Chevrolet Repair: 454 overheating, fan clutch, temp guage

Question
Hi Van,
Here's my question...My 1989 454 which is in a 34 ft. motorhome pegs the heat gauge when I am idling.  The temp is fine going down the road.  My local mechanic thought it might be the fan clutch and installed a new one along with a new thermostat. He has also flushed the rad. and put all new belts and hoses on.  It did not help, it still gets hot.  The fan definitely kicks on (about every 3-4 minutes) while traveling and is deafening, but does not come on while I am just sitting.  Why would it not?  No one has been able to explain that to me.  Thanks for your help.
Nancy  

Answer
Hello Nancy,
This is just a thought.
Perhaps your water pump isn't moving enough coolant through the system while it is idling to cool the engine, but the small amount that gets to the radiator is thoroughly cooled by the air passing through the fins. That air, after passing through the fins, then passes through the fins of the fan clutch. The temperature of the air hitting the fan clutch fins is what engages the clutch.
Since that air is rather cool, relatively speaking, the fan clutch doesn't engage.
As the engine is sped up to drive down the road, more coolant is circulated and gets to the radiator....heats the air stream, etc.
I would be removing the thermostat, temporarily, and making sure the water pump is capable of moving coolant.

I would also measure the temp with an accurate temp guage, if there is any chance your existing one might not be exact.

Van