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Hello Canalworld - I've been a liveaboard boater for the last 5 years and have been reading this forum for about the same amount of time. I've never needed to post before as most of the problems I've come up against have been experienced by someone else and answered already. This time however I'm stumped! I have an Isuzu 55. When we are running the engine the temperature comes up to 81 degrees and will sit there all day long. I've been on the Thames punching against the tide and after about an hour it steadily heats up and if left will trigger the overheat alarm. This is easily rectified by dumping the hot water from the calorifier. The point is I know that my cooling system set up is good enough. This summer we were out cruising and for the first ten days everything was as usual, engine temp constant at 81 deg. Next day the temp was steady at 81 but at some point the temperature quickly crept up to 95 degrees, stayed there for about a minute and then dropped back down to about 75 degrees before returning to 81 degrees. I thought nothing of it but the same happened the following day. I tried running the engine without the filler cap and this stopped the problem from occurring, however it also stopped the operating temperature being reached. THis is now a recurring problem but is now getting more and more extreme. The temperature often shoots up 110 degrees and then drops back down to 60 degrees. The way this happens looks to me like a blockage that clears, or a coolant pump getting stuck. If I run the engine at idle it doesnt overheat, it will sit at 81 degrees as if there is no issue, it could be that the calorifier is operating as a good enough heat sink to deal with idle. I have never run it for long enough to heat the calorifier up. I thought this might be a sticky thermostat so I changed it - the stat was gunked up but putting the new one in didnt solve the problem. I thought it might be the coolant pipes, so I have changed these, although not the pipes to/from the skin tank - which I guess might be my next thing to do. I think I probably need to check my coolant pump as well but from what I have read this is not something I want to attempt without ruling out everything else first. Is this the right approach? Anyone seen similar behaviour? Any pointers would be greatfully recieved. Jared