1986 325es radiator overheats in traffic

Hello All, happy Friday! My 1986 BMW 325es runs hotter in traffice and when I am on highway (or motorway for you Europeans!) it cools down a little bit.
The car has 215k miles on it and eversince I put a performance chip in it (bought it online) it runs great (have to use 91 octan fuel though) and I do get aroung 26MPG on highway.
When I am driving in traffice the gage shows it is at 1/2 marker point but on highway it moves down a little bit and stays in between 1/4 and 1/2.
I just drained the coolant and put new antifreez in the car thinking that it might help but it did not. Could it be the thermostat? The fan blades look normal and raditor does not leak at all.
The car runs fine even in traffice but I do get the antifreez overheating smell that you usually sense when cars overheat. Smell is obviously stronger when you open the hood.
Any ideas anybody?
Thanks in advance.
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Yes, it could be.

Does the fan feel stiff when you turn it? A common failure is that the fluid clutch on the fan will fail and become loose, so the fan does not turn at the proper speed when the engine is hot.
Don't forget to pressure-test the radiator cap.

Could be anything, but check the fan clutch, replace the radiator, and test or replace the radiator cap before you swap out the radiator or pump. There isn't much else to go wrong.
If you pull out one of the sensors and you see a layer of mineral buildup on it, you may want to consider doing an acid flush on the system too. Note that when you do the acid flush, anything that is a little leaky is going to turn into something very leaky almost immediately, so be prepared to do some replacement if you risk the flush. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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You have described the exact problem that arises from having a worn out fan clutch.
While in traffic, the air flow through the radiator slows, and the temp rises. The fan clutch (viscous coupler) heats, the viscous material (a sort of jell-like material) expands and causes the clutch to lock, which physically attaches the fan directly to the water pump pulley which is driven by a fan belt. The result is that the fan draws air through the radiator to bring the heat down.
When the clutch fails, the fan never locks to the water pump pulley, and the result is insufficient air flow and an associated heat rise. when the vehicle speed increases, cold air is forced through the radiator, cooling the viscous material, releasing the fan from the pulley and freeing up a bit of horsepower and reducing noise at the same time.
Your fan clutch is toast. You can buy one from the auto parts store, the price range is rather broad, and you can expect the price to be $100ish. It is not difficult to replace.

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snipped-for-privacy@gmail.com wrote:

Sounds like it's underheating not overheating. BMWs sit at halfway on the gauge and don't budge once they're warmed up.
It probably is your thermostat - sticking open. When you're moving faster there is too much airflow cooling the radiator and the thermostat should be closing to compensate, but isn't.
That's if your temperature gauge is working properly.
Dan
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