2007 bmw 525xi check engine light is one because the thermostat is not opening and closing properly

Is you check engine light on...if it is you need to check for trouble codes...if no check engine light...try checking for vacuum leaks

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Radiator Replacement
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I am really suspicious of that thermostat.
How it is supposed to work is, there will be "no" coolant flow until the thermostat opens up , and it will not open until it reaches a certain temperature. (Like 160, 170, 180, 195, etc.) If it is working properly, it will close back up when subjected to colder water. This valve will keep the engine at that temperature. The things that can interfere are the heater hoses, or by-pass hoses., which route the the coolant other places. From your description, it sounds like the thermostat is not working.
You might take it out of the engine, and get yourself a candy thermometer, and put the thermostat in a pan of boiling water on the kitchen stove, and with the candy thermometer, check the thermostat and see if it is opening and closing properly as you change water temperatures.
God bless your efforts.

Did you replace the thermostat too? The thermostat is notoriously bad on the 80s 3 series. Also, be absolutely sure you added enough coolant. You need to fill the radiator, then wait until the thermostat opens, and add more. If didn't do this, wait until the engine is completely cool before adding the second batch. If you add coolant while the engine is overheated (or completely warmed up) you could crack the block when the cold coolant hits the over heated engine).

Could be the thermostat in the engine is stuck open , but your engine temp guage would show that the engine is running cooler than usual. Could be a partially blocked heater core , low flow of hot coolant makes for cool air.

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.

Don't forget to rate this post !!

When they start the inspection, the states computer
will read that codes have been recently deleted. This
will get you an automatic rejection. I know it will in mass. I would stop by an inspection station and ask
if your engine temp. can keep you from passing.
But if you wan't to clear the codes. Go by an autozone/napa or advance auto parts. They will clear
the code for free. Hope this helps. Good luck.

One very usefull tip is to firstly have the front of the vehicle higher than the back. Possibly on a hill. This will encourage any trapped air to the front of the engine. You then need to locate the thermostat housing on the front of the engine where you will see an 8mm bolt. This is the bleed screw, on a car of this age take care as the screw may shear and then you will need to buy a new housing. Once this is opened fill the coolant expansion tank with coolant until it pours from the bleed screw and then close the screw.

The BMW 525I doesn't have a dip stick. The oil level is checked using the "i" menu. Select service status and from there you should choose "check engine oil level".

first check coolant level .if all good replace thermostat .look at water pump drive belt make sure it is not slipping.if all is good i am looking at a bad water pump.