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BMW and MINI DIY – Overheating – Cooling System Diagnostics How To Diagnose

September 4, 2012

One of the most common BMW and MINI tech issues that we help customers with here at Bavarian Autosport, is engine overheating. We asked Otto to write us a detailed “how to” on diagnosing and repairing the most common causes if overheating. His article turned out to be so comprehensive, it could have been broken out into several articles spread out over two or three articles. But the topic is so timely during the summer months, we decided to present the article in its entirety.

Too hot to handle?

‘Tis the season for cooling system woes. We see it every year, as soon as spring breaks into summer, the calls and e-mails center around overheating BMWs and MINIs. Be it a well worn ‘79 320i, a pristine ‘89 M6, a rather new ‘08 X5 or a young MINI Cooper S, they all eventually start showing signs of needed cooling system maintenance or repair.

The cooling systems on our BMWs and MINIs are fairly simple, once you understand what’s involved, but they do include a few different parts and systems within the vehicle. Each of these parts must be doing its job correctly or we end up with a cooking engine and quite possibly a ruined day or worse yet…. a ruined vacation. In order to properly inspect and diagnose the cooling system, we first need to understand it. Here are the major parts of the cooling system that can typically fail and cause problems:

Typical late model BMW cooling system (3 series 92 thru 98 shown).

Typical late model BMW cooling system parts (3 series 92 thru 98 shown).

Typical early model BMW cooling system parts (3 series 84 thru 91 shown).

Coolant – Typically a mixture of water and antifreeze, it performs many tasks:

•    As its first priority, the coolant absorbs heat from the engine, then releases the heat to the  radiator.
•    The antifreeze in the coolant mixture has anti-corrosive chemicals to help prevent corrosion in the radiator and engine.
•    The antifreeze also contains lubricative chemicals to help prolong the life of the water pump.
•    The antifreeze prevents the coolant from freezing in cold weather by lowering the freezing point of the water/antifreeze mixture.
•    The antifreeze helps prevent the coolant mixture from boiling as the engine temperature goes up by raising the boiling point of the water/antifreeze mixture.

NOTE: Unless your climate calls for a greater ratio than 1:1 ratio of water to coolant (or you are using the Evans NPG+ waterless coolant), never use more than 50% antifreeze. Doing so will actually reduce the heat transfer capability of the coolant mixture, due to the fact that the antifreeze does not conduct heat as readily as water. Therefore, the more antifreeze you have in the system, the less heat it can transfer. On the flip side, too little antifreeze will lower the boiling point of the coolant mixture, causing the engine to overheat. Also, it is always recommended to use distilled water in the coolant mixture to help prevent corrosion caused by minerals in tap water. An excellent alternative is NPG+ waterless coolant. (See blue box on page 6.)

Radiator –


*  Transfers heat from the coolant to the air. The heat is transferred from the coolant to the tubes in the radiator. From the tubes, it is transferred to the grid of small fins, where it dissipates into the air.

Radiator/Expansion Tank Pressure Cap –

Radiator or Expansion Tank Pressure Cap

*  The cooling system (radiator) cap not only keeps the system closed, but allows a build-up of pressure in the cooling system. As the coolant heats up, it expands and builds pressure in the system (up to the point of the cap’s rated maximum pressure). This allows the coolant to run at a temperature higher than 212° F (the boiling point of water). For each pound per square inch (lb/ of pressure, the boiling point of the water/antifreeze mixture is raised by 2.7° F.

Water Pump –

*  Moves the coolant through the engine and the radiator.

Thermostat –


*  A valve that opens and closes to control the flow of coolant from the engine to the radiator in order to regulate the temperature of the engine. If there were no thermostat, the engine would run too cool in the winter time. (The thermostat effectively slows the flow of coolant through the radiator in order to keep from over-cooling the engine). It must be understood that the thermostat only regulates the engine from running too cold, as mentioned above: the thermostat does not regulate the upper temperature of the engine. Once the engine is running at a temperature above the normal running temperature, the thermostat is fully open and will not close until the engine comes back down to a lower temperature.  Therefore, a higher temperature thermostat will not cure a hot running engine. (Unless, of course, the original thermostat is faulty in the first place.)

