on a long trip, my VZ V6 overheated. I added a couple of litres of water ti refill the coolant. 20 km later, same problem. Limped to the next roadhouse. Mechanic says "blown head-gasket", after a half-hour of staring at the engine while it was running, with the cap off. He based this on bubbles showing up when the accelerator was pressed. Towed 160km to nearest Holden dealer. I had to continue my journey by other means. A few days later, the dealer says, after exhaustive tests, that the only problem was a faulty radiator cap. Under normal conditions, everything was fine. But high speed (overtaking, 150 kph - I was in N.T. at the time) would force coolant out of the radiator into the overflow - still not a problem. But the faulty cap wasn't permitting the coolant to return. So, each time I overtook, I was spilling a bit more coolant, until I got dangerously low. After I picked up the car, I did 3,000 km in the next five days, and another 40,000 km since then, and I never had another problem with coolant or overheating. So, I'd check the cap before spending money on other possible answers.