If it stabilizes at 1” below the full cold mark, is that not simply your new normal level? Mine is similar to this, it has been sitting steady at maybe 3/4” below for months now. I put a felt marker on the tank where the coolant level stays. If I fill it to the full cold mark, it settles to this same spot.

 

5th bad cap on the same truck?

 

It isn't obvious that coolant should disappear down to a certain level. Why is this expected?

If I add brake fluid, engine oil, power steering fluid, etc.. I don't expect a little bit to disappear and then stabilize.

 

Between the two discos.

 

I sort of understand what you’re getting at, but it’s supposed to rest at the cold mark nonetheless.

 

A tight system holds pressure even overnight, but again that is not necessarily required. People need to stop thinking the LR design engineers knew what they were doing when they designed the cooling system on this truck, they obviously did not. Stop thinking it should do this or that and accept it for what it is. Fill it too full, and it spits out coolant. BMW obviously over complicates everything. The system should have come with a coolant level sensor, a better thermostat system, a bigger radiator ala D1, no octopus cooling hose/plastic fitting, and on and on. The sooner you stop trying to make it work the way BMW thougth it should and make it work for you life gets a bunch easier.

 

Jason I’m definitely not a cooling system expert but what I think happens is that when the coolant gets hot and expands, the excess blows out of the overflow tube. The full cold line was a guess by the engineers that after the excess blows off, and the system cools and contracts, it will sit at the line. I think what happens is that on some vehicles the overflow tank won’t hold as much hot fluid as they thought, and after cooling will sit at a line that’s lower than the marked line. If the level stabilizes, then you don’t have a leak, so I don’t think it’s something to worry about.

 

I’d also like to weigh in on cooling system pressure (again I realize I’m not an expert). As Brandon alluded to, there should be virtually no pressure in the system when the engine is stone cold. When you remove the cap to add fluid to a stone cold engine, the pressure in the system will be zero (ambient). Now when you close the cap and run the engine to operating temperature, the coolant expands, and since it is a closed system, the pressure increases. When the engine cools down again to stone cold, the pressure reduces back to zero (assuming the ambient temperature and pressure are about the same as when you opened the cap previously). There will then be very little, if any hissing when you remove the cap again.

 

The valve in the filer cap is a one-way affair, so how does the pressure reduce back to zero? Air (or overfilled coolant) is pushed out as it expands and the system pressurises, but it doesn't/shouldn't reduce again particularly quickly. As Extinct said above, a system with no leaks will hold pressure overnight.

 

Where exactly do you guys think the pressure in your cooling system is coming from? The coolant expands because it is hot and pressurizes the system, when the motor cools back down to ambient temperature there won't be any pressure in the system because the coolant volume has been reduced back to what it originally was.