sand and other dirt from the roads – the fan coupling will not work properly!


2. Radiator

I think, every driver understands – the cooling agent flows thru the radiator, the air cool down the radiator. Everything sounds simple, so let’s move to more specific things.

The cell of radiators, used by BMW, are very fine and clogged (from outside) quite quickly. So if the radiator looks clogged, it has to be cleaned – carefully watch it, using a high-pressure washer. Sometimes this problem cannot be noticed visually, so you have to use technical ancillary tools.


Contactless digital thermometer

or: any simple thermal camera will do perfectly.

With these tools, you will be able to check the performance of thermostat, radiator, and sensors of the electric fan.


How to do that?


The engine has to be heated up completely (I think, there shouldn’t be problems to do that with the engine, which tends to overheat). When the temperature of the engine has reached 90 oC or more, the thermostat has to open. How to check that?

With digital thermometer or thermal camera, we have to measure the temperature in the inlet pipe of the radiator (usually – at drivers side, the top part of the radiator).

In the picture – measuring the temperature, outgoing from the thermostat (inlet temperature of the radiator).

Note – make sure, that the temperature doesn’t drop down significantly thru all pipe, it means, that the cooling agent circulates. A possible drop of the temperature in section “thermostat – radiator” is around 5 oC. If the temperature drop is significant, it means – the thermostat is practically closed, water flow in the contour – tiny. In this situation, the problems with thermostat have to be solved.

Note – N series engines with electronic water pump management can have increased inquired engine temperature (even till 112 oC in ECO mode, idle), so the flow-rate demand of the water pump has to be evaluated, using INPA. If the demanded flow-rate is at least 40 .. 50% – the flow in the contour has to be significant (if the engine temperature is above 90 oC); if the demanded flow-rate is only 10 .. 15%, it means – the engine has decreased the power of the water pump to increase the temperature. In this case, the flow of the cooling agent will be weak – it is normal. To switch the engine (for some time) to “high power” mode with requested temperature around 95 oC, short and dynamic driving session has to be performed.

Temperature measurement, using the thermal camera. Real measurements confirm, that data of both measuring tools have very small differences – not more than 2 oC, which is very acceptable. So you can use any of these two tools – which is more available for you.


If a hot cooling agent gets to the radiator, check the temperature after radiator – pipe in the opposite corner (passenger side, bottom part).


Several measurements with a thermal camera.

In this image – temperature directly after thermostat for the engine N53B30, which works in “high power” mode (with the temperature around 95 oC). Measurement – from the front side (radiator: at the bottom of the picture; in the middle of the picture you can see the alternator).


The temperature at the inlet of the radiator. As we see, a difference of the temperature (range: thermostat – radiator) in idle – around 5 oC. With higher RPM/higher power of the engine, the difference should be even less. Measurement from passenger’s side, radiator – on the right side.


The temperature in the outlet of the radiator. This temperature has to be significantly (at least 15 .. 25 oC) lower. Measurement from the driver’s side, radiator on the left.


If this temperature is not significantly lower, possible defects could be following:

a) the radiator is not cooled down (defect of the fan coupling and also an electrical fan or it’s management);

b) radiator is clogged;

c) deffect of the water pump.


In case of completely clogged radiator temperature of the outlet pipe can be even higher than the temperature of the inlet pipe!


What to do in the case, if the temperature of the radiator pipe’s is not at least 15 .. 25 oC lower?


1. Check the performance of the water pump. For older engines – check the presence of circulation (opening the cooling agent refill cap). For newer engines – check, if there are any error messages regarding the performance of the water pump present.

Attention – cool down the engine, before opening the cooling agent refill cap!


2. Check if the electrical fan is working. For older cars: turning on the air conditioner, the electrical fan has to turn on to at least minimal speed. If it doesn’t happen, solve the problem. For older cars, for example, first models of E39, the electrical fan is managed via a relay and bimetallic sensor, which is placed in the area of radiator’s outlet. Lower speeds are managed via ballast resistors, which are placed near the fan and are cooled down with air flow. The lower speed is turned on by DME, as soon as AC is turned on; next two speeds are turned on when the thermosensor connect the corresponding circuit.

For newer cars, the electric motor of the fan is managed via a serial interface from DME, electric fan can be activated using diagnostic tools INPA or ISTA D.

If the fan turns on, move to the next step.


