We are having problems with our 1986 m25 universal Diesel engine cooling. Running at idle for 10 minutes causes a steady rise of temperature to 200 and still increasing (at which point we kill it). All connections to heat exchanger (part 299049) are cold to the touch after this period, even though coolant is hot. Replaced thermostat, no blockages in heat exchanger (gravity pulls coolant from manifold out other end of heat exchanger). Was able to pump coolant with siphon pump from “to water heater” hose to the thermostat cap. Hot coolant from thermostat not seeming to travel to heat exchanger even though no apparent blockages or air locks. Coolant comes out of the thermostat pet **** when bleeding. Possibly faulty coolant pump? Any other causes for this problem?