Dirty Brown Coolant?

Messages
657
Location
Florida
Checking the fluid levels today in the 98 Expedition 5.4. I noticed the engine coolant has turned from the nice bright green color to a dirty brown color.
The oil on the dipstick looked good, other than being low, and the transmission fluid levels and color still seemed to be good.
Any ideas on what turns the coolant color brown? What should I be looking for in the way of potential problems? I am thinking to drain off some fluid and let it sit to separate to see what settles (or floats).
The last problem I had was a broken radiator that leaked transmission fluid into coolant, but that seemed to be a different color, but then again, red and green makes brown.
Perhaps this is the early stages of that happening again. Ford no longer makes an OE replacement for that radiator, so I used AA lifetime radiator.
The last time I had the problem, I noticed my transmission fluid getting low and stalling out in reverse. Don't have that yet.
 
Messages
22,892
Location
Apple Valley, California
Rusty coolant does not happen over night.

When the addaitives in coolant can no longer protect metal the coolant turns into an acid. This acid starts to corrode metals. The rust is from the iron block.

It also destroys the softer aluminum parts like the heater core and radiator.
 

Fitz98

Thread starter
Messages
657
Location
Florida
Coolant was flushed and changed a little less that 2 years ago when the radiator was replaced, FWIW.
 

Fitz98

Thread starter
Messages
657
Location
Florida
2021-04-27 21.36.29.jpg
 
Messages
67
Location
IA
It does have an oil cooler.
Time for a new one. These will blow head gaskets occasionally, but normally when they do they either leak oil externally or get coolant in the combustion chamber, so while possible it's not likely. Oil cooler, change the oil, and flush the cooling system should take care of it.
 
Messages
1,396
Location
Pacific Northwest
I think I remember that the oil cooler/filter adapter on those has a gasket that can let let oil into the coolant? If your cooling system has a lot of oily gunk in it Cascade dish washing detergent will help flush it out.
 

Fitz98

Thread starter
Messages
657
Location
Florida
Mechanic diagnosed it was a bad oil cooler.
Replacing oil cooler and lines, coolant flush and fill, oil change, also having fan clutch, idler pully and belt changed to help with A/C performance at low speeds on hot days...$1400.00
 
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