My V6 also loses coolant and no clear signs of why. I know of one area worth looking at.
There is a coolant pipe that is supposed to be attached to the battery-cradle. A common problem is that when mechanics temporarily remove the cradle, they don't realise that they should re-attach the coolant pipe afterwards (and sometimes the clip on the cradle snaps off). This fairly common mistake causes the coolant pipe to droop down and rest against transmission cables.
Effects of this are:
1. Wear to the coolant pipe (which might cause coolant loss).
2. Rusting of transmission cables.
3. Perishing of the bungs that attach transmission cables to gears (these came right off on mine..).
When I examine the coolant pipe on mine, I can see the rubber is thinner where it had (historically) rested against the transmission cables. It does not look like a leak, but when hot coolant is forced through the rubber pipe, the rubber will expand.. I don't know if expanded-thinned-rubber will gradually leak coolant? But it probably does leak because something has to cause signs #2 and #3.
Also, the reservoir tank is poorly designed. It traps tiny amounts of coolant between the MIN/MAX markers, which can result in a reading when in fact there is no coolant. At one point my car had lost 2L of coolant (and temperature dial shot up) but the reservoir still showed it as above MIN!! Clearly a lying marker because it was fed an extra 2L of water and then drove on fine..
Last edited by stormbind; 27-07-2012 at 08:20.