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So my 2004 Dodge Neon's transmission cooler failed. (Neons are notorious for this.) This caused transmission fluid and coolant to mix together, both in the automatic transmission and in the cooling system.

The good news: when it failed, it spilled out on the road all at once instead of subtly leaking internally over time.

The bad news: I unwisely let the car sit for 5 months before starting repairs. But at least, according to my mechanic, coolant is denser than transmission fluid, and they don't blend (like olive oil and vinegar), so the coolant should have settled on the bottom of the transmission pan with the transmission fluid resting on top of it.

I just finished replacing the radiator. Now, I'm wondering what the best way to flush the cooling system and transmission is.

COOLING SYSTEM FLUSH:

Yesterday I flushed it using the Chilton manual technique: I disconnected the upper radiator hose, removed the thermostat, stuck my garden hose into the radiator, and ran the hose until the water ran clear.

Is this enough? Because on StackExchange and YouTube (ChrisFix), the recommended procedure is filling the system with distilled water, then running the engine a while to let it circulate, optionally with some radiator flush chemical added.

Would this be overkill? If so, I'd like to avoid it -- temperatures are now just below freezing for 18 hours a day where I live, making water flushes tricky and possibly risky.

TRANSMISSION FLUSH:

Both my mechanic, and various online sources (namely, scores of negative user comments cursing transmission flushes for destroying their transmissions), have warned me away from power transmission flushes, saying the risk of transmission damage is too high.

Instead, my mechanic recommended I flush the transmission by changing the fluid twice, being sure to run the car in every gear for 5 minutes apiece after the first fluid change. (And also to change the transmission filter.) Again, he said that coolant will settle below transmission fluid (like olive oil and vinegar), so emptying the pan should dump most of the coolant out.

What's StackExchange's opinion of transmission flush techniques?

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  • I agree with your mechanic. 2 or 3 fluid changes in the transmission would be my choice. I don't ever use the transmission flush. As for the cooling system, water flush is not terrible, but I usually use distilled water and run the heater to flush out the whole system. If you did not let the thermostat open up and get the water circulating through the heater core, I think you'll find that there is still old coolant in there. If it's freezing where you live, add some fresh coolant to the water to keep it from freezing – David Watson Dec 3 '20 at 17:32
  • I did remove the thermostat -- forgot to mention that. Added it to my post. – Ishmael Dec 3 '20 at 20:32
  • There is nothing wrong with transmission flushes despite the "old wives tales" from uninformed and ignorant people. If you have the equipment available, that would be the best way to get the coolant out of your transmission. Otherwise, I'd change the fluid once, drop the pan and change the filter, then change the fluid again. – jwh20 Dec 4 '20 at 11:14
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I agree with the first two user comments. Regarding the transmission, however, it should ideally be flushed more than 2-3 times. Depending on the car, dropping the pan on the transmission only removes 25-45% of the total fluid capacity. Mixing fresh ATF fluid with old ATF creates sludge, varnish deposits, clogs filters and can ruin the transmission after a few thousand miles. See this link for further explaination.

I have experienced this phenomenon myself over several decades on a few different cars. Worn ATF does not return to the bright, cherry red color of fresh ATF after 1-2 changes. Five fluid changes is what actually restored the fluid back to normal. I know that's expensive at a shop, so I've changed the fluid myself in between filter replacements by using this hand-held pump through the dipstick.

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  • How much time (or how many miles) do you give it between transmission fluid changes? – Ishmael Dec 5 '20 at 7:20
  • Only 15 minutes of run time on the first AFT exchange, shifting through all the gears. Longer on the second change, say an hour. Add one hour or 100 miles with each successive fluid change until all five are completed. The idea is not to allow time for clumps of sludge to form, which is what some shop manuals recommend on a neglected vehicle. Just before the third change, I would use this flush fluid for 15 minutes at idle to remove any contaminants in the tranny. – Carguy Dec 6 '20 at 23:59
  • On the first exchange, I wouldn't actually drive the car. Here is the link to the flush fluid I recommended before the third change: amsoil.com/p/engine-and-transmission-flush-flsh Remember to shift through all gears. – Carguy Dec 7 '20 at 0:11

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