If expansion tank was low on coolant, will this cause my bmw to overheat? and where is the expansion tank located on my car?

There's trapped air in the cooling system.

• You need to remove the bleeder screw on top of the radiator when adding water to the expansion tank.
• With the engine running, pour water into the expansion tank till you see air go out through the bleeder screw's hole.
• Just keep adding water until no more air bubbles come out then just put the screw back in place.
• Then fill the expansion tank to it's normal level/line.

replace sensor. Its part of the expansion tank.

Have a cylinder leakage test performed to check your head gasket. It dosn't have to overheat to have a head gasket problem. A leakage test checks if looseing combustion gases to the cooling system which would presurise the system higher than normal which causes the tank to overflow.

not the pump but air in the system or a broken wire to the coolant level sensor
that sensor at the top of the radiator tank has to be completely immersed in coolant or the circuit is broken and the light comes on
same effect id the wire to that sensor is broken in the insulation

I WOULD CHECK BOTH RADIATOR HOSES TO MAKE SURE THEY ARE HOT. THERMOSTAT COULD BE STICKING SLIGHTLY CLOSED.CHECK COOLANT LEVEL MAKE SURE COOLANT NOT LOW.IF COOLING NOT BOILING OUT OF THE EXPANSION TANK. I WOULD HAVE CAR HOOKED UP TO A DIAGNOTIC SCANNER AND CHECK COOLANT TEMPERATURE SEE IF TEMP. OVER 230 DEGREES IF SO RADIATOR COOLING FANS NOT WORKING.COOLANT THERMO SWITCH BAD BOTH FANS SHOULD BE RUNNING AT LOW SPEED.IF ALL LOOKS GOOD YOU COULD HAVE FAULTY LOW COOLANT SENSOR IN EXPANSION TANK.MAKE SURE COOLANT IS IN THE EXPANSION TANK BECAUSE COOLANT LEVEL COULD BE LOW.ADD COOLANT IN THE EXPANSION TANK UNTIL YOU AT MAX LEVEL LINE ON THE EXPANSION TANK.DONT OVER FILL THE EXPANSION KEEP ADDING COOLANT UNTIL COOLANT LEVEL STAYS AT THE MAX LEVEL LINE ON THE EXPANSION TANK.

Are you talking the expansion tank? If so - Drain coolant first by going underneath the car and take off the protective shield and unscrewing the blue plastic screw (make sure the car is NOT hot!), under the reservoir you'll see an electronic sensor - carefully twist is a 1/4 turn counterclockwise and gently pull downward to remove this sensor don't force it, you don't want to pay $150 to replace it. Disconnect the hoses on the top and bottom of expansion tank by pulling up the metal tabs upwards then pulling the hose away from the tank. I believe there are two or three screws which hold the expansion tank bracket in place, unscrew these and pull the bottom of the tank away from the radiator then lift upwards on the bracket (the bracket and expansion tank should then come out). Get the old expansion tank off the bracket and replace. You want to take off the bracket because getting a new tank on the bracket is a B*TCH!

Reconnect in reverse order.

Make sure to bleed the cooling system properly to remove air pockets (please use BMW coolant and distilled H20 in 50/50 mix - it's not the same as your Prestone or other mass produced coolants). Last replace the expansion tank cap because that might have created the expansion tank cracking.

Good luck and hope this helps

We have had some similar problems with some BMW's,the last one we replaced the expansion tank.Before we attached the upper radiator hose to the radiator we filled the engine with antifreeze through the hose.Install the hose and fill your expansion tank to the proper level and run the engine to operating temperature.

replace your coolant reservior cap; the rubber ring has become brittle and "steams" the coolant out as you drive

I wonder why the overheating caused you to change the engine instead of checking with the cooling system. Some time back my 323i overheated and coolant expansion tank burst open, but that probably due to ignoring the coolant level low light for a couple of days. I replaced the coolant expansion tank, thermostat and few hoses connected with the tank and it worked fine for me.
Try to check you coolant level - may be you should have replaced your water pump and not the engine.

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.