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Change Coolant & Radiator Heater Core Flush (Antifreeze + RMI Additive)

Coolant Change on Buick Grand National
Antifreeze Change on Buick Grand National
Radiator Flush on Buick Grand National
Heater Core Flush on Buick Grand National
add RMI-25 cooling system treatment in Buick Grand National

These are all of the things that we are going to do today to the 1987 Buick Grand National.
[which is equally applicable to whatever version Turbo Regal you may have]

All part of the same job, as if you’re going to do a task like this, you might as well do it properly and completely.

Get all the gunk and junk and old anti-freeze out of your cooling system (since who knows how long these liquids [and whatnot] have been floating around in there).


The last time we changed out the coolant (a few years back), we simply drained out the radiator, filled it back up with a gallon of anti-freeze, the rest with tap water, and called it good.

This time around, we want to totally flush the system.
Plus add some RMI-25 additive, since we’ve been hearing good things about it (for a long time now), and want to actually try it to see if it really lowers the temps just a bit.
[and get the bonus of it cleaning out the entire cooling system automatically]


Need to see what type of coolant to use for your Turbo Buick?


[note, there’s products out there you could use – “radiator flush” – however, since we’re adding the RMI-25, which is suppose to clean out the system by itself over time, we didn’t feel the need to use anything else like that right now, we’ll let the RMI do its job, and simply flush the system out for now with plain water]

[side note: if you’re wondering what the RMI additive looks like, imagine a slightly thick creamy cloudy lemonade drink]



* 1 gallon Prestone anti freeze
(full strength, concentrate)
($13-$18 depending where you get)

* 1 bottle RMI-25
(8 oz) – (about $16)

* tap water



* pliers (helps unscrew tough stuck radiator drain plugs)
* garden hose
* drain pan


Just FYI, the petcock valve
(radiator drain plug)
is located on the passenger side of the car,
at the bottom of the radiator.

(the radiator shown in the photos is an
f-body radiator swap
we did on this Buick GN)



[note, you’re probably not going to completely get out all of the old coolant unless you have access to a flusher. the method we’re presenting will do an ok job]

[you can also remove the lower radiator hose, on the radiator side, and lower it down as far as you can get it, to get even more coolant out of the system]


1. place a drain pan under the radiator drain plug.
2. remove radiator cap (helps it come out faster).
3. loosen drain plug.
4. after all the coolant drains out, close drain plug, refill radiator with water.

[you can leave the radiator cap off while you are flushing the system. When the water level gets low, which it will, as soon as the thermostat opens (you can actually see the level drop, a lot!), add more water]

[we’ve also stuck the garden hose inside the radiator to assist in flushing the junk out during the draining process. and yes, all liquids will come spewing out of the radiator top by doing it this way, but you will get a constant flow of clean water running thru the radiator which helps clean it out. make sure you are parked in an area where anti-freeze will be ok sitting (spilling) on the ground. BEWARE anti freeze has a sweet taste to it and animals may want to drink it – WHICH IS VERY BAD FOR THEM!]

[DO NOT leave the hose running in the radiator while the engine is running, the Buick Regal will never get up to operating temp (because of the cold tap water), which means the thermostat won’t open! and hence, no “flow” through the engine, to flush it out!]

4. start Gbody Regal up.
5. turn on the heater (to get the fluid circulating thru there).

[the process shown herein should flush the heater core at the same time as the rest of the system. if you know your heater core needs a GOOD cleaning, you should start with that by itself, (with the vehicle OFF), by removing both hoses from it, then stick the garden hose on one side and let it drain out from the other side. once that is done, reconnect the hoses and continue to flush the rest of the system]

6. after 10 minutes or so, shut off engine.

(give the car time to warm up so the thermostat has time to open up, so the water can actually circulate!)

[when you see green coolant in the radiator, and after the thermostat has opened up, you may want to again stick the hose in the radiator for a minute or so, while the engine is running, to benefit in flushing as much of the old coolant out as possible]

7. loosen drain plug.
8. after all the coolant drains out, close plug, refill radiator with water.

[you may want to again, stick the garden hose in the radiator and let it flush out in that area]

9. start Turbo Buick up again.
10. after 10 minutes or so, shut off engine.

(again, cold tap water will require some time for the thermostat to open)

11. loosen drain plug.
12. after all the coolant drains out, close plug.

[you could also now remove both hoses from the heater core, blow air thru it to get more fluid or water out of the system – but we didn’t]

[flushing the system twice should be enough to (mostly) clean out the entire cooling system, but judge your own vehicle by what’s left in the radiator after the second flush (see lots of green coolant? do it again). repeat the flushing process again and again if necessary, until you see mostly clear water in the radiator]

13. refill radiator with 1 gallon anti-freeze, entire bottle of RMI, and fill up the rest with water.
14. start Turbo Regal up once more.

15. when the thermostat opens this time, fill the radiator again completely with water.

[when the thermostat opens, the fluid level in radiator will drop, and you’ll need to fill it up]

16. put the radiator cap back on.
17. let the Buick Regal run for another 10 minutes.
18. shut off engine.
19. let it cool down for 15-20 minutes before opening radiator cap again.
20. if the coolant level is low, fill it with more water.

21. your fluid level should remain ok after this point, but it wouldn’t hurt to recheck it one last time after you take it for a long test drive.

22. good to go now!

[until the next time that your Gbody Buick Regal needs it done again!]

[do note, that over the course of the next few months, the RMI additive should make any sort of debris float over and into the overflow bottle, so check it every so often for junk within. strain/remove it from tank.]



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