Keith---There is a good possibility that there is no (hot) water passing
thru the intake manifold. Long, highway runs will flow a lot of air/fuel
and cool off the carburetors and intake. The engine will then act as a
'cold start'. Running thru the city, or at lower speeds, allows engine
bay temperatures to normalize the intake manifold area. You can easily
check by feeling the water tube as it leaves the manifold, by the heater
hose junction. I have seen crud pile up at the corner of the return
tube, blocking the flow. Disconnecting the return tube at the water pump
housing should show a good flow of coolant, with the engine running.
Dick
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