W123 changed t stat now it overheats

[New2MB]

So I changed my thermostat on my '83 240d and also drained and filled the coolant.

The lower radiator hose is cold but the engine is getting up near 95.

I took the new thermostat and verified it's function in boiling water.

I tried changing the orientation of the thermostat.

I drained the coolant out of the block and filled the system through the heater hose on the driver side.

The system seems full, coolant comes out of the bleeder on the t stat housing, heat works very well.

I filled it on an incline, I squeezed the upper rad hose to burp the air.

Any ideas what is going on?

[79Mercy]

It seems most new t-stats I have been getting are making these motor run at about 90-95C. Its still within spec(100C MAX), however its hotter than the older t-stats were keeping the motors.

What brand did you get? The last i had was a Behr stamped 80C which i put in a customers car, the motor actually ran at about 90-92C.

[Zacharias]

Originally Posted by New2MB

The system seems full, coolant comes out of the bleeder on the t stat housing, heat works very well.

A w123 240d has a bleeder for the coolant system? I was not aware of that.

[New2MB]

Well I'm calling it a bleeder, the 19mm nut on top of the t stat housing.

Put the old t stat in and it works just fine.

I wonder if the new t stat did not have a vent on it as there was no arrow.

[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

[Stevo]

I have a hand held infrared temp sensor from HF that I use when theres a question about over heating. The gauge in my 85 240D will read high after a couple hours on the road, a smack on the dash above the instrument cluster will bring the temp down

[Brian Carlton]

Originally Posted by 79Mercy

It seems most new t-stats I have been getting are making these motor run at about 90-95C. Its still within spec(100C MAX), however its hotter than the older t-stats were keeping the motors.

I agree with this conclusion.

The SD runs 95-100C under all conditions...........summer at 100F...........winter at 20F.

The infrared gun measures 91-93C. at most places along the cylinder head on the driver's side.

The dash gauge is not the culprit for the discrepancy as a second gauge from a different cluster reads identically.

I might swap the sending unit to see if I gain 5-8C. or so. But, it isn't going to be significant.

BTW, the SD has a new radiator and it made no difference.

[MarkM]

Do you mean to say that thermostats that we are getting nowadays...that are stamped 80 deg. C......are making the cars run at 90-95C, and that this is correct and OK?

I can understand overheating when towing, or when its 100 degrees outside and AC is on, but I can't see such a big discrepancy ....80 deg. thermostat, and 90-95 deg coolant temps.....under normal driving conditions. I would think that under normal driving conditions, with an 80 deg. thermostat, normal coolant temps at the sensor should be low to mid 80's.

I'm not questioning you....I would think that the IR gun readings on drivers side of engine are accurate....and you confirmed readings with another dash gauge. Is it just that the SD is a bigger heavier car than the 123s?

Mark

[Brian Carlton]

Originally Posted by MarkM

I can understand overheating when towing, or when its 100 degrees outside and AC is on, but I can't see such a big discrepancy ....80 deg. thermostat, and 90-95 deg coolant temps.....under normal driving conditions. I would think that under normal driving conditions, with an 80 deg. thermostat, normal coolant temps at the sensor should be low to mid 80's.

I would think the same thing.............but the data tells otherwise


I have six data points with two different thermostats..........two different dash gauges..........two different radiators............ all doing the same thing.


BTW, the figure of 80C. is the lowest temperature that the cooling system would attain when the thermostat is fully closed and all the coolant is on bypass. This is just at the point of thermostat beginning to open. Fully open would be 94C. In theory, the engine is operating somewhere within this range under all conditions.

In my case, it operates between 92C. and 101C. on the dash gauge under all conditions. The cylinder head measurement of 91C. was taken with the dash gauge showing approx. 98C. I'm fairly sure there is some discrepancy caused by the sending unit, but the predominant discrepancy is with the thermostat itself. The engine should be operating very close to 80C. when at idle during 50F. ambients. But, it doesn't.

[Zacharias]

Originally Posted by Brian Carlton

BTW, the figure of 80C. is the lowest temperature that the cooling system would attain when the thermostat is fully closed and all the coolant is on bypass. This is just at the point of thermostat beginning to open. Fully open would be 94C. In theory, the engine is operating somewhere within this range under all conditions.

THANK YOU for clarifying that.

Based on my searches, there's a number of people on the board who are under the impression that the gauge is supposed to register 80 in normal driving.

[Brian Carlton]

Originally Posted by Zacharias

THANK YOU for clarifying that.

Based on my searches, there's a number of people on the board who are under the impression that the gauge is supposed to register 80 in normal driving.

They believe it registers 80C. in normal driving because that's what their dash gauge says. More than likely the gauge is reading low, either due to the display unit or the sending unit.

Doesn't stop them from speaking like they know something factual.................

[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...

[tangofox007]

Originally Posted by Zacharias

Based on my searches, there's a number of people on the board who are under the impression that the gauge is supposed to register 80 in normal driving.

Based on my observation, the majority opinion on this board is wrong about as often as it is right.

In the case of the "normal" temperatures, consider that an 80 degree thermostat does not even begin to regulate coolant flow until the temperature reaches 80 degrees. That should make it obvious that 80 degrees cannot be the "normal" temperature. Also be mindful that the main function of the thermostat is to "enforce" a specified minimum temperature; its ability to regulate higher temperatures is limited by the ability of the cooling system to transfer heat. The fact that the thermostat does not fully actuate until 94 degrees would tend to suggest that the designer engineers were content with that temperature.

[tangofox007]

Originally Posted by Brian Carlton

BTW, the figure of 80C. is the lowest temperature that the cooling system would attain when the thermostat is fully closed and all the coolant is on bypass. This is just at the point of thermostat beginning to open. Fully open would be 94C. In theory, the engine is operating somewhere within this range under all conditions.

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

[gatorblue92]

I believe that the gauge can be faulty... both of my cars have a dancing temp gauge... i have checked both with the infrared gun and they are both running right around 85