I've had this problem over an year, coolent runs out of reserve in 3 months and the car overheats. No drop leaks by the way. I took it to a mechanic after and 1 day of serious checking he told me there was small concentration of coolent leaking into 1st(far right) spark plug. So he put leak fix powder in radiator and some coolent on top and I thought problem would be fixed.

The other day I had to stop at red light for about 10 minutes and the temperature guage suddenly went up again. I checked reserve tank, it was full.

Seems my car seem to suffer heating especially when I'm stopped at a signal light for longer period. Why and any suggenstions how to fix the problem?