Very strange temperature problem!
 

Hi everyone,
I have searched all over this forum looking for an answer before I started this new thread, but I haven't come by anything that is similar to my symptoms. Hopefully someone out there can help :)

Anyway, I have a 1985 500 SEL and it runs fantastically. Lately however, when I start the car in the morning, the temperature guage rises steadily as usual, but it does not stop around 82-85 degrees like normal. The temperature continues to climb until it is almost into the red and then immediately, as if the thermostat were just opened, it drops down to normal operating temperature in a very fast drop (from almost 120 degrees to 82 degrees in about 8 seconds). After this behavior, the temperature is fine all day... or until I leave it for a few hours to cool off completely.

Initially, I thought that my thermostat was being a bit lazy and not opening when it should... I thought perhaps the springs had gotten a bit sticky or perhaps it needed a bit higher temperature to "kick-start" the thermostat into opening. So I replaced the thermostat all together, but the problem still persists. It really isnt an emergency because the temperature always comes back down after only briefly being too hot, but I fear that the problem might get worse with time and I would like to nip it in the bud now before something horrible happens. Perhaps there is something electrical going on? Or perhaps there is a second thermostat that I am unaware of? Perhaps a faulty temperature relay?

Any suggestions would be greatly appreciated! Thanks!

-Vaden

On the thermostat that you replaced...did it have a brass like arm extending from it. If so, I would try cutting the arm off then reinstalling the thermostat. Sorry I can't remember if the 116 or 117 engine had that style thermostat.

possilby failing fan clutch, and the auxiliary fans are kicking in and saving the day.

Tom W

erratic temperature indicator
 

In the cheap-to-free category of troubleshooting, check your grounds to make sure that they are making good contact. I had a similar situation with my 560sel and it was traced to a loose main grounding strap near the starter.

PL

Bad temperature sender. Do any of the fans come on at anytime during the rise in temperature?

I concure it sounds as if the fan clutch is worn out. Pesonally I would not drive the vehicle till the problem is rectified. Over heating could cause head warp-age.

Is it possible the thermostat was installed backwards, with the temp sensing mechanism on the radiator side? Could explain the engine getting hot - very hot - then once the thermostat does start to open the rush of very hot fluid past the mechanism would force it open very quickly. As you are driving, of course, the thermostat would operate normally.

That sounds very logical.:idea3: I would check on that first. Also check if the fan stops almost inmediately after shuting the engine off. If it does, the fan clutch it's good.

Take a broom handle to the fan while it is spinning - if the clutch is shot, you will easily be able to stop the blades. Just did this trick about a month ago on one of my cars and it worked - fan clutch was bad!

you can also take a shop towel bunch it up and then try to stop fan clutch while engine idling.this is not dangerous and works well.if clutch is bad a good temp fix is to use a large screwdriver to force open the bi-metallic strip at the valve end and slide a ceramic fuse in beside the pin this will lock it up to engine speed as long as it hasn't lost too much of the silicon.