The specialist who reported Toyotas with high cylinder-head pressure but low block temperature had the right clues but not the cause.
My friend John Cahill, who runs a recycling business in Hoosick, N.Y., explained the problem to me on a 1990 Toyota four-wheel-drive pickup.
Above 65 mph, the temperature gauge climbed rapidly, but cabin heat went from hot to none.
At that point, the water pump, which is driven from the back side of the serpentine belt, had stopped turning because of a combination of insufficient belt tension and centrifugal force throwing the belt away from the pump pulley. Slow down, and things return to normal.
To my knowledge, Toyota is the only engine builder that still relies on initial adjustment of the belt with no tensioner to compensate for service wear. Since initial diagnosis, it has proved to be rather common.