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Discussion Starter #1
Hi guys,
I am a new for Bimmer. My car is E39 M52 1998 Eur automatic version.
After I change a new rad cap and try to bleeding the coolant by myself, there are something weird at my coolant level although my car works fine and the engine temperature seems pretty good .
My observation goes as below:
(a) I open the cap and top it up to the cool/hot level(transparent expansion tank)

(b)After I drive 50 km, I look at my transparent expansion tank and the level will goes
below which I've topped up.

(c) For the next few day after driving, I observe it sometimes the coolant level will still goes down than yesterday.

(d) When I see the tank is nearly empty and try to refilled the coolant, once I undo the cap, I can hear hiss and the level is raise to hot/cold level ( I operated it while the car is completely cool down.)

My puzzles are:
(1) It doesn't lose any coolant but the hissing sound is a little bit make me nervous.
In my opinion, if the gasket has damage and the coolant MUST lost a lot, instead of none. I wonder that is this situation normal or not? Is that residual air trapped in cooling system cause my level go down sometimes?

(2)A stupid question is how to check the coolant level correctly? I am quite confused whether should I remove the cap to check or not? Because the level is different when I open the cap........


Thanks
 

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Coolant level is checked when the engine is cold, overnight.

There should be no change in level unless there has been recent cooling system repairs, or a leak.
 

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Discussion Starter #3
Coolant level is checked when the engine is cold, overnight.

There should be no change in level unless there has been recent cooling system repairs, or a leak.
Thanks! I got it......and that's what confused me...

Normal case for changing the rad cap only shouldn't be affected the coolant level or even caused the level low to the tank.

Anyway, I think might be caused by residual air trapped in the system....
 

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Coolant expands as it soaks up the heat......

the hissing you hear is because the system is pressurized.

If you just refilled it and drove it, wait for it to cool down before opening up the cap and check like edjack states, as air will work it's way back to the expansion tank and top off from there.
 
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