BMW coolant systems are not the most well renowned for being reliable. They have lots of plastic parts and BMW engines tend to run hot overall. This makes it especially important to maintain your BMWs coolant system properly, and one of the most important things to do is bleeding the air out of the coolant system.
Air in the coolant system is a common issue with BMWs, but luckily it is an issue you can resolve in under twenty minutes! In this article, I will discuss what air in the coolant system is, problems it can cause, and the procedures for bleeding the BMW coolant system. There are two different procedures, one for older BMWs equipped with a mechanical water pump, and one for newer BMWs equipped with an electric water pump.
If you would like to watch a video DIY on bleeding a mechanical water pump, I have included one in the mechanical water pump section.
What is air in the coolant system?
“Air in the coolant system”, “air pocket” and “air bubble” all refer to the same thing, air has gotten into the coolant system. Air gets into the coolant system in a variety of ways. Oftentimes whenever you change a part on the coolant system such as a radiator hose, water pump, or thermostat you will need to bleed the system of whatever air you introduced. Air can also get into the system through a faulty water pump gasket or through a faulty radiator cap. Air bubbles in the coolant system can also be a symptom of a significantly more problematic issue such as a blown head gasket.
Basically whenever you change any components on a BMW coolant system and you have to add coolant, then you also have to bleed the coolant system.
What problems can an air pocket cause?
Air bubbles in the coolant system can cause the vehicle to overheat and then drop back down to optimum temperature. Air bubbles may also prevent constant heat from getting through the vents. Finally, air bubbles can cause a rough idle, the idle will fluctuate up and down.
What has happened is an air pocket has gotten trapped somewhere in the coolant system and is preventing the coolant from flowing properly. An air pocket certainly is not the end of the world but it should be removed as soon as possible to prevent damage to the engine due to overheating.
If you suspect that you have air in the coolant system and the needle is passing the middle CUT THE VEHICLE OFF. BMWs are known for blowing head gaskets due to overheating engines.
Bleeding a BMW Coolant System
Use a Buddy
It’s best to have a helper with you when bleeding the coolant system. I use one person to watch the coolant gauge while I’m under the hood. The second person should be watching the coolant needle. If it ever passes the middle, then the second person should cut the engine off immediately! The air pocket can disrupt the flow of the coolant, and cause the aluminum head to overheat. BMWs aluminum heads are susceptible to warping, and a blown head gasket entails an extremely expensive repair.
This is an obvious one but it makes me feel better to mention. Never open a coolant fill cap when the engine is hot. Coolant gets extremely hot and is under pressure after the engine has been run for a while. Opening a coolant cap can cause a fountain of extremely hot water to shoot at you. Even if the temperature gauge is not in the exact middle do not take the chance of injuring yourself, wait for the engine to cool down. That being said, it’s also worthwhile to mention that coolant and nice clothes do not make very good bed partners. So if you just got home from work at the law office and want to mess around with the coolant system, change into something you don’t care about getting stains.
BMW recommends that you use distilled water mixed with coolant, not tap water. Plenty of people use tap water, however, the official guidance from BMW is to use distilled. Why? Because distilled does not have any minerals. The minerals in tap water tend to cling to metal surfaces. This can worsen heat exchange and even clog a passage in older/high mileage vehicles. That being said, there are plenty of BMW owners, I chance to say most that use tap instead of distilled.
How to bleed a mechanical water pump (older BMWs)
Tools You’ll Need:
- Jack (depending on model)
- Jack stands (depending on model)
- Screw Driver
- Car Key
How do you know if you have a mechanical water pump? If your BMW was produced pre-2004 you almost certainly have a mechanical water pump. BMW began installing electric water pumps on the 3 Series in 2004 with the E90. Every 3 Series before that, including the E46, E36 and E30 are belt-driven mechanical water pumps.
These older model BMWs must be manually bled. Although it’s a pretty quick and easy process, it does take more time and patience than the “self-bleeding” electric water pump models. These vehicles feature reservoirs that are mounted on the side of the radiator and they have mechanical water pumps and thermostats.
**For these vehicles I use a jack and jack stands or ramps to get the front of the car in the air. Why? Because this makes the radiator and the bleed screw the highest point on the coolant system and will facilitate the movement of air. Although plenty of people do not do this, I have found that it’s the most effective way to ensure that all of the air bubbles are removed from the coolant system.
**If your heater does not work this bleed process will still work just fine. I have performed this process numerous times after someone’s a/c control module had gone bad. Just skip the step about turning the heat on.
- Jack up the front end/ put it on ramps. (This step is recommended but not required)
- Open up the bleed screw and fill the coolant to the fill line. Leave the fill cap off.
- Set the heat at max temperature, and fan speed.
- Start the engine and allow it to run up to operating temperature. As it heats up loosen the bleed screw to release air and then re-tighten.
- Put the fill cap back on and tighten it back down. Put your foot on the gas pedal and rev the RPMs up to 2000-2500 RPMs. At this point, if your heat works it should be blowing hot air on full blast. This shows you that the heater core is full of coolant.
- Tighten the bleed screw, put the car down, and drive the car around for a bit, you are done!
Check out this YouTube DIY video on bleeding BMW mechanical water pumps!
Does your BMW still overheat?
Finished bleeding and the car still overheats? About half the time it’s just because you did not get some air bubbles out of the system. Older BMW coolant systems can be finicky, and that’s why I jack the front end up to help ensure I get all of the air out.
The other half of the time you still have a coolant problem. The first thing to check is the coolant level. If you are losing coolant then you have a leak somewhere. If you recently did some work on the coolant system I often find that it’s just a hose that I did not tighten enough. 9 times out of 10 it’s the lower radiator hose clamp. This clamp is in a hard to reach place so it often doesn’t get tightened enough.
Alternatively, there may be a more significant mechanical problem such as the thermostat or water pump gasket is leaking. There is also some hoses running underneath the intake manifold that sometimes crack and leak.
How to bleed an electric water pump (newer BMWs)
How do you know if you have an electric water pump? If you have an E90 BMW (2004-2013) or anything newer then you have an electric water pump. You can also pop open the hood and take a look at the water pump. If the pump does not have a serpentine belt on it then it is electric.
I have heard some people think that “self-bleeding” means that once you finish whatever work you were doing on the coolant system you can simply fill the coolant back up and drive away. This is inaccurate, the procedure is simple and takes twelve minutes but it must be done to prevent any air bubbles from overheating the engine.
Tools You’ll Need:
- The key
**For newer model BMWs (E90+) with electric water pumps and thermostats the process to remove air is extremely simple. There is no need for any tools or even to start the engine!
- Insert the key into the dash and hit the starter button but DO NOT START THE ENGINE
- Set the heater to the highest temperature and set the fan on low.
- Hold down the accelerator pedal for 10 seconds then release. The electric water pump will be activated and it will run for about 12 minutes to vent all the trapped air into the coolant reservoir. You will hear it making a noise kind of like a washing machine.
- Don’t open the reservoir cap or bleed screw while the water pump is venting.
- Once the water pump stops after 12 minutes, open the reservoir cap (it will hiss a little) and add coolant until the measuring stick is at optimal coolant level. Check the owner’s manual if your uncertain what the level should be.
That’s it! You’ve already bled the system! Sometimes people will perform this a second time, and I often see a small amount of additional air come out of the reservoir cap on the second try, but it’s not required.