I did a radiator and coolant flush on my 04 corolla about two months ago. I flushed 6 times with distilled water. About 3 liters each time. And I tried getting it to full heat with heater and blowers on between each flush to get the heater core flushed also.

I then drained the last flush and got about 3 liters out.

I then filled up the radiator through the cap with 100%official red coolant and filled the reservoir also with 100%.

I put about 3.5 liters of 100% coolant total into the radiator.

The book says it is like 6 or 6.5 liters total capacity. So I figured this should get me close to a 50/50 mix once everything mixed from the heater core.

2 months later here, and I've been measuring every few weeks with one of those peak hydrometers, where the triangle pin goes to a temp. It still goes to the point of 100 percent coolant.

I've tried driving it a lot with heat on high. But still 100% when measuring at the cap or in the reservoir.

So it seems it isn't mixing with the heater core. Even it I never flushed the core originally, I should be near a 75% mix now if it was mixing. But I'm not.

I'm about to drain half out to put in water, because I'm worried it's killing my water pump.

Why wouldn't it mix. I know I only put in about 3 liters of 100% red coolant. But after months of driving, it still seems to be 100%. I've had no overheating problems through the hot summer, and I have good heat from my vents.

What should I do? To either get it to mix or should I just drain some and add water?

1 Answer 1


Just to be sure try another hydrometer to measure coolant mix. If it still reads 100%, then yes drain some and add distilled water. Test with hydrometer again, 50-70% mix is safe.

  • Any idea why it wouldn't be mixing? Should I be worried that it doesn't seem to be mixing into the heater core? My concern is I drain some and add water to get to about 50/50, but then it eventually mixes with whatever potential water is in the core, and all of a sudden I'm at way less than 50.
    – Nertskull
    Aug 15, 2016 at 11:41
  • I don't why you are fixated on the heater core, since you are measuring AF in the radiator? Turn the heater on to flow water through the heater core, it it outputs hot air through the vents then water is flowing,
    – Moab
    Aug 15, 2016 at 21:32
  • I guess I'm fixated because I don't understand how I can flush the heater core, and the entire system 6 times with distilled water. Then fill it up with 100% coolant at half the engine capacity, and then have that remain at 100%. I'm definitely going to do as you suggested and drain some and put in water. I'm just also confused what happened to all the water I flushed it with. So not fixated really, just trying to understand if I did something wrong in the flush
    – Nertskull
    Aug 15, 2016 at 22:50

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