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Brown/tan "mud" in bottom of coolant reservoir

In the bottom of my reservoir I noticed some buildup of something. Brown/tannish. Looks like mud. Kind of has the consistency of mud. Do you guys think that is rust or oil?

What kind of vehicle are you talking about?

And when was the last time the cooling system was serviced?


It’s a mini cooper. I just bought it so idk when it was last serviced. The owner kept meticulous records so I should be able to find it. My heater wasn’t working and I started looking into the reservoir and found this as I was washing it out.

Take the vehicle to a shop and have cooling system flushed, and cross your fingers.

Here’s the machine I use to flush cooling systems.


Isn’t this the coolant reservoir you poured oil into?

Bad sign…be sure to tell the mechanic this.

If this is the “oil in my coolant reservoir” car, fess up. We need all the information we can get to help you.

Noted that your heater isn’t working. That suggests that the coolant has run below the height of the heater core. That supports the possibility of a headgasket breech.

Whatever it is, it looks like the contaminant and water are not mixing. Oil and water don’t mix, but there are other possibilities. It could be contamination from combustion gasses being blown into the water jacket through a headgasket breech… and into tapwater that the prior owner was using to fill a constantly-emptying cooling system. Combustion gasses blown into coolant will form a brown cloudy stuff, but blown into tapwater they might just look like exactly what you have.

Let me process my thoughts for a moment…
Okay, your heater isn’t working, you have contamination in the cooling system that doesn’t mix with water, and apparently plain water in the cooing system.

Conclusion: blown headgasket. Start with a compression leakdown test Tell us the results.
Or, fill the cooling system with water, purge the air, start the engine, and look for bubbles coming up out of the radiator fill hole. If they’re there, they’ll be combustion gasses getting blown into the water jacket and migrating up to the highest point in the system… the radiator fill hole.

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I don’t think it was plain water in the coolant reservoir. It looked muddy but it smelled like antifreeze. I was thinking maybe since the antifreeze was low maybe it had gotten some rust from the radiator. Correct me if I’m wrong because my logic may be off here, but if there is any coolant in the reservoir wouldn’t that indicate that the radiator itself is full since it is an overflow tank? Also, I took the reservoir off and cleaned it thoroughly and put it back on. I then filled it up in the reservoir and drove around for like 10 minutes to make sure it wasn’t going to run hot. I brought it back, let it cool and checked again and the reservoir was still full and I loosened the release valve on the hose in front but I didn’t have a long enough extension to get to the other one. It bubbled under the screw but it never came gushing out or anything. Did I do this correctly?

I didn’t flush the whole system though and this was my first time messing with all this stuff. I’m colorblind but I asked my girlfriend what color the old antifreeze was and she said, “orange, rusty colored”. Hope any of this helps

Yes it is. If it helps this is a picture of what came out of the reservoir. This was after the oil was put in but I let them separate and all of the oil appears to be at the top15079353785461505293570

The rust is from aerated water passing through the iron block, the radiator is aluminium, won’t rust.

With the engine cool, loosen the bleed screw and add coolant into the reservoir until all the air is expelled from the bled screw.

But what if the reservoir is already at the max line?

Did you remove the cap while the bleed screw was open? The level should drop as the engine is refilled with coolant.

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Nope lol thank you

What you have is coolant that has been in service way too long. As the coolant wears out, it goes from clear to cloudy to muddy. Cloudy is too old, muddy means there may be damage or clogs in the coolant system.

I am not a fan of flushing a coolant system, but in your case, I would recommend you follow @Tester’s advice. Do not try to cheap out and DIY with a chemical flush from the parts store. Get it done professionally.

Update in case anyone was wondering. I went through the receipts of all maintenance done by first owner (I am third) and discovered that she was told by dealer 2 years ago that heater core was plugged and she turned it down. She took it in because the heater was not blowing “as hot as it used to”. Does a plugged heater core necessarily mean that you need to replace it? I put this on both my posts

The answer depends on what your goals are

If you want heat in the cabin, you’ll need to replace the heater core, or figure out a way to regain flow through that plugged heater core. One example would be flushing it out, but I’m guessing that’s already been attempted

If you don’t care about heat in the cabin, you can figure out a way to bypass the heater core. That means looping heater hoses and/or pipes together, so that coolant doesn’t go to/from the heater core.

It’s just weird because I’ve never smelled antifreeze nor seen any drips or anything in the cabin. I just know how dealers are about upselling you things you don’t need

Peyton , I only see these choices: 1. Find a local mechanic to replace heater core 2. Call who ever you purchased the car from and see if they will help on cost ( dealers will not try to repair this just replace and upsell all you have to do is say No 3. Have a cooling system flush and hope for the best

The first thing I would do is disconnect the heater hoses at the engine. If you have a source of compressed air, then blow compressed air into one of the heater hoses and see if you can get anything to come out the other. If no air comes out the other, try blowing into the other hose. If you are successful in getting air to go through, then run plain water to flush it out.

Sometimes the crud builds up in the hoses so if you are not successful in getting air through, then remove the other ends of the hoses and inspect them. If one or both are clogged up, just replace it/them.

Check for an inline valve in the heater hoses. Occasionally these are vacuum operated and for whatever reason, they get stuck closed. That was a common problem on old Fords. If it is cable operated, make sure it works.

If you are successful with any of this, then follow @Tester’s advice for a professional flush and after this, service the cooling system every 5 years with a universal long life coolant.