I've just done a short test drive of my new build and the temperature gauge is going right up. The pipe from the (external) water pump to the block is hot and the pipe TO the pump is hot upto about half way from the rad. So I undo the thermostat housing on the top of the head and COLD water comes out. The thermostat hasn't opened. I took out the stat and tested it in a pan of water. It works beautifully. Why would a thermostat that works, that is sat on the top of a hot engine not open?

Update: I've just run it without the thermostat. The rad got hot after a little while after the coolant had circulated. Why would the temperature sensor, which is right next to the thermostat, get hot and yet the stat, which works, not open?

[Edited on 8-8-2012 by smart51]

sounds like the pump isn't pumping

Mmm. I thought of that, so I ran the engine briefly with the stat cover removed. Fountain!

is the pump pumping in the right direction? is there a little hole in the thermostat plate to let a bit of water through so that the hot water can get to it to make it open? just thinking out loud

There's a small hose from the head where the stat and temperature sensors are. It goes to the inlet of the pump. I guess I could whip the hoses off and check. The pump is on the end of the crank so I can't believe it is going backwards, but something isn't right somewhere.

No by-pass flow, thermostat and temperature sensor can only get heated by conduction you need a smaller bore pipe between an outlet on the head before the water pump and the water pump inlet.

You can get some by-pass flow by drilling the thermostat ( 3 off 2.5mm dia holes) valve plate.

have you removed the heater circuit? you should put a link pipe between rather than block them off. that cured my overheating problem back in the days when i had a pinto.

It has a scooter engine, so there is no heater. There is a small pipe from the hot side of the stat to the inlet of the pump so there should be no problem with cold circulation. It's always worth checking though.

[Edited on 9-8-2012 by smart51]

Pipe has to go from ther hot side (ie engine side ) of stat too pump inlet.

Sorry BT, my mistake. Edited the post above. The small pipe goes from the hot side of the stat back to the pump so that when the stat is closed, the pump just circulates water round the engine.

A guy at work had an idea which sounds good. The engine has been sat dry for quite a while perhaps the diaphragm was siezed, either by gunk or by corrosion. Cooking it in a pan may have shocked it open. I'll reassemble later and try again.

Update: reassembled and run up to temperature. No change.

[Edited on 9-8-2012 by smart51]