Got a similar problem on mine.

Rebuilt it 1300 miles ago and left sitting for 30 seconds / a minute the temperature gauge rises quite quickly and the fans kick in. Removed the plugs which look normal, done a block test which has no traces of exhaust gasses in the tank, to me it just doesn't seem right. Boosts fine with no problems.

Some say that's normal cossie behavior, other says their temp gauge never moves. Unsure what to believe.