Q: Car is overheating, usually when it's hot and the engine idles.

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I replaced catalytic converter, coolant tank, brakes so far. The car drinks coolant like crazy when it's hot outside, and then it overheats of course. There's brown sludge like stuff in the coolant tank. It doesn't burn oil, so I don't think it's a head gasket. It overheats only when it's hot and I'm idling. If I keep the car in neutral and rev the engine, it's fine for a while. My fans work; both of them come on normally. The heat works, so not thermostat issue.

My car has 182000 miles.
My car has an automatic transmission.

You most likely have a transmission cooler that is leaking into the coolant inside the radiator. The transmission cooler is built inside the side tank of the radiator, and if it blew out, then you will have transmission fluid getting into the coolant and cause it to overheat. You will have to replace the radiator and flush out the cooling system to remove all the oil in the system.

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