Why would a percolating heater on a truck not cycle, and how to solve?

[todd]

I put a percolating heater on a 1985 toyota truck to make starting easier up here in Alaska. The temperature rises and the hose heats up to the 3-way that comes from the heater and splices to the heater core and to the block but doesn't continue to the block because the perc. heater shuts off. the system has worked before, so i was curious if it was vapor locked and how to correct the problem