I did a test by putting a digital thermometer probe under the hood near the air horn on my Air Filter Housing.

What I found is that the temp went up when the car was idling and not moving. When the car is moving the temp drops to near what the outside temp is very fast and stays that way even at Freeway speeds. When you get off the Freeway an idle at a Traffic Signal the temp goes up again until you start moving.

I think that unless you have some sort of belly pan to keep the colder air from coming up into the under the hood area when you are moving there is no hot air available except when you need it the least; at idle.

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84 300D, 82 Volvo 244Gl Diesel