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Flush and refill coolant


A good flush and fill machine is the most practical way to get out at least 90% of the old coolant. You don’t have a machine and want to do a conventional drain and fill? Let’s evaluate this approach.
If you drain the system, top up with plain water, run the engine to circulate and mix the coolant, you’ll dilute the antifreeze. A second drain and top up with water will further dilute the mixture.
A long-quoted rule of thumb is that three drain-and fill operations with top up and coolant circulation will get out about 90% of the old coolant.
The fact, however, is that it depends on what kind of a drain you do, and how much old coolant comes out with each drain. Here’s the math:
Assume that draining the radiator only removes 30% of the total coolant (fairly close to typical). If you start with a 50-50 mixture, the first drain and refill will leave in 70% of the old coolant (35% antifreeze). After a top up with water and engine operation to mix, the second drain will leave in 49% of the old coolant (24.5% antifreeze). A third operation will leave in 34% of the old coolant (17% antifreeze).
You’d have to go through a fourth, fifth and sixth drains-and-top ups to get down to under 12% (6% antifreeze). That would ensure that most of the dirty coolant was out of the system and that the used antifreeze percentage was very low.
If you drain the radiator and block, and that approach removes 50% of the coolant, the system would be down to the same level of old coolant with just three drains. That’s still not a quick proposition.
Bottom line: without a good flush and fill machine, we suspect few shops will get out enough coolant by doing the multiple drains (each followed by engine warm-up to circulate the coolant for mixing); it’s just too time-consuming. And a lot of shops don’t go beyond draining the radiator – that’s really just do-it-yourselfer level service.
When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.
You can E-mail us at or visit to find a Mobile Air Conditioning Society repair shop in your area. Visit to find out more about your car’s mobile A/C and engine cooling system.
Many thanks to MACS member RTI Technologies of York, PA for the photo of their coolant exchange machine that accompanies this post.

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