About a year ago my transmission cooler failed and sent coolant into my transmission and transmission fluid into my radiator, resulting in transmission failure. Mechanic replaced radiator with a new one and bought a used transmission from local scrapyard. About $8k later and it’s been a year now. Last week I did my routine check up of the radiator cap due to PTSD and voila I see a light red mixture of oil. Called the mechanic right away took it over and his diagnosis was that it’s just some leftover transmission fluid in the system from the previous incident and that the cars running completely fine. I asked for a radiator flush and he said nah don’t worry I don’t wanna waste your money. He topped up some coolant and said it’s all good. Couple days go by I check the cap again and see it’s a much thicker milkshake color. Not sure if that’s because he added more coolant but I’ve noticed my overflow tank took some coolant as well. Not sure if it’s just me overthinking but also nothing a difference in shifting.

If anyone’s got any ideas regarding this I’d greatly appreciate it, really in the dark here. Gonna take it to another more reliable place to do a flush this week. Wondering if anything else could be wrong here.

thanks for anyone’s time in advanced

 

Unless you're in the artic circle, you'd better start running an external cooler. Bypass the internal radiator one .
If you have to replace the radiator again, you can use a manual one.

Hopefully your Trans is OK.
The 5 speed auto is known to overpressurize the cooler lines. Best to install a shift kit, this not only fixes that circuit, but improves other transmission logic parameters as well.

Not sure if the 7 speed is Prone to line overpressorization or not. But it's worth looking into.

 

i got the 7spd. Coupe xs. If the transmission is ok I’m absolutely installing an external one. I don’t know the odds of it being a one way return leak rlly hoping it is. I’m wondering what the diagram that cooling system looks like was thinking it could be an old seal going bad from the pervious coolant trans fluid mixer incident. No piping was replaced and I’ve noticed they are looking very weak

 

If I were in your shoes, I'd have the transmission fluid drained to see if there's coolant present. It's more dangerous to have coolant in your transmission than ATF in your cooling system.