Well, when i first got my car, some moron had removed the stock air box, installed the stock overflow bottle there and moved the filter back to next to the engine, go figure. So anyway for some reason I was too stupid to look into where the stock overflow bottle sits and removed it and installed a smaller bottle (it was actually a drink bottle i hooked up LOL) and moved the air filter up behind the headlight. Now although I rarely see my temp rise above 100 deg C (it normally sits on 88-92deg C) I was constantly finding the bottle out of coolant. This weekend just passed i found out where the standard bottle fits and reinstalled it and filled it up with coolant. That same day I went for a hoon up one of the mountains around and it was fine. The monday after i'm walking out of the mazda shop after just buying some new badges and notice that the overflow bottle overflow line (if that is what it's called) is leaking on the road, i drove home temps were as normal but i checked the bottle and it's empty again. What the hell is going on? Why does this bottle keep draining itself?

 

I'd pressure check the system. Some thermal expansion is normal so the level can change a bit. Any white smoke?

 

Nah no white smoke, and the engine only has about 10000kms on it since i had it rebuilt. And the engine isn't drinking it, its coming out the overflow tube

 

Try a new pressure cap first.