It could be from a number of causes.
From a stuck solenoide valve to a bad temp controller.
First check to see if the solenoide is stuck. make sure the machine is relatively cold, and the t-stat is turned to max. Disconnect the power to the valve and check two things.
1) does flow through the valve stop?
2) is their power(12 volts) at the wires?
If there is constant flow, with it disconnected, the problem is with the valve. If there is power- on a cold machine with the t-stat turned up, you have an electrical issue to be traced.
It would also be helpful to know whether the unit is diverting (exhaust) while dumping.