Truck Repair: follow-up question, fan clutch, vacuum control

Question
could the fan clutch be a factor to my problem?-------------------------
Followup To
Question -
my delivery truck overheated on the interstate. fluid backed-up through the resevoir.we replaced the thermostat. truck only overheats at speeds above 60mph. can drive at 55 or so with no problem. when we get up to interstate speeds it gets hot. i can let it run at idle forever with no problem. belts appear tight, circulation is good. this 350 has 135'000 miles on it.
Answer -
Hi Randy,
Hopefully, it is something like a restricted exhaust. I had several older models that had a screen in the tail pipe that rust flakes would stack up against, and not let the exhaust out. Knock that screen out, and the power returns also.
Another thing that gave problems was the heat riser valve vacuum control. If it doesn't open, the exhaust from the right side is forced through the crossover port in the intake manifold, and out the left side. This was intended to assist the engine during cold starts, but I had four of them, and all had the same problem. Disconnect the vacuum to the valve in the right manifold, and it defaults to open.
I think that valve was eliminated in 88 on pickups, but check for it.
Another remote possibility is a head gasket just about ready to blow.
Good luck,
Van

Answer
I doubt the fan clutch would be the problem. The fan is needed for city driving, but once you get on the highway, you can take the fan off and throw it in the back. The airflow from going down the road is more than the fan pulls in.
But the extra load on the engine from 55 to 60, might just be enough to need a little extra airflow. You can hear clutches engage on some trucks going up long hills, but usually not at 60.
It's easy to check though, and if it happens to be bad, it can cause damage to the water pump.
Van