Heath Diesel’s 6.5L Diesel Heavy-Duty Cooling System Upgrade

Volume 2 Issue 2 - Diesel Articles

6.5L diesel cooling system temperature gauge

The GM 6.5 L diesel truck has a reputation of overheating when towing or hauling heavy and uphill. Those models built prior to 1997 were supplied with a cooling system that, even under the best of circumstances, provided only a marginal performance. Most of these vehicles were not able to maintain coolant temperature at or below the maximum – in my book – allowable 210°F. The normal accumulation of bugs, dirt and crud in the radiator and A/C core made a bad situation worse. Beginning with the ’97 models, GM increased the water pump flow, added a dual thermostat setup and a slightly improved fan clutch in an effort to make things better. These improvements affected a bit of an improvement.

Through years of experience with the 6.5 L diesel, working it hard under towing conditions, Heath Diesel has determined some important and effective remedies for the cooling inadequacies that plague these trucks.

Heath Diesel had long known that retrofitting the ’97 high-output water pump with the dual thermostat onto older models did not provide a meaningful improvement in temperature control and, while we found ways to improve the cooling system performance, the system as a whole did not deliver good and reliable temperature control under tougher towing conditions.

A Thorough Investigation

Years ago, we investigated these engines for temperature variation by outfitting the block and heads with temperature probes. This study clearly demonstrated that the distribution of coolant to the two banks of cylinders and heads was not equal and that the passenger-side of the engine was being starved for coolant and overheated.

A more recent investigation focused on coolant flow-conditions in the cylinder block and heads. Testing proved that coolant flow was reasonably equal – side-to-side – only at low RPM. As the engine speed was raised, the flow increasingly favored the driver’s side of the engine. In testing a 1995 model engine with factory issue (low-volume) flange type water pump, the coolant flow was biased 76 percent on the driver’s side when the engine was at 2500 RPM. This bias in flow, combined with high engine load conditions, can have a detrimental affect on passenger-side cylinders. The factory issue, low volume coolant pump used on 1992-96 engines was found to produce approximately 75 GPM at 2,500 RPM. Again, only 24 percent of that flow moved through the passenger side of the engine.

Testing a 1998 dual thermostat setup showed no improvement in bias of flow. We did find an increase in volume at 94 GPM.

This means that while the older pump pushed a miserable 18 GPM through the passenger side of the engine, the later, improved style was only marginally better at 22 GPM. The numbers help one to understand why retrofitting the ’97 style pump and dual thermostat set up onto the older engine does little to affect an improvement in cooling performance.

We decided to test both configurations of water pump: the older flange type as well as the later threaded stub type. We also tested both low and high volume versions of the flange type pump. In addition, we evaluated several brands of service replacement pumps from a variety of suppliers.

6.5L Diesel with 9,500-pound trailer

Heath Diesel 6.5L Cooling System Upgrade Kit (single stat) Heath Diesel 6.5L Cooling System Upgrade Kit (dual stat) Diagram of water pump and related mounting hardware

(Click on any image for more information or two enlarge.)

6.5L Water Pump

At the conclusion of an exhaustive evaluation, we had determined a clear and decisive winner in the factory service replacement water pump: part number 88894035. This GM pump proved to be vastly superior to all others in terms of bias with a much improved and nearly equal coolant flow, side-to-side at 2,500 RPM. This represents a tremendous improvement in flow to the passenger side under heavy operating conditions. Where the flow rate on the passenger-side had been a paltry 22 GPM with the ’97 high volume flange type pump, the 88804035 pump provided 48 GPM!

During this work, we performed evaluations of both single and dual thermostat systems with the good pump. We realized the best cooling system performance when we combined this improved pump with the older style, pressure-balanced, single thermostat type system. This combination creates desirable pump-pressure in the coolant passages of the cylinder block and heads that helps to prevent steam pockets. Another very important characteristic of this type thermostat is that it simultaneously blocks the water pump bypass port when fully open in order to route all coolant to the radiator under high load conditions. This is our favorite combo.

In comparison, the dual thermostat setup offers no feature for blocking the water pump bypass port. Since a good portion of the heated coolant is allowed to re-circulate back through the water pump and into the engine block, coolant flow through the radiator is not encouraged. Further, the desired pressurization of the coolant passages is not attained in the dual system. The Heath upgrade kit for the dual thermostat engines includes a special, restrictor orifice necessary to the creation of water pump pressure in the coolant passages of the dual thermostat engine like the single thermostat system.

Having settled on the big 88894035 pump, the next challenge was to find a fan clutch that would perform on long uphill pulls.

6.5L Diesel Fan Clutch

The factory issue clutches are built with an internal working-surface area that is relatively small. The result is poor lock up and a great deal of slippage during full, attempted engagement. Driving at only 50 to 60 percent of water pump shaft speed, this high rate of slip causes the light duty clutch to give up, to ‘kick-out’ or freewheel after a period of time. We think this is the result of temperature breakdown of the viscous fluid used inside or a cavitation of that fluid – or both. In any case, most of these light duty, factory issue clutches will stop working after a few minutes under full load conditions. This will always result in a serious overheat condition unless the truck is stopped for a cool-down.

Through years of towing with the 6.5, we have found that aftermarket replacement fan clutches, while representing an improvement over the factory issue unit, still cannot deliver performance good enough for longer, steeper uphill pulls.

When the factory clutches proved inadequate in the heavy towing applications, we switched our focus to...

In this article...

  • Heath Diesel’s 6.5L Diesel Heavy-Duty Cooling System Upgrade
  • Installation of the Cooling System

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