Why is there brown foam in my coolant reservoir?
Brown foam on the radiator cap accompanied by boiling sounds and bubbling after parking indicates an unknown substance has mixed into the cooling system. Drivers are left trying to determine if this mysterious substance is caused by an oil cooler leak, transmission fluid, or accidentally mixed oil.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1changing the thermostat
- 2checking the oil dipstick and oil entrance
- 3driving on the highway to check for overheating symptoms
- 4flushing and draining the entire cooling system
- 5checking transmission fluid and engine oil condition
- 6starting an arbitration or return claim with the auction
In their words
Unedited, most upvoted first, each linked to the thread it came from.
“I have never seen something like this and dont understand if someone mistakenly put oil in the coolant or what happened.”source ↗
“Does anyone know what this substance can possibly be?”source ↗
“when I park my car, even at 92 or 95 degree Celsius, after 2 or3 minuted i hear bubbles pop and the car is boiling.”source ↗
“on the radiator cap I found brown foam.”source ↗
“Now I am breakkng my head. Transmission fluid, is that possible ? The Oil cooler leaking ?”source ↗
Where this came up
People with this problem also raised
- 2Why is my engine coolant brown or gold?
- 2How to fix a coolant leak and sludge in an old car
- 13Why is my car losing coolant with no leaks?
- 7Engine rattling noise and sudden coolant leak while driving
- 5Is a clear fluid leak from my car or heat pump dangerous?
- 5How to temporarily fix a leaking car radiator