There is no senor on the expansion tank. Check coolant level in the radiator. Or the sensor may be in the top off the thermostsat housing. Check the senor has power connected to it. If all correct remove senor and fill with coolant, it my have gatherd an air lock under the senor.
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im guessing that maybe your thermostat is sticking. im not sure what else would make your coolant rush out like that. if im right, then that would definitely make your car overheat at speed, but not at idle. that isassuming that the thermostat is at least partially open. if that doesnt fix the overheating, then u may want to consider checking the ignition timing for being too far advanced, or even the camshaft timing.
time to change the radiator fluid also flush out the system good the brown stuff is surface rust no big deal just have it flushed or do it yourself. make sure you have the proper coolent these days there are 2 main typs of coolent, to put it simply the green stuff and the orange stuff. my chevy truck takes the orange stuff. now you could be seeing old orange stuff and it may look brown to you so make sure you replace it with the correct fluid because they dont mix and you will do damage to your car. also if you find out you have orange coolent in your car you CANNOT MIX THIS WITH WATER.. it will also damage your car..if you have orange and decide to flush out the radiator it might be a good idea to have it done at a shop.. they will know how to do it correctly. if it takes green fluid its simple to flush and repalce it yourself because you CAN MIX WITH WATER. i hope this helps...
If the coolant in the expansion tank is at the proper level, and you are still getting a coolant warning light, odds are that you have air in the cooling system causing surges in the expansion tank which make the float on the water level sensor go up and down and set the light.
If you have never replaced the expansion tank, it is most likely crumbling inside, letting air get sucked into the cooling system. Actually, if it was replaced once a decade ago, it may be due to be replaced again.
If you have had this car for a while, you know that keeping up with the plastic and rubber parts as they harden and crack on an older German car is a never-ending chore.
If the water level sensor has failed, you will have to disable it or cut the plug off the car and fabricate a plug connection for a new style sensor, because BMW no longer sells the water level sensor with this style plug.
I have an 2002 Impala, which should be the same (What year is yours?). The coolant sensor is on the passenger side radiator tank, about 10 inches down from the radiator cap. on the engine side of the radiator.
I don't know how to test, but make sure your radiator is full, not just the plastic bottle.
I used to have the coolant light go on pretty regular. Under warranty, I think the dealer put in some fancy stop-leak. It hasn't been on since.
u will have to re bleed the cooling system this can be done in a number of ways
1 if your coolent hoses have bleed valves open theese 1 at a time and run car until stream of coolent/water is constant no gaps then close valve continue prosses until finnished start at radiator end working towards bulkhead
2 remove water expansion cap and run car with heaters on until fan cuts in wait for car to cool replace cap
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
there can be a yellow light in the cluster that looks like a small radiator or box with waves inside of it to indicate water. This light means that the coolant level sensor has detected a low coolant level. Typically the expansion tanks inside of these cars will crack from heat and age. A simple diagnosis is to check your coolant level and top it off. Do not forget to bleed it by unscrewing the bleeder screw found on the upper radiator hose while filling up the reservoir found on the right hand side of the radiator. once the coolant has been filled, tighten the bleeder screw back down but be careful not to over tighten. and finally screw on the expansion tank cap. run the vehicle to operating temperature and use a flashlight to inspect the engine bay for leaks. If there is a leak, it will need to be repaired. possible leaks are a cracked expansion tank, leaking radiator hoses, or a leaking water pump or thermostat.
First check the coolant expansion tank and radiator for correct level. The radiator should be full (take the cap off when the engine is cold, doing this on a hot engine will cause hot coolant to spray out and possibly cause major burns). The expansion tank should be marked level-wise, either min-max, or min only, or max only. If the levels are right, your level sensor is probably bad. This sensor is located somewhere in the expansion tank.
I would begin to suspect that the water pump is not working properly. Is the bottom radiator hose hot too, and do you have the type of expansion tank which allows you to see the water going round?
Of course, if coolant is being pushed out of the system that would point to a blown head gasket but you haven't mentioned that, so I would suspect the water pump or the belt that drives it. Do you get a squeaking noise when you start from cold?