How much water do you have to add each month?
 

I seem every 1500 miles or so when I open the hood to check fluids my coolant reservoir is low. It seem like I have to add a cup or 2 every 1500 miles or so. The temperature gauge shows the car is running cool and I have never found any leaks. I replaced the reservoir cap about 2000 miles ago but it doesn't seem to be helping.

So how much water do you add each month to keep your reservoir full? I know it probably depends on temperature but just wanted to know if this was normal. Thanks.

Definitely not normal.. I've never added water and the level dosen't change.

In the year and 8 thousand miles I've had my 240D I've not had to top it up from normal use.

- Peter.

On mine if I fill it to the "level line" on my reservoir, which is new, it eventually goes down to being about 2.5" from the bottom, and stays there, forever. It only takes it maybe 2-3 drives to "loose" that other fluid, but once its down a little, thats where it stays. Always has done that, for all 51k miles I've driven the car. It has new hoses, reservoir, and radiator....and it runs rock solid at 80-81C no matter the outside temp.:confused: Who knows. Probably no big deal. Does yours continue to fall once it has gone down a bit? If so its probably leaking....I have no clue why mine does it.

That happens to me too. Exact same thing... I think the coolant reservoir leaks.

The vent line on the reservoir has a hose and is open. Maybe it is leaking out that hose when I drive. That hose suppose to go anywhere?
It seems to go down and stay but have not let it go that long since it is low and end up adding more water.

I have never had to add anything between coolant changes. You probably have a head gasket leak, unless you can find an external leak. Send a sample of the oil out to the lab and see what they say.

water!!! are you nuts........

it rusts your motor out from the inside out........ use anti freez..........

I just had to top off about a month ago with about a cup ... but my cap was really pretty tattered ... there were cracks/tears in the gasket. Replaced it and hopefully won't see any level drops from there. I had the oil tested about 3,000 miles ago and it came up great, no coolant in oil at all. I was careful to use 50/50 MB coolant and distilled water. In my time with the car, I have never had to top off before this, withthe exception of one time when the overflow tank ... well, overflowed. Not sure if the 617s have that tank, though. Mine is a 603 ...I didn't know that tank existed until coolant started coming out near the passenger wheel well.

Same here. If I fill it to the line, which is about an inch or so above the seam, it'll loose that bit and level off at the seam and stay there. The level will rise and fall with the temp but when its hot its always at the same spot and when its cold its always at the same spot. After I had the revelation to just let it be, I haven't had to had any in months.

Water pump, "Weep-Hole"
 

:idea2: Check the "weep hole" on the water pump after driving the car a while. It's behind the fan pully, on the top of the water pump shaft housing. If it's leaking a few drops every now and then it will usually evaporate on the engine itself before ever making it to the ground. Look all around it, for water stains, on the casting of the block, and pump etc.. I bet that's your issue. It means that that the water pump, is on it's way out, and will need replacing very soon. The seal in there goes bad, and that's what the "weep hole" is there for...So the bearings in the pump, don't get wet, the water can escape before making it to them.
Remember, "Weep-Hole", behind fan pully, after driving a while. Check it now and then...

Look for white streaks on the engine and wet spots on the radiator.

Buy a new cap.



one with a good seal.

I'll second that-fixed mine, along with tightening a few clamps. Does the cooling system build pressure fast when cold-upper rad hose stiff after only a minute or two of running-that's the telltale sign of a blown head gasket in a diesel along with tiny bubbles in the coolant (we'll miss you, Don Ho!)

The original post said "add water" and "runs cool".

The higher the ratio of water to coolant, the more efficient the heat transfer. You should add a 50/50 mix of coolant and water, not just water. Who knows what your actual ratio is now. You also should never let the ratio of coolant get below 45% per factory specs. Coolant lubes the water pump.

Might be a good idea to drain and start over.

Steve