Motorcycle Repair: Engine over heating, freezing point of water, coolant water

Question
I have a a 1985 Honda Shadow vt 500 after I run it about 4 miles it gets hot. It has 4000 miles on it. I have replaced the themostat and the sending unit to the fan at the bottom of the raditor. The fan won't run unless I take the wire off the sending unit and ground it on the bike. I have left the fan grounded so the fan will run all the time but the bike still gets hot with the fan running. The Honda shop said with such few mile the water pump still ought to be in good shape. What else could be wrong with the bike? I got the bike off e-bay so I don't know the history of it. I have just gotten my motorcycle license so I was wanting to use this for a beginners bike. Hope you can help.
             Thanks...Robert


Answer
Hi Robert,

I suspect air is in the cooling system. It is designed to be a air-free system. Much like salt changes boiling and freezing point of water. Air would lower the boiling temperature of the coolant. If air gets trapped at the water pump, coolant will not circulate.

Other considerations: restricted or obstructed radiator. This is more common with ATV's.

Radiator cap is defective.

Improper coolant/water mix ratio.

Wiring to any sensor in the circuit, or any components.

If you drain and replace the coolant in any liquid cooled vehicle, purge the air from the system. Mity-Vac make a tool to do this called the Air-Evac tool. I bought the tool for $89 though it retails for about $120 USD. I can't express the importance of using this tool. It is mandatory. See it at J.C.Whitney's website.

Respectfully,
Mark Shively