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I believe they are an always on type, as long as they are plugged in. Hence why I use a outdoor timer for mine, only turns it on a few hours before I go to work.
You can get engine heaters (block or in-line) with timers or external thermostats for more selective on/off times, but they all cause a certain amount of circulation of coolant based on the temperature differential so you're not highly likely to overheat one area and there's a lot of coolant in a block to keep the heater pretty busy on a cold day. I like the in-line heaters that attach to the heater hose - it helps in warming up the cab a lot quicker to keep my hands and backside from freezing, but unless it's really cold, I don't bother using these heaters and just use an electric cab heater on a timer to warm things up before I need to jump into the truck.
I believe they are an always on type, as long as they are plugged in. Hence why I use a outdoor timer for mine, only turns it on a few hours before I go to work.
This is a Good idea, think I might do the same. Currently I dont use it, ever. haha
for decades chrysler had for at least some car/trucks factory heaters in the soft plug. BUT they are made of steel, ie rust. i liked the one with a magnet to hold it to bottom of the oil pan.