just got my block heater working. i have a few questions. how long do you guys leave yours on? is it best only to leave it on a few hours before you need it or just leave it on all the time? also what does it heat? oil?water?both? thanks in advance

The heater heat the coolant only. There has been much discussion on how long to leave you car plugged in. Most people plug it in for a hour or two before leaving. Timers are an easy way to do this when you can get out to plug it in. I find that I only need to plug in the car for an hour to get it to start quickly. If you want heat right away plug it in for a couple of hours and you should be good.
Try searching the forum using the search function and you can find lots to read.

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You can leave it on all night if you don't have a timer. But, it will cost you about $0.35 to do it. It adds up pretty quick over an entire winter.

The engine typically needs only 2-3 hours on the heater unless it's bitter cold (below 0° F.) Then it's a different story and more time is probably necessary.

The coldest it gets here in central OK is maybe below ten once or twice a winter. Usually when it got below freezing I'd plug it in to a timer to come on at five or so and it would fire right up when I left at seven thirty. I imagine if I lived somewhere where it was considerably colder at night I'd leave it on longer, if not all night. It really just depends on where you live.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo

my engine doesn't need the block heater unless the ambient temperature is below around +12deg F. 3 hours with the block heater on does the trick every time at any temperature below that.
glow plugs, starter, and battery all good.

it does make a big difference in both starting and time before heat is available
in the cabin.

remember our engines are not affected by the wind chill factor luckily
only ambient temperature counts.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

One of the 81 300SD I got in New York, does not have a block heater.
I went and bought a magnetic oil pan heater for a Tractor Trailer.
I find it works well, but you have to keep it pluged in all night. Well
I have too because I am living on a island in the north atlantic.
Jay

81 300 SD 138 Miles " Black "
81 300 SD 134 Miles " Diamond Blue Metallic "
81 300 SD 237 Miles " Diamond Blue Metallic "
83 300 SD 166 Miles " Cream "