Would a "bad" thermostat- run cool when cold out also run hot when warm out?
 

As I have mentioned before, I think I have a bad thermostat and so I am told by others, as my car (85' 380SE) was running very cool on the highway when it is cool out... the cars gauge is like @ 60°C. Now that it is warming up, I have noticed the gauge is going much higher than it did last summer when idling. Tonight, I was setting in a drive-thru and had to idle for about 5 minutes. It was 58° outside. When I looked at the gauge, it was up to about 105°C! :eek: It was not in the danger zone, as I would not have allowed that, but I "revved" up the engine to about 1500 RPMs (to get the fan to speed up) and the temp gauge went down almost instantly down to 85°C, which seemed odd it went down so quickly. With that, does it sound like the water pump getting weak, or will a faulty thermostat also make an engine run too hot when idling, when at the same time run too cool when cold out? Today, when I was driving when it was 67° outside, it was around 80°C, which seems normal, but it goes up and down quite a bit as I stop and go. Last summer, it never went above 95°C, even in 97°F weather, but seems like it may be worse this year.

I would like to add this info and be sure before I replace the thermostat.

Thanks in advance!

that is how it would run if it had no stat.

tom w

Buy a thermostat and replace the old one. As stated above, there likely is not a thermostat installed. Do not run the engine without a thermostat.

Thanks. Well, its been like that since I have owned it (almost 2 years), so I bet the PO had removed it or something. Probably was getting too hot when the auxillary fan stopped working. It does not work (keeps blowing fuse). I am wondering if I put in a new stat without the fans working, will it likely overheat?

I bet your gas mileage is low with that kind of temperature.

you will probably find that the thermostat is broken partially open which will account for your symptoms.

Your car will typically only need the aux fan during the summer with the AC on, and not moving (ie sitting at idle or in a traffic jam). Please replace your thermostat.

Just about an hour ago, I replaced the thermostat on the 2003 E320 wagon. All is good now. Before the new thermostat, the temp would not reach 80 degrees Celsius. Now it goes there quickly and above it. The thermostat is rated for 87 degrees.

From my personal experience with Mercedes thermostats (4 of them), they tend to last for about 40k - 50k miles. After that they will stuck slightly in the open position. The symptom is that the temp will not go above 80. This results in too cold running in normal driving then too hot running in the summer. (A partially stucked in open thermostat probably cannot fully open either.)

That's been my experience as well.

Jeff Pierce