Cooling is a frequent issue on the 6.5. If you haven't already since you bought the truck, pull the rad and clean out the fins as best as you can, as well as the AC condensor and the area between the two. All sorts of crud gets jammed in the cooling stack on our trucks and will cause temperature issues.

The stock calibration on many 6.5's doesn't have the fan engaging until temperatures are already climbing into the 210+ danger zone - mine didn't engage until about 210 before I cleaned the fan clutch - again, another important area. Remove the fan and clutch and clean all the crud out of the little fins and the bi-metal coil - often just cleaning up the clutch itself along with the cooling stack flowing more air after being cleaned as well will cause the clutch to engage earlier - it did with mine, it now engages around 200F. There is also a mod that can be done to the factory fan clutch to lower the engagement calibration a little - search in the 6.5 FAQ's here and you'll find the details. It's an option.

Either way, you have a problem somewhere - empty and unloaded, with ambient temps in the 70-80 range you should be running as cool as a cucumber at the thermostats - probably the 195 range.

It's also a very real possibility that your fan clutch has just bought the farm - it happens. In that case, replacement is the only option, but you need to start with the basics and clean the cooling system first off. The same could be said for the rad itself - rads sometimes clog and wear out and simply can't transmit heat efficiently anymore.

 

Also make sure you have an OEM AC Delco thermostat in there, everything else is crap, more on that here http://www.dieselplace.com/forum/63...esel-engines/21-6-5l-diesel-engine/220481-all-single-thermostats-not-equal.html

 

The fan should really never come on when your driving so you have a heat transfer issue, like said, your rad could be plugged up inside and out. How many miles? As said, the aftermarket thermostats are junk,

I would take out the rad, clean the outside, new OE type stat and see what happens. If the fan clutch is covered with oily dirt, its worn out and needs replacing.

 

Before you start sweating, find out if theres even a problem... Check the gage temp against the ECM temp, they should be close.
Also, check the ACTUAL engine temp against the gage. You'll probably find that its way off, hopefully its reading much hotter than actual... If so, all you likely need is a new CTS for the drivers head...

Using an infra red temp gun, take the engine's temp at the base of the CTS (drivers cylinder head).

Again using an infra red temp gun, you can also check the radiator for "hot spots" and overall efficiency.

 

Fan Clutch Diagnosis

Noise
Fan noise is sometimes evident under the following normal conditions.
-When the clutch is engaged for maximum cooling.
-During the first 15 seconds to one minute after start-up until the clutch can re-distribute the silicone fluild back to its normal disengaged operating condition (after overnight settling).
Fan noise or an excessive roar will generally occur continuously under all high engine spped conditions (2500 rpm and up) if the clutch assembly is locked up due to an internal failure. If the fan cannot be rotated by hand or there is a rough grating feel as the fan is turned, replace the clutch.

Looseness
Check a loose fan assembly for wear and replace as necessary. Under various temperature conditions, there is a visible lateral movement at the tip of the fan blade.
Approximately 6.5mm (1/4 inch) maximum lateral movement measured at the fan tip is allowable. This is not cause for replacement.

Silicone Fluid Leaks
The fan clutch operation is affected by fluid leaks which may occur in the area around the bearing assembly. If leakage appears, replace the clutch.

Engine Overheating
1. Start with a cool engine to ensure complete fan clutch disengagement.
2. If the fan and clutch assembly free-wheels with no drag (revolves more than 5 times when spun by hand), replace the clutch. If the clutch performs properly with a slight drag, go to step 3.
- Testing a fan clutch by holding the small hub with one hand and rotating the aluminum housing in a clockwise/counterclockwise motion will cause the clutch to freewheel, which is normal condition when operated in this manner. This should not be considered a test by which replacement is determined.
3. Position a thermometer so it is located between the fan blades and the radiator. This can be achieved by inserting the thermometer sensor through one of the existing holes in the fan shroud or by placing it between the radiator and the shroud. On some models, it may be necessary to drill a 5mm (3/16 inch) hole in the fan shroud to insert the thermometer.
NOTICE: Check for adequate clearance between the fan blades and the thermometer sensor before starting engine, as damage could occur.
4. With a thermometer in position, cover the radiator grill sufficiently to induce a high engine temperature, start the engine, turn AC on, and operate at 2000 rpm.
5.Observe thermometer reading when the clutch engages. It will take approximately 15- 20 minutes for the temperature to become high enough to allow engagement of the fan clutch on diesel engines. This will be indicated by an increase or roar in fan air noise and by a drop in the thermometer reading of approximately 3-10*C (5-15*F).
- If the clutch did not engage between 65-96*C (150- 205*F) the unit should be replaced. Be sure that the fan clutch was disengaged at the beginning of the test.
- If no sharp increase in fan noise or temperature drop was observed and the fan noise level was constantly high from start of the test to 88*C (190*F), the unit should be replaced. Do not continue the test past a thermometer reading of 96*C (205*F) to prevent engine overheating.
6. As soon as the clutch engages, remove the radiator grill cover and turn AC off to assist in engine cooling. Run the engine at approximately 1500 rpm.
7. After several minutes, the fan clutch should disengage as indicated by a reduction in fan speed and roar. If the fan clutch fails to function as described, it should be replaced.

 

My fan clutch seems to be locking in the engaged fashion. This thread doesn't tell much info on that aspect of it.

How can you tell if the clutch is 'frozen'?

If I turn truck off for a 10 minutes it will still be engaged when I restart the truck. The truck is at a mid range of temperature, not a cold start and not at operating temperature, and the clutch won't disengage until I get up to high speed or high rpm. High rpm makes it Louder, and eventually the speed catches up and it disengages.

Is it normal for the fan to be engaged at those times? It engages when idling in traffic on a hot day.

Seems to be working, and not slipping, but stays on too much? Hurts my mpg

 

This thread doesn't tell much info on that aspect of it.

See item #7 in HeavyChevy95's diagnosis post above.

How can you tell if the clutch is 'frozen'?

I would say if you cannot move it by hand when the engine is cold, the clutch is frozen and needs to be replaced.

 

I can't stress the importance of removing the radiator and cleaning it and the AC condenser. Get some professional coil cleaner from an HVAC supply house and follow the directions. As it foams up it forces the debris out of the radiator.
The other thing you can do is have the radiator hot tanked and tested while its out.

 

When the engine has sat overnight, the fan clutch will move but it will have resistance due to the silicone gel getting hard. If idling at normal temp after driving and its on, its junk.