Engine Fan –

Engine Cooling Fan

*  Pulls air over the radiator when the vehicle’s speed is too low (e.g. stop & go driving or idling), to force air through the radiator. Note: 3 series 6-cylinder models 99 on with manual transmission (as well as many later model BMWs and all MINIS, do not use an engine fan.

Radiator Fan Shroud –

*  Improves the engine fan’s efficiency by directing the fan’s airflow fully through the radiator.

Fan Clutch –

Thermostatic Engine Fan Clutch

*  A BMW’s engine fan is mounted to a thermostatically controlled clutch (mounted to the water pump pulley). When the air coming through the radiator is below the clutch’s temperature rating, the clutch allows the fan to freewheel. This creates less noise and uses less horsepower. As the air coming through the radiator gets hotter due to reduced airflow (e.g. stop & go driving), the clutch thermostat engages the fan so that it pulls additional air through the radiator.

Auxiliary Fan –

Auxiliary Fan

*  Electrically operated fan that is mounted in front of the radiator and A/C condenser (on BMW and MINI models that do not have a standard engine fan, the auxiliary fan is mounted on the rearward side of the radiator). On BMW models up until the early ’00 model years the auxiliary fan is operated via thermostatic switches in the radiator (earlier models have two separate switches, later models have one switch with two temperature settings).The switches will close and turn the fan on as the coolant temperature rises. As the temperature of the coolant rises above what the system can handle, first the low speed switch comes on. If the coolant temperature continues to rise, the high-speed switch will come on.  Later BMW models and MINIs will operate the auxilary cooling fan via control input from the engine management system.

Ok, now that we have a better idea of what the parts are and how they interact, let’s look at some typical problems and the associated diagnostic steps. But before attempting to diagnose ANY overheating problem, verify the following. (Any of these points could cause any or all of the symptoms described later.):

1.    The system is full of coolant and the mixture is no greater than 50% antifreeze (when using a water/antifreeze coolant mixture).

2.    There are no broken blades on the engine fan.

3.    There are no visible coolant leaks.

4.    The radiator fan shroud is in place.

5.    The accessory drive belts are in place, tight and not fraying. (Slipping water pump belts can give symptoms similar to corroded radiators.)

6.    Finally, make sure that the front of the A/C condenser and the radiator are not clogged with debris (which will limit airflow).

Common overheating issue #1: 

“My BMW overheats at idle or in stop-and-go driving. However, it’s just fine when I’m driving along at 40 mph or more, even if it’s very hot out or I’m going up hills.”

When moving at a slow pace, there is little forced airflow through the radiator and the heat transfer from the radiator to the air is very poor. Conversely, when moving at faster speeds, there is a very good forced airflow over the radiator and the heat transfer is very efficient. Therefore, we are dealing with a problem of low airflow at the low speeds. This is where the engine fan and the auxiliary fan come into play. The purpose of these fans is to mechanically force air through the radiator. If either is not functioning properly, we see problems at low speeds.

Possible Cause A: The engine fan thermostatic clutch may be faulty and allowing the fan to  freewheel at all times – test the clutch as follows. As noted above, some BMW and MINI models do not have mechanical engine fans.

1. With the engine and radiator cold, start the engine and run it for a minute or so.

2. Take a rolled up section of newspaper and gently push it into the back of the rotating fan. With the
engine and radiator cold, it should be easy to stop the fan.

3. Let the car run or go for a short ride to get the engine hot. Let it sit and get to midway on the temp gauge.

4.    Using the rolled up newspaper, gently try to stop the fan again; it should be noticeably harder, or nearly impossible, to stop.

5.    If you can stop the fan just as easily as when the engine was cold, you need a new fan clutch.

Possible Cause B: The electric auxiliary fan may be faulty.  On the earlier BMW models, the fan’s low speed resistor may be faulty or the radiator temp switches (that control the fan) may be faulty. Test the switches, resistor and the fan as follows.