3. Check, if the radiator is not clogged. Turn on the ignition, turn off the engine, turn on the electrical fan (for older engines – turning on the AC, for newer engines – using test module of the fan via INPA or ISTA D). Check, if the flow of hot air can be well felt in the motor room. This test is especially effective for older engines, for whom the electrical fan is placed on the opposite side of the radiator. If the air flow can be felt very well, the radiator is in technical order, if it’s weak or not present at all – the radiator has to be cleaned.


4. For older cars – check the fan coupling. Turn on the engine. At first – reach the temperature around 80 .. 90 oC in both pipes (if the temperature doesn’t reach required – cover the radiator with a sheet of the board for some time, till the temperature increases). When the temperature reached, take off the board (if it’s installed), check the air flow in the motor room. The air flow has to be very hot, after a moment the fan coupling has to turn on – the air flow has to become very strong; increase the RPM to 2000 .. 3000, the performance of the fan coupling has to be heard. If the airflow doesn’t become very strong – clean and/or replace the fan coupling.


5. For older cars – check the performance of the bimetallic switch of the electrical fan. If the car is fitted with an electrical fan of “old type”, for example, first-year models of E39:

a) take off the plug of the sensor, connect the contacts with wire (according to the scheme). The electrical fan has to turn on speed 2 or 3 (according to connection). If it doesn’t happen, check the ballast resistor (for speed 2) or power connection (for speed 3);

b) restore the connection with termo-switch, cover the radiator with a sheet of board, heat up the engine till the moment, when the temperature in the area of sensor reaches around 90 oC, check, if speed 2 and 3 are turning on. If it doesn’t happen – replace the sensor.


6. For newer cars – check the indications of radiator’s outgoing temperature, using INPA and digital thermometer. If data are incorrect (displayed by INPA – lowered) – check the connection of the sensor, if necessary – replace the sensor.


And finally – for the engine cooling system to function normally, it has to be filled with a cooling agent.


So here are several nuances:

  1. Very common myth: if the inlet/outlet pipes of the radiator are hard (there is a pressure in the system) – the “head” of the engine is damaged. On the contrary – there SHOULD be pressure in the system (for the heated engine), the pipes HAVE to be hard. The radiator cap takes care of normed pressure. Its pressure relief valve is even marked in its name!

Increased pressure (above one atmosphere) is critically important! The higher the pressure, the higher the boiling temperature of the cooling agent! In case of the pressure of 2.0 bar, the boiling temperature increases for several tenths of degrees. If the warm engine doesn’t have the pressure in the cooling system – look for the cause of the problem (non-airtightness).


2. If the refill cap of the engine in working temperature (open the cap very carefully) is opened, the liquid doesn’t have to “grow up” – if it happens, the engine is damaged. Wrong! If the engine is in working temperature, the liquid has to “grow up” – when the temperature rises, the volume of the liquid increases; locally the liquid starts to boil (because of the pressure drops). For this reason don’t open the cooling agent refill cap, before the engine is cooled down! It is very dangerous – you can get serious burns!


3. If there is an air bubble in the cooling system, the engine will overheat, squeeze out the cooling agent. Correct. If there is an air in the cooling system, several problems appear – the volume of air, while heating, increases quicker than the volume of the liquid, accordingly – it tries to squeeze the liquid out of the system (excess liquid will be pressed out via cap/overflow valve); in some places (closer to air bubble) the engine locally heats up to the boiling temperature of the cooling agent, and in these places the cooling systems become a steam locomotive! A long time ago there was such vehicle – when the water changes it’s physical condition from liquid to gas, it’s occupied volume increases rapidly (this effect was used to move the piston in the steam engine or do another task).


Some nuances, filling up the cooling system:

1. The saloon heating system turns on to the max. If the max temperature is chosen, even in warm weather, not taking into account the temperature in the saloon, the system will open temperature regulation valves. Accordingly, when filling up the cooling system, the cooling agent will flow into the saloon heating radiator and there will no air bubble left.

Attention – after filling up the system, make sure, that the hot air flows into the saloon. If the saloon heating radiator valve(s) is (are) damaged, the saloon heating radiator will not fill with a cooling agent!

2. Open special venting screws, when filling up the cooling agent – for example, M52/M54 engines (and others with circulation container) have 2 screws: by the thermostat and filling cap;

3. The final stage of the filling has to be done for cool, turned on the engine, refilling the cooling agent slowly and without hurry;

4. While refilling, don’t be afraid to spoil the cooling agent, especially for systems with circulation container (where the refilling container is placed next to the radiator, not on the top). These systems “like”, if the cooling agent reaches the top of the container – then it quickly fills all cooling channels and presses out the air, which is in the system, via venting screws.