The coolant change procedure for my 9th generation UK Honda Civic is to start the engine, set the cabin heater to maximum, turn off the engine, drain and refill the coolant, bleed air from a nipple, and then warm the engine up (until the radiator fan comes on twice) without using the air-conditioning, before finally adjusting the coolant level.

I turned on the air-conditioning for a couple of minutes when warming up the engine, thinking that the additional load would speed things up, until I remembered that I wasn’t supposed to do that.

What could be the issue with doing this?

  • The engine doesn't need to be warm prior to draining it. However, a scheduled errand before draining hot coolant would be killing two birds with one stone if you insist of a hot coolant drain, Refill with water then a short drive around the block before draining this and fill with new coolant. Warm up the engine and top off coolant in the driveway or drive around the block again, then top off either after engine cool down or carefully, slowly unscrew the coolant cap to ensure hot pressurized coolant doesn't spray out suddenly to scald you. No rules apply, using ac for coolant procedures.
    – F Dryer
    Aug 26, 2023 at 19:47
  • Getting the engine warm before changing coolant may not be a good thing to do. If the engine is warm and you drop cool/cold coolant in on it, you can run the risk of warping something. I know this depends on how warm the engine is, but realistically, there is really no need to warm the engine before draining the coolant. Aug 26, 2023 at 20:20
  • I didn’t warm the engine prior to changing the coolant. I started it, adjusted the cabin heater, and turned it off again.
    – William
    Aug 26, 2023 at 20:26

1 Answer 1


Turning the air conditioning on may cause a valve to close that stops hot coolant going to the heater core in the passenger compartment. Doing this would therefore stop the heater core being bled of air.

I don’t know that this is the reason, but it would make sense.

Here is a guy describing the valve on a Honda, but I have seen these on other cars - https://www.youtube.com/watch?v=6MVnlI8oyi4

  • I'm not sure what you are talking about when you state a valve may be turned off? When you use the defroster, both the A/C and the heater core are used together to clear the windscreen. I don't believe one has anything to do with the other (A/C causing the shutting off of flow to the heater core). Aug 27, 2023 at 11:56
  • @Pᴀᴜʟsᴛᴇʀ2 When the AC is cooling, some cars have a valve that shuts the heater core off so that it doesn’t heat. There is no mention in the OPs comments about the defroster. It makes sense that before you finally top up the coolant you make sure that the air con wasn’t cooling to make sure the heater core is getting water pumped through it.
    – HandyHowie
    Aug 27, 2023 at 12:01
  • I'm not aware of any vehicles which do this. I agree you'd want to purge the heater core along with the rest of the cooling system. Most vehicles have an unfettered flow of coolant through the heater core. The core doesn't stop getting fluid. The way heat is controlled into the cabin is through the blend doors directing air over the core or bypassing it. The only vehicle I'm aware of which doesn't do this is older VW Golf diesel, with which when the thermostat was closed, it kept all coolant internal to the engine. Not saying this doesn't happen elsewhere, I'm just saying it would be rare. Aug 27, 2023 at 12:11
  • @Pᴀᴜʟsᴛᴇʀ2 For example - youtube.com/watch?v=6MVnlI8oyi4
    – HandyHowie
    Aug 27, 2023 at 15:39
  • @Pᴀᴜʟsᴛᴇʀ2 Here is a discussion about one on a civic - civicforums.com/forums/379-air-conditioning/…
    – HandyHowie
    Aug 27, 2023 at 15:43

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