NOTE: Most BMW models ‘99 and later (and all MINIs) do not have the radiator-mounted temperature switches. These models have various system inputs to the ECM (Engine Control Module) which controls the auxiliary fan and thermostat electrically. Diagnosis on these models requires checking the ECM for fault codes (using a code reader & reset tool like the one offered by Bavarian Autosport, or the BMW MoDiC or DIS machines at the dealer).  Consult the applicable Bentley repair manual.

Testing the radiator temperature switches is a very important part of the diagnosis procedure. Although the testing is fairly easy and straightforward, the detailed description for the diagnostic steps is rather lengthy. We provide the basics here. For more detailed step-by-step instructions, consult a Bentley or Haynes repair manual.

1.    With the engine running, turn on the A/C.

2.    Look through the front grill and see if the auxiliary fan is turning. If it is, the resistor and the fan are OK and you should test the radiator temperature switches. If the fan is not running, either the resistor or the fan are faulty.  We will make a determination as to which is faulty while testing the switches.

3.    Locate the thermostatic fan control switch(es) on the radiator. Most models through ‘88 have two switches; later models have one dual-range switch (with three wires).

4.    With the engine running, jump the wires from the radiator temperature switch(es) to determine if the fan comes on at low and high speed. (For greater detail, consult your Bentley or Haynes repair manual.)

5.    If the fan does not run with the wires jumped, use a voltmeter to test for 12 volts at the power wire on each switch. If there is power in one wire for each switch, your auxiliary fan is faulty. If there is no power at any of the wires, you either have blown fuses, faulty auxiliary fan relays or a fault in the wiring.

6.  If the fan does not operate on low speed, but does on high speed (and did not operate when the A/C was turned on in the prior steps), the low speed resistor is faulty. If this is the case, ignore steps 7–9 until the resistor is replaced.

7.  With the A/C turned off, perform steps #3 & 4. The fan should operate at both low and high speed.

8.  Plug the temp switches back in, let the engine run and keep an eye on the temperature gauge. The low speed temp switch should turn the fan on at low speed somewhere between 1/2 and 3/4 on the temperature gauge. The high temp switch should turn the fan to high speed at close to the 3/4 mark or just before the red section of the gauge. DO NOT LET THE TEMPERATURE GO PAST THE BEGINNING OF THE RED SECTION OF THE GAUGE. Turn the engine off if it gets this hot and the fan does not come on.

9.  If the fan did not come on at all, it is likely that the temp switches are bad.

Upgrade Notice:  The original temp switches are set at relatively high temperatures. Bavarian Autosport offers lower temp switches that will turn the fan on at a lower temperature (to help prevent runaway overheating), for most applications. This is a good idea even if your original switches do eventually turn the auxiliary fan on.

Lower temperature auxiliary fan temp switches. These will cause the auxiliary fan to come on at a lower coolant temperature than the original switches. This can keep the cooling system from getting into a “runaway” overheating situation.

Common overheating issue #2:

“My BMW is fine at slower speeds (but not at faster speeds) or is OK at faster speeds as long as I don’t have to climb any hills or as long as it’s not real hot (even when going over 40 mph). In other words, it overheats when it’s working harder, but can be fine on all but the hottest days or as long as I don’t go too fast or climb a big hill.”

In this scenario, there is plenty of airflow through the radiator as the vehicle is moving at 40 mph or more. Therefore, we do not need to address the fans at this time. (At 40 mph or more, we should be able to remove the fans from the vehicle and never know the difference.) Our problem now revolves around a lack of heat transfer from the coolant to the radiator or the radiator to the air (although we know it’s not an airflow issue). We could also be dealing with a thermostat that is not fully opening and is limiting the flow of coolant.

Original BMW coolant

Possible Cause A: You may have more than 50% antifreeze in your coolant mixture. Test the coolant mixture with a hydrometer. Drain and replace with the proper mixture if needed.

