Starting on page 103 of the 6.2 handbook is some good info applicable to the 6.5s..
"General Motors will not recommend the use of Number 2-D diesel fuel below 20°F unless it is “winterized” .
Temperatures below 20°F will cause the “ non-winterized” fuel to thicken which may keep the engine from running.
The colder temperatures will cause number 2 diesel fuel to thicken or cause a wax build-up. This wax buildup could plug the fuel filter and keep the engine from running."
COLD WEATHER OPERATION (DIESEL ENGINES)
Diesel fuel is sensitive to temperature. All diesel fuel has a certain amount of heavy paraffin-like components, which are high in energy value and help improve fuel economy. But, when temperatures are less than about -7 °C (20°F), these heavy paraffin components begin turning into wax flakes. If temperatures are low enough, these flakes can build up on the fuel tank filter or the engine fuel filter and stop fuel from reaching the engine.
At low temperatures, wax flakes are more likely to form in Number 2-D fuel than Number 1-D (or a “winterized” 2-D) fuel. For best operation at temperatures below - 7 ° C (20°F) use Number 1-D, or Number 2-D which has been blended with Number 1-D for winter use.
If you are driving in temperatures less than - 1 8 °C (0°F) and do not have Number 1-D or “winterized” Number 2-D fuel in the fuel tank, kerosene can be added to reduce waxing. Kerosene should be added at a ratio of one gallon of kerosene to two gallons o f diesel fuel. Because of the lower energy value of kerosene (and reduced fuel economy) it should be added only when anticipated temperatures are less than - 1 8 °C (0°F). Once kerosene has been added the engine should be run for several minutes to mix the fuel.
The addition of kerosene will not unplug a filter plugged with wax. Warming a “waxed” filter 0 °C to 10°C (32°F to 50°F) will return the wax to solution. Filter replacement is not normally required.
To improve cold weather operation, an engine block heater and fuel heater are on your diesel engine. (See “Cold Weather Starting” under “Starting the Diesel Engine” in Section 1 of this manual for information on the block heater.) The fuel heater is designed to come on when the fuel temperature is less than 4°C (40 °F). It warms the fuel and helps stop wax flakes from building up in the fuel filter.
FUEL PICKUP AND SENDING UNIT
See Figure 4-4. The fuel pick up, commonly known as the “ sock” has three functions:
1. Strain out large solids.
2. Act as a strainer to prevent entry of water.
3. Act as a wick to drain fuel down to the bottom of the tank since all pickup pipes do not reach the very bottom of the tank.
The tank filter is a Saran (Polyvinylidene Chloride) sock and is fastened to the fuel inlet line of the in-tank fuel filter and fuel pick-up assembly.
The fuel tank filter sock has a bypass valve which opens when the filter is covered with wax allowing fuel to flow to the fuel heater.
Without this sock fuel line heater would be ineffective because the fuel would be trapped in the tank. Since the bypass valve is located at the upper end of the sock, fuel will only be drawn into the waxed sock if the tank contains more than approximately 4 gallons of fuel. Therefore, it is important to maintain a minimum of 1/4 tank of fuel when temperatures are below 20 degrees F.
The Saran sock material has a nominal pore size of 130 microns. In addition to acting as a particle filter for the mechanical lift pump, the Saran tank filter acts as a wick to pick up fuel from the bottom of the tank and as a water filter; water is excluded on the basis of the difference in surface tension between the water and the sock material on the one hand and the fuel and the sock material on the other.
By law in many states, water in fuel should be no more than 1/2 of 1%. That quantity of water will be absorbed by the fuel. Periodically, station operators check for water by putting a special gel on the dip stick. If it turns color, then water is present and it can be pumped out. Unfortunately, not all station operators are responsible and this prompted the use of the Saran sock.
The fuel pickup tube doesn’t reach the bottom of the tank. However, since the sock acts as a “wick” the fuel level can actually be lower than the level of the tube and fuel will be drawn out right down to empty. Also, with this design, the level of water in the tank can be much higher before water enters the fuel system. This is about five gallons. Water that gets into the tank will eventually be absorbed by good fuel and will pass harmlessly through the fuel system. Water will be absorbed at a rate of about one gallon per 1000 miles.
— IMPORTANT —
The 6.2L diesel fuel tank sending unit is a 3-pipe assembly (main fuel, fuel syphon, and fuel return). The 4.3L V-6 and 5.7L diesel sending unit is a 2 pipe (main fuel and return). The gasoline fuel tank sending unit is a two pipe assembly (main fuel and canister). The canister pipe has a .055'" orifice in the end of the pipe.
It is important that these two units not be interchanged. If the gas unit is installed on a diesel, it will cause intermittent problems with idle and power loss. If the diesel unit is installed on a gas car, the fuel vapors to the canister will be uncontrolled. The ground wire on a diesel sending unit is a different color than on a gas unit for identification purposes. The tubes are also different sizes: main fuel 3/8 inch, gasoline fuel tank sending unit canister tube is 5/16 inch and the diesel sending unit return tube is 1/4 inch.
"All vehicles using diesel engines have a sock with a bypass valve in the top end. This bypass valve is designed to open up in the event that high cloud point fuels are used in cold weather and the sock gets plugged with wax crystals. The fuel level should be kept at a 1/4 tank, to make sure you do not run out of fuel."
