The single most important factor in starting a diesel engine in very cold temperatures is cranking speed. This is because a diesel achieves fuel ignition by heating the air in the cylinder by compression. If compression takes place too slowly, heat of compression bleeds off into the cold walls of the cylinder, piston and head before ignition temperature is reached. Glow plugs help by warming the air in the cylinders before compression, but glow plugs do not directly ignite the air-fuel mixture. Some engines such as Dodge Cummins diesels have no glow plugs, but have electric air heaters through which the intake air passes.
So how can you maximize cranking speed?
Use the proper engine oil for your temperature range. Check your manual and use the motor oil with the lowest viscosity that is approved for your car at the expected temperature range. This may be a full-synthetic oil with an SAE rating of, for example, 0W-20. (The number before the W is the cold-weather viscosity.)
Use a car battery with the highest "Cold Cranking Amps" (CCA) rating that will physically fit in the battery tray. Although your car's recommended CCA might be 700 or 800, there is no such thing as too much CCA. If an 1100 CCA battery will fit with no clearance issues, you can use it.
Have the battery tested each fall (or test it yourself if you have the equipment) before the coldest weather sets in. A proper battery test involves subjecting the fully charged battery to a heavy load of hundreds of amps for up to 15 seconds, and testing battery voltage at the moment before the load is disconnected. There is no substitute for heavy a load test. It will reveal problems that are impossible to detect with a simple voltage check. If the battery fails the test or is just marginal, replace it before winter.
Avoid short trips that will run down your battery. Living near the arctic circle, most driving in winter involves headlights, seat heaters, heat blower on full and maybe a rear window defogger. After running your glow plugs and cranking your starter, your battery needs voltage, current and time to recharge, and all these accessories running while the battery is trying to recharge will lengthen the time required. Short trips kill batteries.
It goes without saying that you need to keep your engine in top condition with all required maintenance if you expect it to start reliably. Quality, winterized diesel fuel is also essential to prevent wax particles from clogging your fuel filter. If extreme cold, say down to -40F or lower is expected, you can use a diesel fuel additive that is intended to prevent fuel gelling. You'll know that your fuel has gelled if the engine starts and runs fine for a couple of minutes, but then it stalls for no apparent reason.
TIP: If your engine has glow plugs and a "wait to start" light on the instrument panel that stays lit while the glow plugs are heating up, you can "double glow" in extreme cold if you have a good battery. Turn on the ignition, wait for the "wait to start" light to go out, turn off the ignition, turn it back on and wait for the light to go out a second time, then start the engine immediately.
(I've been driving diesel cars and trucks for 35 years, so I speak as a graduate of the School of Hard Knocks! My daily knockabout car is a model year 2000 diesel sedan which still starts and runs great. It has been down to -18F here in New Hampshire in the years I have lived here.)