Possible Cause B: Your water pump drive belt may be slipping due to looseness or glazing of the belt running surfaces. Tighten or replace the belt.

Possible Cause C: The radiator is possibly corroded internally which will reduce the efficiency of heat transfer from the coolant to the radiator. The radiator must be replaced.

Possible Cause D: The radiator is possibly corroded externally which will reduce the efficiency of heat transfer from the radiator to the air. Run you finger along the thin fins that are between the coolant tubes, on the radiator. Do this in various places on the radiator. If the fins easily fold over, or even crumble, the radiator must be replaced.

Upgrade Notice: BMW coolant is specially formulated to prevent internal corrosion and the build-up of silicate compounds on both the radiator and the engine internal surfaces. The BMW coolant is available at Bavarian Autosport. We now also offer NPG+ waterless coolant.

One of the biggest troublemakers in your BMW’s cooling system is water. It can be a cause of: • water pump cavitation. • localized hot spots, boil-overs and vapor pockets. • internal corrosion due to the inherent minerals. • oxidation of aluminum and metal. NPG+ from Evans Cooling Systems is an innovative product that takes water out of the equation because it is used at full strength – you never mix it with water. It does not boil until 375° F, and it doesn’t freeze at -40° F like a 50/50 coolant. As a result, NPG+ eliminates a number of the problems outlined above. Plus, when installed in a properly drained and sealed system, it can remain stable for 500,000 miles – no more coolant changes! NPG+ has been used in racing for years.

Possible Cause E: An exception to the above is 6-cylinder engines on 3 series 92 on, Z3, Z4 and 5 series 91 on. The water pumps on these engines were made using plastic impellers (the part that actually pumps the coolant). There is now a long history of these impellers either slipping on the pump’s drive shaft or actually breaking apart. The engine may act like it has a stuck thermostat, or it may idle OK but get hot if you try to drive it. The water pump must be replaced.

Upgrade Notice: Bavarian Autosport has an upgraded water pump that uses a metal impeller instead of the failure-prone plastic one. This part will eliminate the broken impeller problem once and for all.

Metal impeller water pumps for most 6-cylinder engines in 3 series 92 on, Z3, Z4, 5 series 91 thru 03. These better-built water pumps replace the original factory water pumps that have plastic impellers. The plastic impellers are known to crack and slip on the pump’s drive shaft or to completely break apart, leading to potentially catastrophic failure.

Upgrade Notice: Bavarian Autosport has an upgraded thermostat housing.  The original housing is plastic and tends to crack.  The upgraded part is cast aluminum.

Aluminum thermostat housings for 6-cylinder engines in 3 series 92 thru 98, Z3 thru 98, 5 series 91 thru 98. Replaces the crack-prone, plastic thermostat housing that came on your car.

Common overheating issue #3:

“My BMW overheats no matter what I do. I can start it cold, and the temp just continues to go up until it is in the red in a few minutes.”

With a couple exceptions, this would typically be a thermostat that is stuck closed and is not allowing the coolant to flow to the radiator.

Possible Cause A: Is the radiator still cool, even when the gauge is reading very hot? If so, the typical problem would be the closed thermostat. You’ll have to replace the thermostat.

Possible Cause B: Refer to Possible Cause E above.

Possible Cause C: Another exception can be a failed cylinder head gasket or cracked cylinder head.  Things to look for would be:
•    Excessive white smoke from the exhaust.
•    The coolant has visual evidence of oily deposits in it and smells like oil or fuel.
•    The engine oil has a tan or gray looking foam in it.

Possible Cause D: There may be air trapped in the cooling system, preventing the thermostat from opening or causing the water pump to cavitate.  Either way, the coolant does not properly flow to the radiator in this case.  However, this should not happen unless the system has been opened up due to the replacement of a part, such as;  thermostat, hose, water pump, radiator, etc.

Common overheating issue #4:

“My BMW doesn’t seem to be overheating initially, but it does keep losing coolant, which eventually causes an overheating situation.”

