TLDR: What portion of the cooling system is rust formed in (I.E. Engine block, radiator, all... etc)

I did a coolant flush in a 94 Ford Ranger 2.4L at 226K. It was a consistent brown color and the radiator had brown residue inside the pressure cap. I would not be surprised if the coolant had over 120K and 10 - 15 years on it. After four complete flush cycles it was still coming out brown. I did not have time, so I just filled it with coolant/water mixture and called it good for the next couple months (plan on replacing the radiator/water pump soon). This all got me wondering, is there a specific portion of the cooling system where rust is typically formed, or is it all the metal surfaces in the system.

Might be an obvious answer but I am new to cooling systems.

Lastly, the engine oil is not a cream color so I am not worried about the coolant discoloration being from a head gasket leak.

  • It is mostly from the radiator and metal pipes around the block
    – method
    Commented Aug 1, 2017 at 5:37
  • Er.. wait just a moment. Did you say water (as in tap water?) Ouch. You need to use distilled water mixed with antifreeze. The ions in tap water cause corrosion.
    – zipzit
    Commented Aug 1, 2017 at 6:24
  • It's all the metal parts especially dissimilar metals in electrical contact and the rust itself is down to the water jacket in the head / block as most radiators are aluminium.
    – Solar Mike
    Commented Aug 1, 2017 at 7:35
  • @zipzit good looking out, but thankfully no, I did enough research to know that I should use distilled water.
    – jwmorse
    Commented Aug 1, 2017 at 13:50

1 Answer 1


The three most common metals in automotive engine cooling systems of the last 30 years are iron, steel, and aluminum. Iron and steel rust through oxidation and aluminum corrodes but aluminum oxide tends not to. Aluminum oxide actually creates a barrier against further corrosion and slows down the oxidation process which is why it is common to see high performance radiators made out of aluminum.

In short, all the iron and steel parts that the coolant touches will begin to rust when the molecules in the coolant begin to break down or leaks introduce excess moisture and air into the system.

Some cars have some more exotic metals like magnesium which behaves similarly to aluminum but corrodes significantly faster so it is not used in high heat/moisture parts of the car.

If you replace coolant before it breaks down, you will see little to no rust.

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