Originally Posted by tangofox007

Provided that the cooling system has excess capacity to transfer heat under all conditions.

Originally Posted by tbomachines

Sounds simple but make sure the t.stat isn't installed upside down (comments on "changing the orientation"...did you spin it or check for it being right side up?)...I would imagine that would cause a closed condition rather than open. It has been the culprit to more than one thermostat thread in recent memory on here...

Originally Posted by WNC123

the thermostats i've either pulled out of parts cars, found in my car (cheap chinese POS that was in the center console), or have bought (Behr) have had an alignment mark.

Originally Posted by tangofox007

Have you seen a Wahler 616/617 thermostat with an arrow?

Originally Posted by Brian Carlton

What's your thoughts on orientation?

Originally Posted by tangofox007

Have you seen a Wahler 616/617 thermostat with an arrow?

Originally Posted by E150GT

I just went through this. My car ran at 80 when at fully warm operating temperature. It ran to cool with the heater on the heater sucked. I got a new Behr thermostat and did a coolant flush. Now it runs at 95 and its cold outside.

Originally Posted by joshhol

Similar situation. My brother's 85 123 running hot. He had a near overheating/low coolant situation in June. His mechanic replaced the water pump and thermostat and ever since then, it has run 90 to 100. Today we replaced the radiator due to a leak, still running hotter than historically but no leak.

Questions:
Has anyone pulled out the T-stat completely and run the car for a while to see what it's native operating temperature is?
Has 80 always been the initial point of opening with 94 full open, even in the "old" design t-stats that seemed to keep the car closer to 80 all the time? In other words has 80 ALWAYS meant the point of first opening?

Lastly, I'm suspicious that these T-stats are purposely making our engines run hotter for emissions reasons and they haven't disclosed that fact to us.

This is all speculative. The guys that have used alternate measuring devices know way more about those methods than I do. Perhaps this explains everything. What I don't know is why a thermostat change would [I]suddenly[I] make the temp gauges be out of calibration.

Originally Posted by MarkM

I have had two MBZ 123's since 1997....84 wagon is still my daily driver. I do all my own repaire, and I have experienced many of the problems that owners of these cars experience. I have had the exact same problem with the car overheating, thinking that the thermostats are bad. For many years, no matter what the outdoor temp was or what the driving conditions were, the temp gauge on the car would always read slightly above 80...maybe a little hotter (85?) on a real hot steamy day with the AC on. But then at some point, it seemed that the car would overheat, with no apparent reason. This happened gradually over time, but at some point, I realized that the coolant temp dash gauge is always around 90-95, vastly different from before when it was always reliably a little above 80.

So, over a few months, I bought several new thermostats to troubleshoot and correct the "problem"...I now have a collection of 4 spares...two Whalers (Germany), one Calorstat (France), and one marked DFT (China). I can't remember what brand I now have in the car. They are all stamped 80 deg. C, and with each of these, the car temp. gauge would read 90-95 deg. C when up to running temperature, even on cool days.

5 new thermostats cant all be bad, so the thermostat must not be the problem. I also did not believe the overheating was the result of blockage in the cooling system, air lock, or something like that....just very very unlikely....I have always maintained the system (flush with commercial radiator flush every 2 years). Also, I refill coolant by first detaching upper radiator hose from radiator, fill as much as you can into the hose to get coolant "behind" the thermostat, reconnect hose, then go through normal process of filling through the coolant reservoir as the engine is running and warming.....no chance for air lock, and no need to drill little weep hole in the thermostat as some have suggested here in previous posts.

So, I now figured that the coolant temp. sensor on the engine (drivers side, between glow plugs 1 & 2) must be reading incorrectly. I had a spare from parts I salvaged from my previous 123, (it was actually relatively new, bought from Fastlane). I changed the sensor, and temp gauge read the same 90-95. I got a third sensor from a salvage yard, and temp gauge read the same! Conclusion...it can't be the temp. sensor.

Very frustrating! But, I realized that this system is very simple.....three main components...thermostat, coolant temp. sensor, and third, the TEMP GAUGE...."I wonder if the temp. gauge is reading incorrectly?"

Well, easy to test....I took my multi-meter (nothing fancy....$30 Extech) with a temperature sensor...it's a wire that plugs into the multimeter, with a thermocouple tip on the end (looks like two strands of exposed wire connected with a bead of solder at the tip). On a hot summer day I ran the car with AC on till coolant temp was at highest...gauge on the dash read about 95. I quickly stopped the car, placed the tip of the thermocouple onto the metal collar on the side of the engine into which the coolant temp. sensor is screwed, and held it in place tightly with the eraser end of a pencil....waited several seconds, and low and behold, the temp. reading on multimeter was about 83C. I repeated this a few more times over the next few days, and got the same result...measured coolant temp in low 80s, dash gauge low to mid 90s.

CONCLUSION....THE REASON FOR "OVERHEATING" ON THESE CARS IS THAT THE DASH TEMP GAUGE GOES OUT OF CALIBRATION!

So I figured I would pull out the dash cluster, hoping that there was an obvious calibration adjustment screw on the temp gauge....maybe a screw with a dab of lock tight on it, or some other obvious means of adjustment. Well, I found no obvious calibration adjuster.

FINAL SOLUTION (very eloquent and sophisticated fix!)....I figured maybe I can just rotate the gauge needle a little on its shaft. Nope! That needle is on there solid (I didn't want to force it and break something). So, my final solution was to just grab the needle and rotate, causing the shaft to twist and bend a little, so the needle would read lower (pretty tough to "fine tune" the gauge using this method....just eyeball it). You obviously have to do this when engine is not running…in order to get at the back of the gauge, you need to pull cluster, and disconnect oil pressure line, as well as electricals. So you have to just eyeball the adjustment, install cluster, and hope for the best. If unsatisfactory, go through the process again.

Now, when I run the car up to temp., dash gauge reads in the low 80s (where it's supposed to be), and thermocouple test confirms that coolant temp is about 82.

Check your coolant temp with a thermocouple and you will see. Make sure you check at the collar into which the temp sensor is installed...this gives most accurate reading of the temperature that the car's temp sensor "sees". Don't worry about possible inaccuracy due to being an inch or so away from the actual coolant temp. sensor...heat conductivity of metal is very high, so the temp you see at the at the collar of the temp sensor is essentially exactly what the temp sensor sees, and is exactly what the coolant temp is. For our purpose here, don't test temp. elsewhere on the engine...there will be wide variation in temp....very hot near exhaust manifold, much cooler on valve cover, etc.

Sorry for the long write up, but I wanted to share details of my experiences for a problem that I have seen many on this site have inquired about.

Good luck,

Mark