Change coolant hot or cold?

[Ed_Flecko]

When you change your coolant, should you do it after the motor is warm (and the coolant is, therefore, hot) or when it's stone cold? I sort of think that changing the coolant while it's hot might do a better job of flushing any debris out along with the old coolant, but maybe it doesn't make any difference?

Also, I've seen some YouTube videos that say, after you've refilled with new coolant, set the heater to hot and...

1. Don't turn the fan on
2. Turn the fan on low
3. Turn the fan on high

Does turning the fan on make a difference and, if it does, what speed setting should I turn it on?

Thank you,
Ed

 

[JeffKeryk]

Depends on whether you wanna burn yourself.
If it is a regular service, I say cold is fine and just do a drain and fill.
If it is dirty or has the wrong fluid, you may want to drain the block.

 

[Kawiguy454]

My 02 ford had a radiator drain cock so i would mount a tube to a bucket and open Hot to blast any crud out under pressure. Burning yourself badly is a real risk so cold change is 99.99% same result with none of the risk.

Heater on full fan and make sure to KNOW if you have any air bleeders to purge air pockets. Failure to bleed air pockets can cuse real problems ....dont ask how i know this. :-( ...Another thing i wish i didnt know is never use Water Wetter! it turns to Brown Goo as it decays thus the drain under pressure approach.

 

[atikovi]

If you're doing a flush, it has to be hot so the thermostat opens, unless you remove it first.

 

[AutoMechanic]

Cold only.

 

[atikovi]

Cold only.

What's the guy with the air cooled car know about this anyway?

 

[atikovi]

My 02 ford had a radiator drain cock so i would mount a tube to a bucket and open Hot to blast any crud out under pressure.

I thought your couldn't say that on a family forum.

 

[AutoMechanic]

What's the guy with the air cooled car know about this anyway?

I’m a mechanic lol I work on cars everyday. If someone is draining antifreeze hot they are out of their mind hot anti freeze is worse than hot oil.

 

[atikovi]

I’m a mechanic lol I work on cars everyday. If someone is draining antifreeze hot they are out of their mind hot anti freeze is worse than hot oil.

Umm no. Motor oil temperatures can reach over 300 degrees. Coolant maybe 250 and just when under pressure, maybe 230 outside the cooling system. Of course you, wait 10 or 20 minutes if the engine is hot before changing it. And wear gloves. Do you change your oil cold too?....Wait, better not answer that.

 

[Zaedock]

Maybe 300* on track day. Around town, about the same as coolant or less under low stress.
I think he was referring to the danger of opening a pressurized cooling system.

I change my coolant and oil at the temp I like my beer..........cold.

 

[tasch72]

When you change your coolant, should you do it after the motor is warm (and the coolant is, therefore, hot) or when it's stone cold? I sort of think that changing the coolant while it's hot might do a better job of flushing any debris out along with the old coolant, but maybe it doesn't make any difference?

Also, I've seen some YouTube videos that say, after you've refilled with new coolant, set the heater to hot and...

1. Don't turn the fan on
2. Turn the fan on low
3. Turn the fan on high

Does turning the fan on make a difference and, if it does, what speed setting should I turn it on?

Thank you,
Ed

I do it with the engine at operating temp, heater on full hot, full speed, with defrost on. This is the best way IMO to cycle the heater core. If you want to go above and beyond, you can reverse/back flush everything

 

[WylieCoyote]

You can do it cold if you take out the thermostat. But the re-filling & burping process will still expose you to warm coolant. But that's better than scalding hot.

 

[Pew]

I’m a mechanic lol I work on cars everyday. If someone is draining antifreeze hot they are out of their mind hot anti freeze is worse than hot oil.

Or trying to take off the radiator cap for an overheating humvee in the middle of a national forest. EPA is gonna have fun with that one

(All the bad things you think that could happen did happen.)

 

[SHOZ]

Turning the fan on only delays the heating up of the new coolant for it to cycle the tstat. The water is going though the heater core circuit no matter what the fan is doing.

 

[Rand]

I dont let mine get bad so.. drain and fill coolant cold..
Oil esp in winter I'll at least try to get warm 120f-140f min.

 

[JeffKeryk]

Umm no. Motor oil temperatures can reach over 300 degrees. Coolant maybe 250 and just when under pressure, maybe 230 outside the cooling system. Of course you, wait 10 or 20 minutes if the engine is hot before changing it. And wear gloves. Do you change your oil cold too?....Wait, better not answer that.

Well, oil is not under pressure like coolant is. So there's that.
I prefer to change my oil cold, unless I am using the MityVac. Getting to a hot oil filter is some cars, especially our '06 TSX is a messy PITA.

 

[rob1715]

Drain cold. Fill with distilled water and run it with thermostat out until hot. Allow to cool. Repeat until water runs clear.

 

[slacktide_bitog]

Cold!

Drain the coolant when it is cold.

Then, refill using the Lisle funnel. Turn the car on, then turn the heat on (except Honda*), wait for it to warm up, wait for the bubbles to stop, put the radiator cap on, and you're done

Put the stopper in the funnel and do what you want with the leftover coolant in the funnel. Put it back in the bottle or put it with the old coolant for recycling.

*When you change the coolant on a Honda, turn the heat temperature to full hot but keep the blower off.

 

[atikovi]

Cold!

Drain the coolant when it is cold.

Then, refill using the Lisle funnel. Turn the car on, then turn the heat on (except Honda*), wait for it to warm up, wait for the bubbles to stop, put the radiator cap on, and you're done

You do know that leaves about half the old coolant in the system unless you drain the block which most people won't do.

 

[slacktide_bitog]

You do know that leaves about half the old coolant in the system unless you drain the block which most people won't do.

Most people never change the coolant in the first place

So even doing the half with the radiator is already more than most people do

Anyway, there is usually no need to drain the block, and even if you wanted to, it's impossible because the freeze plugs won't come off. That's why they're called freeze plugs: because they're frozen!

When I changed coolant was on a 2008 Civic, I only did the radiator and didn't bother draining the block. The simple drain and fill was around 1.5 gallons, the number listed in the owners manual. You only miss about 1.5 qt, or 20% of the old coolant in the system. See page 297