Look at the diagram above and imagine about 1/2 inch of air in the top of the radiator when it's cold. Here's why: To some extent this system is self-bleeding, thanks to the expansion and contraction of the coolant as the engine heats up and cools down. That works, of course, but so do many other ways for refilling and bleeding the system. When the engine cools air will be sucked back into the top of the tank and the coolant should be at the right level. As the coolant expands excess coolant will be blown out the cap. Then to put the cap on, replace the bleed screw, and run the car.
#Coolant cap bmw manual
The Bentley manual for my car (2002 540i) says to open the bleed screw and fill the system all the way up.
#Coolant cap bmw full
It's the fastest, easiest way to get the radiator full of coolant. That's why BMW uses a bleed screw in cases like this. Pouring more coolant in the tank would not raise the level in the radiator unless the trapped air had a way to get out. Imagine what would happen if the expansion tank and the radiator were both half full. Here it's easy to see how air would get trapped at the top of the radiator core. I'm not sure about all models, but eventually the engine, head, heater cores and other coolant passages all get filled until coolant comes back to the radiator through tubes 2 and 4, and then the radiator core starts filling up. BMW re-fill procedures call for getting the coolant to flow through the engine and the heater core with the secondary pump (and/or starting the engine, as I like to do it). As coolant is poured into the open Cap 1, in flows into the tank and out Tube 6 to the water pump. Here's how the system would work, filling it from empty and then running it. Other than that mistake I think this diagram is pretty good for this discussion. I'm thinking it should be shown along the top of the radiator, near the top of tube 3, where it would let air out. (I'm happy to be corrected or use a better diagram is anyone has one.)įirst, I should point out that I think there's a big omission in this diagram - the bleed screw is missing. This came from an AutoZone page and it's supposedly for an e36.
Generally that means the level of the coolant in the tank must be below the level of the coolant in the full radiator.įollowing is the best diagram I found for this after a fair amount of Googling. But things get tricky when a manufacturer wants to put the expansion tank beside the radiator, at the same level. And it's easy to put a pressurized tank above the level of the radiator. It's easy to put an unpressurized expansion tank at the same level as the radiator, or above or below it. I think that's why BMW went through all this. I hope others will weigh-in over time and this thread will eventually provide most if not all the answers.Īlso, I have some images and insights to share that I haven't found elsewhere on this site, that will be new to some readers here. I'm starting this thread because I'm curious and I know all of us are smarter than some of us. (For example, if you loosen the bleed screw on a cold properly-bled engine, the coolant level in the tank will magically rise.) And when we do things to them they can behave in unexpected ways. Many of us would call them over-engineered. We can probably all agree that these cooling systems are more complicated than others we've dealt with. Radiator-Expansion-Tank-Cap-for-BMW-3,-5,-6,-7,-X5,-Z3,-Z4-by-HELLA-Germany-12.jpgĪnd if your BMW has an expansion tank that is mounted as part of or beside the radiator, as in the examples below, then this thread is for you. If your BMW has a coolant cap that looks like this (with two O-Rings and a washer), then this thread is for you.
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