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In some countries it is quite popular to cover up the front grill in winter (especially on diesel engined cars). As I understand the intention is to inhibit airflow to the radiator, but why would you do that if coolant flow to the radiator is allowed by the thermostat only when operating temperature is reached? As far as I can tell it all it could do is cause harm, such as overheating and lost intercooler effectiveness.

What is this???

I've actually seen special plastic or fabric covers sold for specific models, so it must have SOME kind of use:

Do you really need this thing?

So what's the catch, when do you need one if your cooling system is in order? I've heard of people saying that sometimes the interior heater is so effective that the engine is having trouble keeping the temperature up, but how would covering up the non-functioning radiator work? Is cold air hitting the outer shell enough?

  • I assume this is more common in areas with cold climates. Am I right? – Zaid Jan 20 '16 at 16:38
  • Yes, during cold weather only. – I have no idea what I'm doing Jan 20 '16 at 16:55
  • From my limited experience with a VAZ-21011 (probably with a faulty thermostat) when the temperature outside was below -30, the coolant never ever got into the big circle. The cabin heater didn't heat anything but the lower part of the windshield. Installing the cover (just a piece of cardboard) did fix that problem. – Quassnoi Jan 20 '16 at 19:18
  • It could also be to protect the radiator. Very cold air flowing over the thin veins of a radiator could drop the temperature lower than the freezing point of 50/50 antifreeze. – HandyHowie Jan 20 '16 at 20:18
  • @Quassnoi I understand the limited use in the case of a faulty thermostat, but buying a new thermostat is cheaper than buying one of these fancy covers. – I have no idea what I'm doing Jan 21 '16 at 7:36
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I can see a few reasons to do this:

  • The weather in some places will be so cold that even with the thermostat fully closed the cooling system isn't able to warm the engine up to operating temperatures. Inhibiting air flow through the radiator will reduce the amount of heat rejected to the surroundings, allowing the engine to get up to temperature. The cold environment lends to a reduced risk of overheating.

  • Preventing air flow through the radiator grill helps reduce drag coefficient on some vehicles, which helps improve fuel economy. BMW has introduced a dynamic kidney grill on their 2016 7-Series for this purpose (provided the necessary conditions are met - so the system is never active on a hot day with the engine under load).

  • Because everyone else is doing it and it looks "cool". Though this reason is more psychological than technical :)

I'm not sure if there is anything diesel-specific going on here.

  • Can't say I'm convinced. Why would the engine care if the radiator is covered or not if no coolant is going through? Pretty sure cold wind blowing at the side of the engine outer layer is not going to change the inside coolant temperature much. And these are definitely not used for reducing the drag coefficient, it's used exclusively in winter and without other drag reducing mods. "Everyone is doing it" is definitely true, but there must be some basis. And it's much more often used on diesels, which emit less heat as far as I know. – I have no idea what I'm doing Jan 20 '16 at 16:53
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    @IhavenoideawhatI'mdoing There will still be some minimal amount of coolant flowing through when the thermostat is fully-closed; you can't have a cooling system designed to have zero flow. Inhibiting air flow through the radiator will minimize heat dissipation in the radiator via convection. – Zaid Jan 20 '16 at 17:20
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    @IhavenoideawhatI'mdoing I've been thinking about diesels. They are more sensitive to cold weather since they rely on the engines reaching a certain temperature for autoignition to occur (think glow plugs), At least petrol engines can ignite through spark. – Zaid Jan 20 '16 at 17:26
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    Trolling some diesel forums seems to indicate that they are primarily for getting the engine up to proper temperature faster and having heat in the cabin faster. – JPhi1618 Jan 20 '16 at 17:39
  • @JPhi1618 yes, same results. Root cause=impatience....in many cases. I want to be warm!!!!! – DucatiKiller Jan 21 '16 at 7:00
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This is old, but some of these comments annoy me. I live in Fairbanks AK and drive a 90's diesel Chevy. It takes a 15 minute idle period (in addition to at least an hour with the block heater plugged in) and 3-5 miles of highway driving for the coolant temp to get over 120 degrees when it's 10 degrees or colder. When the temp is below -20f or so, the coolant will never reach the proper operating temp, even pulling steep hills at 70mph. My rad cover lets my temps be in acceptable operating range in less time, which reduces idling and engine wear. Enough cooling takes place to prevent overheating, though I will take it off if I need to tow when it's that cold. It's also not true that closed thermostats prevent all water movement, they are not water tight.

  • On my 90s Dodge with the Cummins 5.9L, I can't even idle it up to operating temp. Cummins explicitly states that, in cold weather, you should not idle it for more than 10 minutes (that's even after it's up to operating temp) because fuel washdown will starve the cylinders of oil and dilute the oil in the pan - at idle in sufficiently cold temps, the cylinders don't stay hot enough to burn all the fuel, and it collects in the cylinders. I have to run it at 1500rpm or higher to get it to move the needle, and if its cold enough (under 15 degrees F), it never warms up like this. – Dr. Funk Jan 16 '18 at 15:24
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An anectodal answer to this...

Back in the '70s I owned a Moskvich 412, a Russian car built to survive in Siberia. This had a brass venetian blind arrangement in front of the radiator that could be opened and closed from a lever beside the steering wheel.

The handbook stated that this blind should be closed when starting the vehicle at temperatures below -10ºC and should be opened when the temperature gauge reached normal. It also said that in temperatures below -40ºC it should never be opened. The reasoning given was that the blind reduced airflow throughout the engine compartment, not just through the radiator itself. This gave the engine a chance to reach an efficient temperature. But of course the blind could be opened if the engine started to overheat.

So, yes, there is a point to blanking the radiator and it doesn't cause damage if you keep an eye on the temperature gauge.

The car had a petrol engine, not diesel. There is nothing particular about blanking the radiator of a diesel. As the weather never gets that cold in Southern England I never closed the blind in anger.

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