Winter may seem like a long time away right now but Fall is right around the corner. That is the time to begin preparing your vehicles for colder temperatures to ensure that they’re ready for what winter has in store. Below is a checklist of items for which bus fleet operators need to be proactive to prepare for extreme conditions, whether you are running diesel or natural gas engines. These procedures, ranging from cooling system maintenance to fuel choice, to selecting the right engine oil, will help fleets keep their buses running safely and reliably in cooler ambient temperatures.

Diesel Engines – What is Effected

  • Verify that all coolant lines and connections are leak-free.
  • Use the proper coolant/antifreeze mixture (ethylene glycol concentration) for route conditions/temperatures. A 50/50 mixture of ethylene glycol and water lowers the freezing point down to about -34°F. If you operate in temperatures colder than this, a 60/40 mixture will reduce the freezing point to about -64°F. Be sure to consult the Owners Manual that came with your vehicle. Always ensure that after any maintenance event, proper filling procedures are followed and that the cooling system is purged of any air to protect the Exhaust Gas Recirculation (EGR) cooler.
  • Add starting aids such as a coolant heater/intake manifold heater and/or oil heater wherever temperatures typically drop below 11°F (-12°C).
  • Use winterized diesel, or blend #1 and #2 fuels.
  • Add a fuel warmer to the fuel system.
  • Double-check Diesel Exhaust Fluid (DEF) warming lines before temperatures drop.
  • Modify the air intake in extreme cold (-25°F and below) to a position adjacent to the exhaust manifold.
  • Check the cold-cranking capacity of the battery. Add a battery warmer in extreme cold conditions.
  • For diesel-powered engines fleets should switch to 5W-30 engine oil for normal winter conditions. When temperatures go below 5°F (-15°C), you may need to consider switching from 15W-40 oil to a 10W-30 or lighter oil. Fleets should consider switching to 0W-30 when encountering prolonged arctic cold conditions. The oil selected must meet Cummins CES 20081 specifications for diesel engines.
  • Use a dipstick oil heater to help maintain oil lubricity and improve the engine’s cold-starting capability.
  • Closely monitor and drain the water/fuel separator by opening the petcock valve and releasing any water that has accumulated in the fuel filter/separator. It is especially critical that operators perform this function when buses are running on biodiesel. Biodiesel suspends a higher amount of water than diesel fuel, has a different additive package and may require more frequent fuel-filter changes than normal. For natural gas buses, ensure that the fuel filter is checked/drained daily to prevent oil and/or heavy hydrocarbons from collecting in the filter.
  • In below-freezing conditions in which an engine typically idles for more than 15 minutes, adjust the “Fast Idle” setting to 1000 rpm to 1200 rpm. A normal idle setting of 700 rpm to 800 rpm may not provide enough heat for regeneration events. Bus operators should use the “Fast Idle” switch on the dash. Refer to your Original Equipment Manufacturer (OEM) operations manual to learn how to set up engine speeds for fast idle. When prolonged idling is required, make sure the RPM is adequate to heat the coolant above 140°F (60°C).
  • If needed, add a full on/off-type fan to aid in maintaining optimal operating temperatures.
  • Insulate exposed lines, filters, pumps, and reservoirs.
  • Add a winter front for conditions below freezing.
  • If operating regularly in conditions under 32°F (0°C), pull air from within the engine compartment. Under -2°F (-19°C), pull intake air from a compartment around the exhaust stack to preheat the air.

Cummins Westport Natural Gas Engines

Cold-weather preparation/operation procedures for Cummins Westport natural gas engines are similar to those for diesel engines (block heater, coolant heater, battery warmer, radiator shutters or winter fronts, etc.). CNG filling stations should include a dryer to remove moisture from the natural gas.

Dry fuel is an important consideration for cold-weather operations. It is important to consider minimizing load on the engine at start by turning off Power Take-Off (PTO) accessories such as hydraulic pumps, etc.

Natural gas fuel systems include a pressure regulator that is kept from freezing with a supply of warm engine coolant. In cold weather, allow the engine to warm to operating temperature before operating under load. Maintain intake air temperatures above freezing through the use of winter fronts and/or warm underhood air. The correct engine coolant, lubricating oil, and fuels must be used for the cold-weather range in which the engine is being operated:

Ambient Temperature 32°F to -25°F (0 to -32°C) – Use 50 percent ethylene glycol antifreeze and 50 percent water for the engine coolant mixture.

Ambient Temperature -25°F to -65°F (-32 to -54°C) – Use 60 percent ethylene glycol antifreeze and 40 percent water for the engine coolant mixture. Refer to Section V (Lubricating Oil Recommendations) of your Owners Manual for correct specifications.

For more information on Cummins diesel engines for the school bus market visit
For more information on Cummins Westport natural gas engines visit
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