I did some fairly major work by my standards (water pump, thermostat,belts, hoses, timing stuff), and I noticed that my temp (86 560SL) is about 5 degrees C higher than it used to be - 85 vs the former 80. I think it's possible that this is tolerance of the thermostat itself (like, +/- 5-10%?). In the meantime, I noticed that my overflow tank was leaking around the neck and making noise. I got a replacement cap recently because the old one was probably 19 years old. When I put the new one on, I noticed that the metal flange around the expansion tank neck was loose. Thinking that any leak is not good, and seeing no practical way to fix the neck, I bought a new OEM MB expansion tank, and installed it. Guess what? The new one leaks around the neck too - but not nearly as much - just a few drops and a little hiss). It is not loose.

Is this normal?

Do you think the cap could be defective (not releiving pressure?). I find this hard to beleive because you would think that a cap would be faulty in the opposite way.

__________________
86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm

possibly the incorrect cap? different pressure?

I'm very certain that the cap is proper and is fitted correctly. The original cap that came with the car was 1.2 bar and marked "120". Mercedes superceded this across the board with 1.4 bar caps ("140") and these are the recommended replacements. I did a little research into this and this seems reasonable. The old "120" cap was also installed while this leakage was going on as was evidenced by massive deposits on the sides of the bottle.

__________________
86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm

Even the slightest loss in pressure, "hissing, drops of coolant" will cause your temps to rise. (from experience).

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190e 2.6 1990. 300,000 KM
C230 K 2000, 130,000 KM

Normal operating temp. is 80-105C and within this range there should be not coolant leakage at the cap. Pull the overflow hose hose off the nipple and inspect it. Does it fit snug on the nipple? Is the inside rubber layer cracked? That could be a source of leakage.

Duke