Possible Cause A: This will typically be due to a leaking hose, gasket, water pump, radiator, heater core, head gasket or cracked cylinder head. Often the leak is small and does not leave a visible puddle under the car, nor does it create a cloud of steam under the hood or, in the case of a head gasket or cracked head, visible white smoke or oil foam. In this situation, you must perform a very thorough inspection of the whole system in order to identify the leak. If you cannot identify a visual leak, the system can be tested with a pressure tester. This attaches in place of the system’s cap and pressurizes the system (while it is cold and not running), which will likely make a leak visible that may only happen under pressure. This is also the proper means to identify a small head gasket or head leak.

Possible Cause B: This can also be caused be a faulty system (radiator) cap. If the cap is weak and not capable of holding the specified pressure in the system, it will allow the expanded coolant to escape through the overflow tube. Additionally, the reduced pressure in the system will allow localized boiling of the coolant at the hottest parts of the engine. This will reduce the efficiency of the coolant and tend to create a “runaway” overheating situation. The cap can be tested at your BMW repair shop on a pressurized tester, or it can just be replaced. Look at the rubber seal on the cap. If it is torn or looks suspect, replace the cap.

Bleeding the cooling system:

After opening the cooling system and replacing a faulty part (hose, gasket, water pump, radiator, etc.), the system must be bled of all internal air pockets. If not fully bled out, these air pockets will cause overheating due to cavitation at the water pump or a tendency for the thermostat to not open properly. Bleeding the cooling system can sometimes be tedious and frustrating (though other times it is quick and straight forward). Here are some pointers on performing the bleeding:

•    It will help the bleeding process if you can raise the front of the car by placing it on on ramps or jack-stands.

•    Locate the bleeder screw. Some models (most through the late ‘80s) have the bleeder on the thermostat housing while others have the bleeder on the radiator side tank or the expansion tank (reservoir).

Early version bleeder screw on thermostat housing

•    Set the heater controls to full warm settings and start the engine.

•    Leave the bleeder screw open while the engine is running (with the reservoir cap closed). Bring the engine speed up to about 1500 rpm and give it a bit more throttle intermittently. If the temp goes up past 3/4 on the gauge, shut it off and leave the bleeder open (assuming air and maybe a bit of coolant are coming out).

•    When the steam and pressure stop coming out, carefully open the reservoir cap just to the point where the remaining pressure (if any) releases. If coolant comes out of the overflow tube or the cap, re-tighten it. If no coolant comes out, go ahead and open the reservoir and add fluid as required.

•    Install the cap and start the engine again. Keep repeating this process until coolant is continuous at the bleeder and the engine temp stays constant.

•    It can also help if you pull the plug on the thermostatic fan switch (on the side of the radiator) and jumper the power wire to the high-speed wire, to turn the fan on. This will help prevent the system from overheating just from revving the engine with no airflow over the radiator.

•    Once you think you have the system fully bled, grab your tool for the bleeder screw and a jug of coolant and go for a short ride. If the temperature goes past the normal operating temperature on the gauge, stop and turn the engine off and open the screw; there will likely be more air. Proceed as outlined above;  add coolant as required. This should take care of the final bleeding.

Tip: doing the above while safely parked in an uphill position (as if the front of the vehicle were up on the ramps) can help eliminate trapped air.

As always, if you have any questions about any of the procedures outlined in this article, don’t hesitate to give us a call at 800-535-2002.

Bavarian Autosport

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  1. Ricky Jackson permalink

    I have no power on my Air condition console in my 325i . tthere’s no light on it therefore my air condition does not work. The blower or fan does not come on too blow air. Is that a fuse problem or something major. I push the up arrow and nothing happens. What can I Do about it?

    • Otto permalink

      You’ll need to have some diagnostics done. A BMW dealer or a properly equipped independent shop can plug into the climate control system and look for faults using the full diagnostic scan tools. You did not mention what year the vehicle is, but if it is an E46 (3-series 99-06), you may just have a faulty climate control panel. If this is the case, just give our agents a cal at 800-535-2002.

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