Answer
Sep 03, 2025 - 06:14 PM
Not gonna lie, brown, oily milkshake in your rad is never a good sign, and the overheating is a definite red flag. but yeah, saw that couple times in my career, so here's a couple pointers:
When oil and coolant mix, the first suspect is usually a blown head gasket or cracks between the engine's oil and coolant passages. But since your engine oil is fine, we can probably rule that out, which is a huge relief.
So, where is the oil coming from, and where could it mix with the coolant then?
If I had to bet (and it's not really betting since I saw it happen several times before), I'd say the auto transmission cooler located inside the radiator his leaking, allowing the high-pressure transmission fluid to be forced into the lower-pressure cooling system. The two fluids mix and create that nasty brown sludge, which is terrible at absorbing heat, causing your car to overheat.
This is a serious problem because not only is your engine not being cooled properly, but your transmission is losing its vital fluid, and coolant might be getting sucked back into the transmission, which can destroy it.
The fix is to replace the radiator (since the cooler is an internal part) and then perform a complete, thorough flush of both the engine's cooling system AND the transmission to get all the contaminated fluid out. This is a big job, but it's absolutely something you can tackle yourself to save a ton of money.
In any case, I'd suggest getting your hands on the repair manual for your Vento. It will provide the specific, step-by-step instructions for safely removing and replacing the radiator, the correct procedures for completely flushing both the cooling system and the transmission, and the specifications for the exact type of coolant and transmission fluid your car requires to do the job by the book.
I hope that helps! 👍
