2004 MCS, 123K. While recently working under the car I noticed on one of the plastic under-body panels evidence of a slight coolant leak, which appears to be coming from where the upper coolant hose connects to the radiator. Anyone else experience this? This connection has not been disturbed in 100K miles, so recent work is not a likely explanation. TIA.

 

It may be a thermostat but you never know with blind diagnosing.

 

Haven't heard of that as an issue. It's a pretty simple connection, and the OEM clamp should be pretty solid. If it looks like it's leaking, first thing I'd do is remove clamp, remove hose, and check ID for imperfections that would lead to a leak.

 

My 02' MCS has 150K, and has been burning coolant for probably 20K. It's not a mass loss (unless I don't add fluid). Keep an eye on it and see what happens. Make sure to have some extra coolant with you after checking all your connections and condition of hoses and clamps like others have mentioned.

 

Split coolant tank, thermostat, waterpump gasket are your usual suspects.

 

My thermostat gasket went not too long ago, causing a leak. Wouldn't hurt to church it out.

 

Check the hoses for cracks etc. if its not that check the water pump gasket. The thermostat is on the other side of your leak. Lift the car and trace the leak, most of the gaskets are pretty easy DIYs.

 

I only have 102K km on my car, 2005 cooper S, and I'm on my 2nd thermostat housing/thermostat about a month ago, found myself topping up fluid in a month I ended up topping up almost 1 ltr of radiator fluid. So I could not see puddles or leaks, but I decided to go under the car, remove the power steering pump fan duct and I discovered that the duct was accumulating enough radatior fluid but not enough to overflow onto the floor. I looked up with a flashlight and found that the leak was coming from above the centre of the engine perfectly lined up below the water pump.

So back the dealer I went and sure enough slow leak from water pump bearing, I had it replaced today.

Does water pump at 102 K km seem too soon? about 65K miles.

 

I have had the same problem on my 04 MCS first with the upper connection and now with the lower. The way the hose connection connects to the radiator is a bolt on assembly with an o-ring on the inside. What happens is that the surface warps if overheated and causes the surface to not match flush. When my top connection leaked it was right after I had broken the serpentine belt and it got hot before I got it shut off. I had to add a second o-ring and gasket sealer to make it hold. Then two weeks ago I lost the lower idler pulley again tossing the belt and again slight overheat, two days after I got it repaired the lower is now leaking in the same fashion so I'm assuming it just finally had enough after this last failure. What really sucks is I just had it in the shop getting the power steering unit replaced under warranty at 146K miles and the very next morning the idler came apart on the way to work.

 

My 2005 Cooper started leaking at the thermostat. Coolant got past the o-ring and corroded the head at two tiny gasket spots. I cleaned it up and added some RTV to the new gasket and so far so good. Admittedly, it is a major effort to check if there is any weeping at the seal so I'm not 100% sure.

 

Why is everyone talking about leaking thermostat housings and water pumps when the OP is talking about the upper hose at the radiator? It's obiviously either a)tired clamp, b) failed hose or c) failed interface of plastic side tank to Aluminum core of radiator.

 

+1
Boards kinda amaze me sometimes....
Plastic do get brittle with age....the necks on the rad crack as do where the radiator's metal core is heat welded (melted) to the plastic tanks....at about 10 years they start to crack....
The factory spring clamp or hose might be getting old weak...but be careful if you try a new clamp...folks often over tighten the worm screw clamps...cracking the radiator side plastic hose....

 

I'm not a fan of Spring type hose clamps and also changed a half dozen of mine with the fancy German screw type ones from Pelican Parts; however, a couple of years ago, I did fail a plastic radiator tube by over tightening a hose clamp. New radiator but just broken off tube. I guess too much of a good thing might not be a good thing.