Hello, have a 2002 mustang, I was doing my routine fluid check, and noticed that the coolant reservoir was up to almost the hot fill line (it has always been under the cold line when cold). Removed the cap and dark brown liquid was inside, I also took radiator cap off while car was running and noticed that green coolant was coming out, no bubbles. Car runs fine, no overheating, no milky oil cap. I have read that this could be head gasket, but could it be rust, bad cap? What is puzzling is the extra fill in reservoir which is troubling. I noticed that the small overfill hose that attaches to radiator comes off very easely.

Any input is appreciated.

 

No one?

 

Cooling system pressure check will help in figuring out if you have a cooling system leak like a head gasket. How old is the coolant? If it’s just aged out, the rust color could just be a sign that your cooling system is corroding.

 

The radiator cap has a valve inside of it.The caps job is to seal the radiator & it also raises the boiling point of the antifreeze/water mixture by 45°.When coolant heats up it expands & this expansion causes pressure to build up.Once the pressure reaches the caps rating (15 or 16lbs) a valve in the cap is forced open and coolant is sent to the reservoir bottle.Once the radiator cools back down,another valve opens in the cap and vacuum pulls the coolant back into the radiator from the reservoir bottle.If the radiator cap is faulty though,the boiling point of the mixture is drastically lower & when this happens,the valve in the cap is forced open at a much lower temp than normal and the coolant leaves the radiator and sometimes overfills the reservoir so much that the level will be much higher in the bottle or even spill out of the vent.So make sure your cap is good.As far as the coolants color is concerned,old coolant can be brown or rusty in color if it hasnt been replaced in a long time.However if its oil that is actually mixed in with the coolant,it should have a really slippery feel/consistency to it.If your spark plugs are dry,your head gasket should be ok.