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Old 04-16-2010, 09:15 AM
cn90 cn90 is offline
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Location: Omaha NE
Join Date: Mar 2006
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Mein Auto: 1998 528i 5-sp 140K+
DIY: The trick on how to fill the M52, M54 coolant

Nothing new, but the cooling system on the 6-cylinder engines has baffled me for a long time, why it is too difficult to bleed the air out etc.

So I will do a short and sweet DIY for those who wonder how to fill the coolant after performing cooling system work (such as cooling overhaul or water pump replacement etc.).

1. Take a minute to study the design of the E39 cooling system (see figure below). When the car is cold in the morning and you start the engine: during the warm-up phase, there is some hot coolant getting into the driver's side hose, "ready" to go through the radiator once the thermostat is open. Think about the thermostat as the "flood gate".

2. Now once the thermostat is open, the flow direction is:
Engine ---> driver's side hose ---> upper radiator ---> across radiator (to be cooled down)---> lower radiator hose ---> through thermostat area ---> Water Pump to cool engine.

- So in a normal engine at operating temp, the lower hose is considered the "Cold" side because the coolant has been cooled by the radiator but it is still somewhat "Hot" to the human hand anyway (maybe around 60-70 deg C, enough to burn your skin!). Here is the direction of coolant flow (from M54 but M52 flow is the same):

Study this M54 layout:

3. After some work in the cooling system is performed, re-filling the system is tricky because of the goofy design.
I just took another detailed look at my car again and here is the trick.

- Normally (if you haven't done any work on the cooling system at all) you want coolant at "KALT" level (or COLD level in the morning), but during re-filling, don't worry about it for now.

- Look at the thermostat housing, there is a small hole connecting the right and left sides of the thermostat housing. This hole allows air trapped in the lower radiator hose to travel through the hole to the driver's side to be bled out the bleeder screw.
(See GREEN arrow in pic below).

- If your car is on level ground, look carefully, you will see that the thermostat housing bleed screw (#1) is located at the same level as the reservoir neck and not at the "KALT" mark on the reservoir.

- Once you understand the above design, re-filling is easy.

a- Fiil coolant slowly. But I will add some tips...

b- Be patient and be patient, the hole connecting the (R) and (L) sides the thermostat housing is very small, maybe 1/16" or something like it. So air bubbles move through it very slowly.

c- Fill coolant up to the reservoir neck until it stops filling. Now some air is trapped in the lower radiator hose. Give it a good 5 minutes to settle there. Fill again until coolant fills to the neck, which is about 3 inches above "KALT" mark.

In the mean time, listen to some music and drink some beer...

d- Now open the thermostat housing 1 full turn (it takes 5 full turns to remove the bleed screw, so 1 full turn is all you need), air will come out slowly because the hole in the tstat housing is very small. When coolant comes out, close the bleed screw.

e- Don't start the engine yet, repeat the steps a through d a few times (another beer won't hurt...) until you are sure all air is bled out of the bleed screw.

f- For the 1st fill, if coolant is at the reservoir neck, then it is OK, air trapped in the engine will be expelled out when you run the engine. It will eventually settle down to "KALT" level anyway. If you are worried, then use a turkey baster to remove a bit coolant so it sits just below the neck prior to running the engine.

This should do it. Be patient and be patient when filling the cooling system!!!

4. The above little trick will help get rid of air outside of the engine.
Air Lock (air trapped inside the engine behind the thermostat) is another issue. Do a search.

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Last edited by cn90; 04-17-2010 at 05:50 AM.
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