# When do I need to use my block heater? **URL:** \ **Category:** Factual Questions\ **Created:** [November 8, 2003, 2:17am UTC](https://boards.straightdope.com/t/when-do-i-need-to-use-my-block-heater/212019 "2003-11-08T02:17:52Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Fuji\_Kitakyusho](https://avatars.discourse-cdn.com/v4/letter/f/f0a364/32.png) [@Fuji\_Kitakyusho](https://boards.straightdope.com/u/Fuji_Kitakyusho)\ **Post date:** [November 8, 2003, 2:17am UTC](https://boards.straightdope.com/t/when-do-i-need-to-use-my-block-heater/212019/1 "2003-11-08T02:17:52Z")
Just curious - when do I need to plug in the block heater? Previous vehicles I have had with block heaters installed, I generally only used them in winter in very cold places (i.e. minus 20 C or so). Around Vancouver, I never used it at all. My vehicle is a 7.3 L diesel with (I think) 15W40 oil - I heard that diesels need the heater more than gasoline engines do, and I certainly have noticed a bit of roughness on cold starts lately, as we have been having a cold snap (by Vancouver standards - not really cold). Do I just use it based on how the engine sounds, or is there a rule of thumb for this? ---
**Author:** ![NurseCarmen](https://avatars.discourse-cdn.com/v4/letter/n/5daacb/32.png) [@NurseCarmen](https://boards.straightdope.com/u/NurseCarmen)\ **Post date:** [November 8, 2003, 2:54am UTC](https://boards.straightdope.com/t/when-do-i-need-to-use-my-block-heater/212019/2 "2003-11-08T02:54:30Z")
It’s used more to keep the oil viscous, it isn’t really to warm the fuel or anything. Friction inside even a warm engine will heat things up real quick. IIRC under 10F oil flows slower, and viscosity starts to go down hill. ---
**Author:** ![chique](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@chique](https://boards.straightdope.com/u/chique)\ **Post date:** [November 8, 2003, 5:12am UTC](https://boards.straightdope.com/t/when-do-i-need-to-use-my-block-heater/212019/3 "2003-11-08T05:12:53Z")
I tend to start plugging my car in the day after the first day it fails to start.