My 2002 MC has had a slow coolant leak for awhile now. I really only noticed it when I had it on the lift and was under it. I notice the green antifreeze dripping off the lower plastic shield on the front under the alternator. I am not sure where exactly it is leaking but the previous owner told me he had the radiator replaced about 2-3 years ago. Regardless I wasn't losing much coolant.

However, it has just started to get cold around here so I started using my heater more. I noticed the awful smell of hot coolant whenever I had the heat on. I didn't think much of it at the time.

Last night just as I was pulling into my driveway I started to smell it really strong. I don't think the heater was really even on. I checked the overflow reservoir and it's bone dry I filled it up about a month ago and hadn't noticed it going down much at all. I also checked the radiator fill cap, which I believe is on the thermostat housing if I'm not wrong and it was plum full. Finally I noticed this morning and have noticed more recently that my windows fog up after I use the car, meaning overnight or a day or two later the windows will be fogged.

After looking online it seems like most people attribute this behavior to a heater core leaking. And said in the MC that it will leak right into the carpets on either side. My carpets only slightly damp but I was out in the rain before I got in and drove home last night so I assumed it was that. Mostly because everyone says that the carpets get soaked in coolant.

Any ideas?

 

Check these areas.

- Hose clamps. Could be when the radiator was replaced it was not snugged back up.

- Coolant tank - Often splits at mold seam and at bottom where you cant see. As preventative maintenance I would replace an original coolant tank around 90K miles. You can check the heat shield below the tank to see if it's leaking out onto it. If it's leaking when that area is hot, the heat sheild will have lost some of it's pressed in factory texture and will have smooth "puddle" circle area about 4 inches in diameter.

- Thermostat. Original brownish colored gaskets were crap. New thermostat comes with an updated opaque/clear gasket. Check area around it for residue buildup.

 

I just had a slow coolant leak that ended up being the thermostat housing leaking. That part is only about $40. The shop recommended putting in a new thermostat while they were at it and that was another $40-ish. the labor was a lot though. Ultimately about $380 total. I got the impression from the shop owner that this is a fairly common issue.