Why do diesel-powered vehicles need a windbreaker in cold weather?

In northern parts of the US in the winter (and I presume Canada as well), it’s common to see diesel-powered vehicles with some kind of fabric shield across the grille, blocking most of the incoming air that would otherwise pass through the radiator. Is that wind blocker a solution in search of a problem? Can the coolant thermostat really not close down enough to prevent the engine from being overcooled while out on the highway?

It’s not just diesel engines. Gasoline engines can also be over-cooled in cold weather. I’ve had it happen to my pickup truck on really cold days. I have also seen people wedge cardboard in front of their radiators (between the radiator and the grill) on really cold days to prevent the problem. Most commonly though you see the covers on big diesel trucks that you refer to.

With both gasoline and diesel engines, running them too cold can make the oil a bit sluggish, which can cause a bit of extra wear and tear on the engine, especially on the piston rings (or so I’ve been told). You can compensate for that by using thinner oil. Some people who live in very cold areas will put different oil in their vehicles in the winter than the summer.

Colder diesel fuel tends to clump more than colder gasoline, so diesels are more prone to unburned hydrocarbons causing buildup inside the engine if they are run too cold for extended periods of time. As I understand it, gasoline engines don’t suffer from this as much.

My old Toyota Camry (1984 w/1.8L turbo diesel) had a problem with the fuel gelling; not in the tank but in the filter/water separator. I had to Rube Goldberg a shield with a clothes dryer duct running from the vicinity of the exhaust manifold to keep things running on below zero days.

So keeping the engine up to temp and fuel gelling problems.

As a general rule a Diesel uses more of the heat energy to push the piston (which why you get better mileage with a diesel) and thus sends less of the heat to the cooling system. Add to that the great big honkin radiators in large trucks and in really cold weather the thermostat never opens. Diesel locomotives in colder climates will have a winterization hatch which blocks off part of the radiator. Diesel and gas engines both run better when warmed up.

When I was younger (early 1960’s and such) small Fords were overcooled. It was common to see a Falcon or early Mustang or even a Fairlane with a piece of cardboard over about half the radiator.

As Toledo Jim says, diesel engines generate less waste heat, especially the reasonably modern ones.

I have a turbodiesel panel van from 2007 (so not particularly new) and live in a cold climate, it actually has a supplementary a supplementary diesel-burning heater that kicks on automatically to warm the coolant enough to provide cabin heat. With temps around freezing this doesn’t kick in and it can take the engine 10 mins of driving to hit 80c / 180F. On a cold day at say -20c / -4F the extra heater is whining and whooshing away otherwise the engine would be very unhappy indeed.

I’m assuming this was a problem with older gas engines but not so much now. I live in northern Alberta and the temperature this morning was -40C. I drive a plain old 2008 GMC pickup and the engine temperature was up to 100C in less than 10 minutes driving. I’ve seen cardboard in the grill of older trucks years ago but I don’t see it at all where I live now, which is full of newer pick-ups.

I had a V8 gasoline car which had a always spinning fan connected to the water pump. The darn thing would not give heat worth a dam in winter without putting cardboard in front of the radiator. Since it didn’t have a temp gauge I can’t comment on the engine temperature aspect for that car.