Cooling systems can be your best friend when operating efficiently. Cooling system repairs . . . your worst enemy if you don't understand how your BMW cooling system works.
Your cooling system performs a critical function. Simply put, it maintains proper engine temperature by circulating coolant through the engine to pick up heat and passing it through a radiator to cool it with air. The coolant passes through a thermostat valve to control flow and possibly over a temperature sensor which controls external air cooling fans.
Cooling systems consist of three main parts:
Cooling System Part #1: BMW Water Pump
Your BMW cooling system's pumping function is handled by its water pump, which keeps the coolant mixture moving.
The main BMW water pump is gear- or belt-driven but, in many cars, a secondary electric water pump is used for improved flow and cooling.
Critical to the pump's operation is the drive belt that turns it. On most newer cars this is the engine's timing belt. On older cars, the pump and belt are external and run off the main crankshaft pulley with a "V" or flat belt.
Maintenance of cooling system pumping is limited to scheduled coolant replacement and drive-belt replacement and tension adjustment (external type). Timing-belt-driven pumps should always be replaced at the same time as the timing belt and tensioner.
Cooling System Part #2: BMW Hose
Your BMW cooling system's piping consists of all hoses, any control valves, the heater core, the radiator and the expansion tank. Because of the materials used and the constant contact with coolant, all parts in this system deteriorate more from time than use.
Maintenance of cooling system piping consists of scheduled coolant replacement, replacement of all hoses on a regular basis and replacement of any plugged or leaking parts.
All hoses should be checked at least twice a year for abrasions, cracks, flexibility and evidence of leakage. Whenever the coolant is drained for replacement or during engine repairs, any suspect hoses should be replaced. All hoses should be replaced at least every few years.
Radiators, expansion tanks, heater cores and control valves are normally only replaced due to leakage or plugging. The condition of these parts should be assessed by a BMW professional since proper functioning is critical to many other systems within your BMW.
Cooling System Part #3: BMW Thermostat
Your cooling system's temperature controls include all coolant temperature sensors, thermostat, radiator or expansion tank cap, cooling fan(s) and fan clutch (if equipped). These cooling system parts function primarily independent of the engine but control the engine either through cooling or by sending control signals to your BMW's electronic systems.
The thermostat is a spring-loaded valve that opens and closes based on the temperature of the coolant flowing through it. A high temperature reading followed by a drop to normal temperature (or a continuously low temperature) is a common first sign of a sticking thermostat. However, many other conditions may cause these symptoms, so you need to know how to eliminate each possibility.
The radiator or expansion tank cap is also a spring-loaded valve reacting to system pressure. It serves to maintain proper system coolant level at predetermined pressures. It must always be replaced with an exact replacement cap with the same pressure setting. Never use other caps except for short-term emergencies!
A belt-driven fan blade for pulling air through the radiator is usually on the water pump pulley and should have a fan clutch to control it. The fan clutch allows the fan to turn with the belt at low engine speed and "free-wheel" at higher speeds. A bad fan clutch either doesn't allow the fan to spin at low speed (overheating in traffic) or doesn't allow it to free-wheel at high speed (potential overheating on highway or reduced gas mileage).
An electric fan can be either by itself (usually front-wheel drive) or auxiliary (used with a mechanical fan). Both types are controlled via a temperature sensor - in the radiator or upper radiator hose or on the thermostat or water pump housing. This sensor is usually an on/off type switch with a fixed temperature setting. (Some vehicles may have 2-3 settings for multi-speed fans.) This sensor is commonly called an "auxilliary fan switch".
Other common temperature sensors are: 1) gauge sender (variable output); 2) warning light sender (on/off type); 3) lambda and/or fuel injection sensor(s) (variable to control fuel injection settings); 4) thermo-time switch (cold start valve control). Your BMW may have other sensors as well.
Temperature control is critical to both performance and emission control. Unfortunately, this system is the most difficult to troubleshoot without proper equipment and diagrams. It's even more difficult with computers that adjust timing, idle speed, vacuum and fuel delivery automatically to make up for potentially faulty temperature sensor signals.
Maintenance of your cooling system sensors is virtually impossible since there's nothing really to "maintain". Keeping them clean both internally (coolant replacement) and externally (engine cleaning) is the best way to ensure trouble-free driving. Checking and replacing all parts at the factory-recommended time or mileage limits helps as well.
A Few Important Things to Remember
Heed these cooling system maintenance tips and you're well on your way to ensuring your cooling system won't let you down:
BMW Engine Compartment Tip #1: Keep your engine and engine compartment, as well as your radiator fins and grill, as clean as possible. A clean engine runs much cooler - and it's much easier to work on.
BMW Coolant Tip #2: Replace coolant at or before factory recommended intervals with the proper type, mixture and volume of coolant. Always allow the coolant system to rid itself of air before installing the radiator cap.
BMW Cooling Hose Tip #3: Replace all cooling system hoses - upper and lower radiator hoses, bypass hoses, heater hoses, manifold coolant hoses and any other hoses on your vehicle - whenever you even suspect there may be a problem. All hoses should be replaced at least every two years.
BMW Thermostat Tip #4: Replace the thermostat with the original temperature setting equivalent. The electronics in your vehicle may use that setting for other controls. Do not substitute under any circumstances.
BMW Radiator Tank Cap Tip #5: Replace the radiator/expansion tank cap with the original pressure setting and OE-type equivalent. Some aftermarket substitutions do not seal and hold pressure properly on foreign-manufactured cars. Again, don't substitute.
BMW Water Pump Belt Tip #6: Adjust or replace the water pump drive belt (external) at recommended intervals or more frequently, if required. Check belts whenever you're working on any coolant system components.
BMW Water Pump Tip #7: Replace your water pump with an OEM/OES pump at the first signs of trouble or when your timing belt and tensioner are replaced. Watch for signs of overheating - you don't want to break down in the hot sun when your water pump fails.
BMW Fan Clutch Tip #8: Replace the fan clutch and/or fan blade as needed (if applicable). Your BMW's temperature gauge is often your best guide as to when your fan clutch needs attention.
BMW Temperature Sensor Tip #9: Replace temperature sensors as required by diagnosis. Leave troubleshooting of your sensors to BMW experts who have the proper equipment and diagrams.
BMW Maintenance Tip #10: Keep your entire vehicle properly maintained because of the effect timing, idle speed, exhaust and other systems have on your engine's temperature. Your BMW's cooling system is designed to function with all other systems operating properly. It cannot make up for a poorly operating or overheating engine condition.
Your BMW Cooling System Parts Shopping List
Here's a list of cooling system repair parts you should consider when repairing your BMW's cooling system: