Would a knackered stat be related to white steam out the exhaust continuing WELL beyond startup? I mean after a 25 mile run to work and then doing a bit of crawling and you can see it in the rear view mirror even though no other cars are doing it? If its stuck open then it cant warm up properly so would you always get the exhaust steam. Its deffo steam, not smoke. I have no performance issues and am not losing coolant either. My heater blows hot no probs. I should note I replaced the stat 2 months ago as the dial never went above 50. I re-used the temp sensor (cheapo) and it now only goes to 70, maybe 75. From cold start you can see it creeping up all the the way to 90 then it does a massive drop to 65 before coming back up and settling on 70/75. Reckon I have a a dodgy stat off the shelf or should I replace the temp sensor or both!?