Let it do the overheat thing then grab the top hose or take its temp. If its cold then the t-stat isn't opening. Could be the clutch on the fan is fouled up and not tightening when hot causing it not to spin faster. You could let the truck get to operating temp, shut it off, and try turning the fan by hand. If it freewheels relatively easy, I'd look into replacing the clutch fan. It should be tight and harder to turn when its hot and freewheel when cold. Only other thing is somehow the plow is obstructing radiator flow but I would think the overheating would be constant with it up then. Possibly some clogging inside the radiator, damn dex-cool!
Only relative tsb I saw....
Quote:
File In Section: 01 - HVAC
Bulletin No.: 00-01-38-010
Date: October, 2000
INFORMATION
Subject:
Proper Positioning of Lower Radiator Baffle When Diagnosing Poor A/C Performance
Models:
2000-2001 Chevrolet and GMC C/K Utility Models
(Tahoe, Suburban, Yukon, Yukon XL)
Prior to performing any air conditioning (A/C) diagnostics to determine poor A/C performance, the lower radiator baffle should be inspected. A contributor to poor A/C performance at idle and/or low vehicle speeds in city traffic, as well as increasing the coolant temperature, may be an out-of-position lower radiator baffle. The baffle is attached to the radiator support assembly and must be positioned between the top and bottom edges of the bumper. This position is critical because the baffle prevents hot engine air from recirculating through the condenser and the radiator. If the baffle is found to be out of position, push the soft baffle up above the bumper's bottom edge. No fasteners are required to secure the baffle. There is no need to evacuate and recharge the A/C system. Evaluate A/C performance following published Service Manual procedures.
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edit:
This explains fan clutch operation. Might have an issue with the fluid not properly heating.
Quote:
The cooling fan clutch may be described as a continuously variable clutch. If the vehicle engine is running, the fan blade is always turning unless the fan clutch is non-functional. The speed of the fan in relation to engine speed is temperature dependent. Maximum fan speed (air flow and related fan noise) through the engine compartment is experienced under two conditions.
1. When the vehicle sits in an unused condition for several hours, the viscous fluid within the thermostatic fan clutch assembly migrates and fully engages the clutch of the fan. After a short drive, the viscous fluid will migrate to the storage area in the fan clutch and the fan clutch will slip, reducing the noise (roar of the fan). This is normal thermostatic fan clutch operation.
2. When the engine is running, and the air being drawn by the fan or pushed through the radiator from the vehicle movement reaches a high enough temperature, the fan clutch will fully engage the fan clutch, drawing additional air through the radiator to lower the engine coolant temperature and A/C refrigerant temperature. When the cooling fan clutch fully engages, fan noise increases (for example; this is the same as switching an electric household fan from low to high speed). Some customers have interpreted this sound increase to be an increase in the engine RPM due to transmission downshift, transmission slipping, or TCC cycling. As the engine coolant temperature decreases, the fan clutch will begin to slip, lowering the actual speed of the fan blade and the resultant sound.
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