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Old 03-13-2013, 05:17 AM
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bluebee bluebee is offline
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Location: San Jose, California
Join Date: Mar 2008
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Mein Auto: 02 BMW 525i M54 auto 130K
This interesting description was on the other thread:

Originally Posted by Aussie528iT View Post
The nipple on the expansion tank is not an overflow tube.

The other end is connected to the radiator via the nipple on the top of the LH/hot radiator tank. The hose goes under the top of the radiator shroud to the nipple on the hot tank. As far as I can work out its a continuous bleed from the hot tank to the expansion tank to remove any air trapped in the top of the radiator.

If you take off the cap you will see coolant flow from the hole in the side of the filler neck. What happens when the cap is in place is one of those questions I haven't been able to work out. The hole exits into the neck between the 2 O rings on the cap. I haven't got around to cutting open an old radiator cap to find out.

You will notice the top radiator hose has an S shaped section between the bleed nipple and the radiator. This is to stop air trapped in the radiator tank from getting back into the head etc.

I have also cut up my old expansion tank to see what is inside. I don't have the level sensor switch (low level instrument panel and you need the high level panel and the "I" data bus etc) but its a Hall Effect switch which is triggered by the magnet on the level float.

I think the cover in the other half of the tank is just a stilling arrangement to stop the coolant level sensor from being activated by water flow from the auxiliary water pump.

Its operating all the time to ensure enough water is pumped through the heater because of the unusual cooling system flow through the head and block. The M52TU and M54 engines cooling systems are completely different to the earlier single vanos M52 engine which has a conventional cooling system arrangement.

They use a minimal water flow through the block with maximum precisely controlled water flow through the head. At high engine speed and load the aux water pump is needed to ensure adequate flow through the heater as most of the water flow is back to the radiator. There is a PDF on one of the E46 boards which explains how it works but I can't find it right now.

Also don't remove the radiator cap if the engine is hot. Apart from the risk of scalding you can end up with air trapped in the system and then its back to square one and the cold bleed process all over again. How do I know? Been there and done that!

Also don't open any of the bleed screws with the engine hot or cold after you have successfully bled the system. All you will do is let air back into the system. You can test this with the engine cold. Take off the radiator cap and check the coolant level then undo the top radiator hose bleed nipple and observe the water level rise in the expansion tank as air enters the top hose and the water finds its own level in the system.

You now have air in the system and have to do the cold rev bleed procedure to bleed the system again. The normal level in the expansion tank is below the water level in the top hose. The water level in the top hose and the engine block, head etc is held there by a sort of siphon effect and opening the bleed screw destroys this and allow air to enter the system.
Please read the suggested threads and add value, either by pictures or by descriptions, so the next person stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need
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