Question about 2005 BMW 325i Sedan
So checked *Fuse OK *radiator fan works fine even when engine is turned off to cool down *radiator hoses no leaks *Expansion tank has no leaks so maybe i need to change the expansion cap but if anyone has any ideas or solution that would be great
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Posted on Jan 02, 2017
have you replaced the bleeder screws? and the expansion tank cap they both have O-rings that compress and will fail over time also check the hose at the top of the exp tank it also has o-rings inside it
Posted on Apr 03, 2009
The water (coolant) in the expansion tank will rise and fall with the engine temperature. What the expansion tank does is collect and return coolant to and from the engine. When the engine warms up the coolant gets hot, builds up pressure and opens up the radiator cap. The coolant then goes into the expansion tank. Now when the engine cools down the pressure drops in the cooling system and the pressure drop (vacuum) pulls the coolant back into the engine via the radiator cap. The radiator cap allows the cooling system to build up pressure and by doing so increases the boiling point of the coolant, but when that pressure exceeds the caps rating the cap opens and the coolant goes to the expansion valve. The cap has another part to it that when the engine cools down and a partial vacuum is created in the cooling system a "valve" in the radiator cap opens and allows the coolant to be drawn back into the engine. I would look at your radiator cap to see if any gunk or build up is on it, and check the rubber gaskets for cracks. It's easiest just to replace the cap because they are inexpensive and easy to replace (2-10 dollars). The expansion tank should have two hoses on it. The one on the bottom comes from the radiator and the one on the top (possibly part of filler cap) runs down and is open to the ground. That way if it is overfilled or becomes overfilled it will slowly leak onto the ground. When and if you change your radiator cap, make sure the engine is cooled down, remove cap and start engine and turn heater to full blast, full heat. Leave the cap off and let it run until engine warmed up. This should burp out any air pockets that may have happened when coolant was changed. Also top off the coolant in the radiator while it is running. Hope this helps and good luck
Posted on Apr 03, 2009
SOURCE: coolant leak and overheating
I had a water leak on my 730i I found it after much trouble.A hairline crack in the plastic thermostat housing which onlly opened up when hot and of course got worse as water level dropped.Hope this helps.
Posted on Oct 30, 2009
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I assume you have checked the cooling system to ensure that first, that it is completely full of coolant (not just the overflow tank) with no air locks and second that there are no leaks anywhere and the radiator cap is in good condition. The heater hoses and those pesky little bypass hoses in the cooling system tend to go hard and split after a few years, allowing coolant loss. Those spring type hose clamps tend to be less effective in clamping as the hoses become hard. Worm drive ones are the best to use.
If there are no leaks or air locks, the over heating could also be caused by a clogged radiator (more than 15% restriction in the radiator flow capacity will cause overheating problems), a faulty water pump or a faulty thermostat.
If there are no leaks and water pump and thermostat are working correctly, but undetected loss of coolant once the engine heats up, then it could indicate a problem with the head gasket (Usually caused by allowing engine to become excessively overheated when cooling system has run dry) allowing very hot high pressure combustion gases into the engine water jacket, which super heats the coolant in the engine block, which then boils off, and is released via the cap. If the head gasket is really bad it will leak water into the cylinders and into the oil in the sump. This is indicated by an emulsion of water and oil mix on the dipstick.
If unsure take vehicle to a cooling system specialist and have them do a pressure test and more thorough diagnosis.
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