Coolant Flush

[RPGR90s]

I have a question. The other day I wanted to do a coolant flush on my 2019 ST. The car was purchased now and currently has 103k miles. It's been a perfectly flawless vehicle.

 

At the 1st coolant service, I had the Ford dealer flush and re-fill. They swapped out the original orange colored coolant for a green colored coolant. 

 

Now I see that the latest Ford coolant is yellow colored, so that's what I purchased. 

 

I found the drain (boy what a fun job it is getting access to THAT!!) and drained the old coolant. 

I then added a coolant flush with distilled water and let the car come up to temperature.

I ran this for the specified interval (10 minutes) and drained again.

 

The liquid coming out was mostly clear so I filled the tank with the new, yellow colored coolant and brought the engine back up to temperature. 

 

I let the car cool down over-nite and in the morning the tank was a bit low, so I topped it off.

 

But what caught my eye was the coolant reverted back to a solid green color. 

 

Did I miss a step in my probess and possibly not get all the old coolant out?

 

Thanks for any advice or direction.

 

RPG

 

[1004ron]

Running the coolant flush for only 10 mins would not have warmed up enough to open the thermostat and you should set the cabin heating to MAX for it to circulate through it.

[RPGR90s]

3 hours ago, 1004ron said:

Running the coolant flush for only 10 mins would not have warmed up enough to open the thermostat and you should set the cabin heating to MAX for it to circulate through it.

If I recall, the engine was already at max temperature and I did have the heat set to Max.

 

Thank You!

[Samurai Edge]

Just remember you are not completely draining the coolant from the engine with a normal flush.  Old coolant will still reside in the engine so this may be the cause of your " green "coolant.

[1004ron]

The only way to get only the new coolant in the system is to do a few drain-fills with distilled water and then the final fill with a higher concentration of the new coolant expecting some distilled water to remain in the engine.

 

 

[Samurai Edge]

One could also use a system like the following to flush out the entire system and then fill with concentrated ( non-diluted ie; 50/50 ) anti-freeze

 

https://www.amazon.com/dp/B0002SRH5G/?coliid=IBXVQ52CJGVOB&colid=1VY88PYQ3KPE1&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

 

[johnmarkp]

Not sure why you recommend pure antifreeze?  Pure ethylene glycol freezes at 10f.  The normal 50:50 mixture freezes at -35f.

[1004ron]

1 hour ago, johnmarkp said:

Not sure why you recommend pure antifreeze?  Pure ethylene glycol freezes at 10f.  The normal 50:50 mixture freezes at -35f.

Not sure who that comment was directed at?

 

I always buy the concentrate and mix it 50/50 and the product details state clearly not to go higher than 60/40.

 

My comment was to drain/fill a few times with distilled water and that way you would have removed about all of the old coolant mix, then go back with say a 60/40 mix knowing that the engine still contains some straight distilled water.

 

I use a vacuum fill kit similar to this. 

Edited August 2 by 1004ron

[Samurai Edge]

2 hours ago, johnmarkp said:

Not sure why you recommend pure antifreeze?  Pure ethylene glycol freezes at 10f.  The normal 50:50 mixture freezes at -35f.

 

I stated concentrated, ie; non-diluted, because when you flush the system as described, you will be left with water still in your system.  By using the concentrate, you will be close to a 50 / 50 mix.

 

BTW, the concentrate is cheaper than the 50/50 mix from the supply store and you are paying for water.

As 1004ron said, I also buy the concentrate and dilute on my own

[1004ron]

I have done two drain-fills on my 2017 Sport.

 

The fact that the new spec Ford coolant is compatible with the old spec there's no issue with not purging all the old coolant and I didn't purge the system on each occasion - the first time it the fluid in the reservoir didn't look anything like the new color, and with the second drain-fill its closer to the new color.

 

Maybe next time I'll purge it with distilled water before refilling.

[1004ron]

On 8/1/2026 at 5:40 PM, RPGR90s said:

If I recall, the engine was already at max temperature and I did have the heat set to Max.

 

Thank You!

Just revisiting this - fairly risky to drain the coolant with it at normal operating temperature and even if that was done, it would be well below operating temperature with the new cold coolant just added and would take longer than 10 minutes to get back up to operating temperature.

[Samurai Edge]

On 8/3/2026 at 7:54 AM, 1004ron said:

Just revisiting this - fairly risky to drain the coolant with it at normal operating temperature and even if that was done, it would be well below operating temperature with the new cold coolant just added and would take longer than 10 minutes to get back up to operating temperature.

 

So, you think getting hot coolant splashed in your face is bad?

 

Check!   🙂

[NzP]

I plan to do this procedure tomorrow. I plan to drain over a drain plug, remove the thermostat, connect the housing back and apply about 15-20psi air pressure trough the expansion tank to get rid of all the old coolant. Then I will install the thermostat back, vacuum the system and fill it up with a fresh coolant. But the question: if I will remove thermostat - it's going to leak there because of no thermostat(like 1mm thick) should I make some gasket(from thick paper or a plastic) and put in next to the thermostat o-ring?

[1004ron]

@NzP  That's a whole lot of unnecessary work removing the thermostat. 

 

The typical home size compressor can provide the pressure but not the volume needed to displace the coolant residing in the engine block - I used a leak blower which was more successful but also cant remove all the coolant.

 

Flushing with distilled water and then adding concentrate to account for the remaining distilled water in the block is a more effective option.

 

 

[NzP]

probably, but I already bought premixed coolant and can't return it. So I am looking for the best way to push the old coolant out(I do have a vacuum tool to fill it).

I can use a big compressor(like 20 Gal) but i don't think I need it because I will not apply more than 18-20psi at the time. I am still not sure about the best way: I have an air adapter to the reservoir, I can disconnect the heater core hoses, plug one pipe on the water outlet and provide pressure to the other one(keeping the radiator drain plug open), I can easily take the thermostat out... 

Edited 11 hours ago by NzP

[STBEAST]

Don't count me in on being a good input on a proper coolant flush. But this seems like a good opportunity to take it to Jiffy Lube and have them do it for you. I never use Jiffy lube for anything, but they seem to be on their game when it comes to this. Years ago, they did a coolant flush on my wife's Mazda 626 and it was well worth the money. Or take it to the dealer and hope they do it right?

Edited 8 hours ago by STBEAST

[NzP]

I will not trust someone working on my car. I used to work as a car mechanic for years. Now I'm not sure which way is the best to get rid of the old coolant and looks like no one knows(here and YT). So probably I will drain 1 gallon and add 1 gallon premixed I got(it says fits for all cars)...

[omar302]

4 hours ago, NzP said:

it says fits for all cars

 

That is definitely not the way to go. There is no one coolant for all cars. 

 

As @1004ron suggested, is the best way to go. Drain/fill with distilled water several times until you have mostly distilled water, then drain and fill with concentrated coolant. Depending on you engine, fill half the coolant capacity with concentrate. 

Between drain/refill cycles, make sure to run the engine until it warms up to ensure complete mix.