I have a 2005 325i, my car started overheating all of a sudden, I noticed that I had a pin hole in the expansion tank. I bought a new expansion tank along with a new thermostat located in the bottom of the expansion tank. I also replaced my main themostat, flushed my system good, and my vehicle is still running hot. I took the water pump out and visually inspected it and it looks good. What could my problem be? The vehicle runs hot after driving only 2 or 3 miles each time.

Hi,
A coolant leak is unlikely, but it is possible that the expansion tank cap is allowing coolant to seep-out.
There would be 2 reasons for this;
1. The seal in the cap is failing & allowing water to escape at too low a temperature.
2. You have an engine overheating problem & the coolant is forcing it's way past the cap seal, in the way it is designed to in the event of overheating.

Check your coolant temperature.
If it's running really hot, find out why.
If it's running at normal temperature, change the expansion-tank cap.
Best of luck.

have a compression test done to check head gasket or cracked head

hi chris does your car have electric fan or viscous. u say it heats up in traffic.If its electric fan can u here when it comes on.

You need the overflow tank functional, use plastic
repair for the tank

Check that your radiator fan is working if not check fan belt if it has one , if not check fuse . Next suggestion if all these are ok you may have a thermostat fault check the wires to this it may need relacing..

might sound stupid but did you check the impeller by holding the drive shaft and trying to turn the impeller asnif its a plastic one they tend to come loose on the metal drive shaft and loose power so they are not circulating the coolant ??

Start by cheking the coolant level first and add as necessary. If it is low, add, and then start and turn on the heat to full and check operation. it is possible that you have a small leak and have not noticed an operating/overheating issue. Let me know if you need anything further.

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

Check this out. Had the same problem. Easy fix. When you pull up at the lights does it blow steam everywhere as your water drips onto the exhaust. Next step is the water gets onto the coils engine light comes on and it mis fires!! Anyway solution. As it is doing it when you look at the bottom of the expansion tank you will see the water dripping. The tanks had a weakness and have I believe now been modified. Take it off and put your fingers over two of the three pipe holes, then blow down the other. Wifes often good at this!!! You will here a hissing as the tank has a pin hole in. It comes out fast as the pressure builds. To remove the tank undo the air box (1 x 10mm bolt and the screw on the large jubilee clip) Then undo the two petrol pipe bolts ensure your engine is cold. You should need a seventeen mm spanner. Then just under the engine lid you will see a 10mm nut which you need to undo. The box then slide forward toward the middle of the car and then back in a fastion!!! Give it a good blow you will see it. If you really must get in the bath with it and do it under the water.
Now get another and replace it.
Good luck everyone..... j 

broken fan belt water pump dont turn water dont circulate its gonna overheat alternator dont turn battery light stay on you said the bottle was full and when it cool down it was on low level that normal radiator took back what it was missing check the belt HOPE IT HELP FIXYA OUR POST THANKS PIERRE