My SD has begun to run over 100 deg C a couple times on the highway, reaching 115 or so for a minute or so. I suspect I am due for a flush and new coolant. I'm also hearing a squealing sound, making me suspect perhaps a water pump bearing or something. The belts look good, and there is no noticable leakage at idle. I did notice that the coolant appeared to be "flowing" (I hesitate to say bubbling, because I didn't see any bubbles, just some movement) inside the overflow tank where the hose from the radiator enters the tank.

I'm wondering if I would help matters by getting some citric acid and putting it into the overflow tank and letting it clean out the cooling system for a week or two until my tech can look at it.

Would leaving the flushing solution in for this long cause corrosion? Just wondering. Any thoughts appreciated. Thanks,

Thanks for the reply, Bill. It sounds more like a belt shrieking / squealing rather than a grinding. I just looked at the belts while it was running, and didn't hear anything. I didn't really check them with the engine off. It has been fairly humid, though not really warm; it's been around 70 degrees F.

I notice the temp pushing 115 maybe for a minute or two, then it'll drop back to 100-105 gradually. I have observed the needle drop by 10 degrees within a split second, leading me to suspect the electrical portion of the sensor's readings.

How fast would the thermostat opening make the guage drop? It happens so fast that I suspect and electrical intermittent/corroded contact. Sometimes when she's running at 100 deg the guage will drop back, very suddenly, down to 90.

I do hear the aux fan running, and it doesn't seem to be running in the wrong direction. I also see the main fan running but don't know how to check the fan clutch.

Thanks again for the reply...

Bill, thanks for the replies. Driving today I kept a closer-than-usual eye on the gauge and it only jumped once, up and down maybe 2 or 3 mm. I always suspected an intermittent electrical connection, but I suppose it could be a mechanical aberration withing the gauge also.

I did have the thermostat changed about a year ago, when the car wouldn't warm up above 60 deg C except after 8 miles or so. I suspect MB thermostats are fairly variable in quality. I may just get my tech to change it along with new coolant, flush, and all new belts the next time it goes into the shop. I've been saving up my $$$ for a new steering box, and I'll have them also check the rear end for some vagueness.

thanks again Bill!