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I have a BMW 328 -98, or in other words, an E36 with the M52 engine. I cannot for the life of me understand how the coolant system works on this engine, and therefore not how to properly top it up after having drained it for engine work. Googling around seems to yield contradictory results, and the Haynes manual also reads unclearly, and nowhere at all can I find any more complete explanation of how the system actually works.

Here are some concrete things that confuse me about it:

To begin with, I do not understand why the coolant system needs a bleed screw at all. Isn't the whole purpose of the expansion tank that the coolant passes through it and therefore air bubbles that come along stay in it? In fact, I have never noticed a coolant system bleed screw on any other car -- does this engine use some different kind of "paradigm" for cooling?

Also, when starting to fill the coolant system, when I have the expansion tank cap off and the bleeds screw unscrewed, I repeatedly fill the coolant up to the top of the expansion tank and let the level subside, at which point it always seems to stay on the "COLD"-mark on the expansion tank. How can this be? When I fill it up more, shouldn't the expansion tank, you know, fill up? How is it that the coolant level always subsides to the same level?

In addition, the Haynes manual states that I should "top the expansion tank up until coolant free of air bubbles is visible in the hole for the unscrewed bleed screw" (admittedly not an exact quote, as I don't have the manual with me here). However, since the bleed screw hole is at the same level as the top of the expansion tank, doesn't that mean that the expansion tank will be completely full when that happens? Do I subsequently need to "drain" the expansion tank down to the "COLD"-mark after having verified coolant in the bleed screw hole? The Haynes manual sure doesn't mention that I should have to do anything like that.

Finally, I've been trying to find schematics of the coolant system on-line in order to be able to understand it, but when I look at images such as this one, it seems to indicate that the top of the expansion tank is connected almost directly to the top of the radiator. That seems absurd to me, since that would mean that the expansion tank would have to be full all the time in order for the radiator to be filled, which it obviously isn't (and isn't supposed to be, as the "COLD"-mark is half-way up on the expansion tank). How is this explained?

  • Is this a saloon, coupe or convertible? Do you know if your M52 is fitted with an auxiliary water pump? – Zaid Jan 15 '16 at 19:10
  • @Zaid: It's a coupe model. If, by "auxiliary water pump", you mean one of those electric pumps that just circulate hot coolant through the climate control system, then it has no such thing. – Dolda2000 Jan 15 '16 at 19:18
  • OK, that makes sense – Zaid Jan 15 '16 at 19:19
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I do not understand why the coolant system needs a bleed screw at all

Because air pockets.

Filling up coolant doesn't necessitate that the entire system will be filled with coolant. There are parts of your cooling system that will trap air when it is refilled. The bleed screw is a means to expel the trapped air from the system.

I repeatedly fill the coolant up to the top of the expansion tank and let the level subside, at which point it always seems to stay on the "COLD"-mark on the expansion tank. How can this be?

Keep on filling. It will eventually stop subsiding.

Remember that you're filling up your entire cooling system, not just the expansion tank. It will take time for the water to fill up the radiator and other subsequent parts of the cooling system.

However, since the bleed screw hole is at the same level as the top of the expansion tank, doesn't that mean that the expansion tank will be completely full when that happens?

Depends on the level, but if what you say is true then yes.

Do I subsequently need to "drain" the expansion tank down to the "COLD"-mark after having verified coolant in the bleed screw hole?

No.

You don't have an auxiliary pump for the heater, so your refilling procedure is performed with the heater lines shut. This means that the coolant level will actually drop the moment you start up the engine and turn the heater on full (which I highly recommend you do for a few minutes, even if your manual doesn't highlight this). Recheck the level after this is done and refill to the appropriate level if required.

  • Zaid.... I found out about the heater core the hard way... He's 100% – Dee Jan 16 '16 at 3:04

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