Why is radiator coolant still green after multiple flushes?
Old coolant can leave lingering discoloration and residue in the system that takes multiple repeated cycles to clear out entirely. This uncertainty makes it difficult to know when the flush is actually complete and whether the system is fully cleaned.
What the numbers show
Counted from the threads quoted below. How we count
- The most recent one is from October 2026.
- This comes up 17.3× more often in r/AskMechanics than across the site as a whole.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1Accepting the current flush condition as good enoughDid this help you?
- 2Repeating the flush process multiple timesDid this help you?
- 3Removing the thermostat, lower radiator hose, and heater core hose to run a garden hose throughDid this help you?
- 4Planning to perform a complete flush of the system while replacing the compressor, condenser, and expansion valves.Did this help you?
- 5Buying pre-mixed coolant instead of mixing it yourself.Did this help you?
- 6Using reverse osmosis (RO) purified water from a grocery store machine.Did this help you?
- 7Looking for distilled water in the baby formula aisle of a grocery store.Did this help you?
- 8Using tap water for the flush and then using compressed air to push it out.Did this help you?
- 9Collecting condensation from a home dehumidifier.Did this help you?
- 10Emptying old coolant, refilling with distilled water, running the car with full heat for 15 minutes, and repeating until clearDid this help you?
- 11Posting on Reddit to get clear answers that resolve conflicting online instructionsDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“I live on a small island and for some reason it's hard to find distilled water for small projects.”source ↗
“Trying to do a coolant flush. Is purified water good enough?”source ↗
“Complete compressor failure on my 200k mile 2015 Sienna. Had the refrigerant drained and found flakes in the filter on the bottom of the drier.”source ↗
“I’m here looking for advice on the flush from the firewall to the trunk. Those are long lines with lots of curves and an evaporator somewhere in between. I’d rather not take apart the dash if I can help it - can I reliably flush that whole system without taking the lines apart?”source ↗
“I’m changing my coolant from yellow to blue and I’m seeing a lot of different things about how I should be doing it. I guess I’m just looking for some clear answers”source ↗
“I’ve also been seeing that you need to run it until the cooling fans switch on but I haven’t seen my fans come on but my car hasn’t overheated or anything.”source ↗
“Also when I refill it with coolant eventually, should I just use the 50/50 or full?”source ↗
“From videos I seen I expected to see muddy but clear water. Not greenish.”source ↗
Where this came up
People with this problem also raised
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