BMW Repair: 2001 BMW 740 Il sport USA, coolant level, bmw 740 il

Question
I was sitting in traffic and I noticed the temp gauge starting to raise up, I started to drive and the needle went right back down to normal, I  Checked my coolant level and it looked a little low so I topped it off. The next day after driving for a hour or so I was sitting with the engine running and all of a sudden  the expantion tank burst right down the side. Needless to say I had it towed home. I replaced the tank with a OEM tank. I drove the car around town to see if that was all that was wrong with it. It ran fine untill i was entering traffic and had to step on the gas. When I did that ,the radiator blew out right at the plastic part on the top right hand side by the hose inlet.
 Do you have any idea what would cause pressure to build up like that?

 Thank you

Erik


Answer
Three thoughts:  

Radiator cap not releasing pressure

Doesn't sound likely though.  Shouldn't be building up enough pressure to open it.



Restricted flow on return from radiator.

Possibility.




Head gasket.

Worst case possibility.  You can get it checked for <$50....call around for a place to "sniff" the coolant while it's running.

If it is this, get back to me.  Ive done one.