Overheated when in the mountains, what could be wrong?
 

Just drove our 85' 300TD across country from NY to CO (3800 miles, was on my bucket list) and only a few problems on the trip. First the cruise control quit on the second day, next the seal on the rear of the SLS pump leaked after only two years after being rebuilt. And lastly it overheated when climbing into the mountains over 9000 ft. The first two things I can understand, and we rebuilt the pump once out in Denver. So not a big deal, but the overheating has me wondering what could be wrong. Had no issues going out west at temps in the 90's with the AC on, but when we started going up into the mountains the temps went whey up and she started to overheat out the overflow tube on the tank. Also the car does not like the thin air out there, was a real dog till the turbo spooled up. Not like here at 500 ft. above sea level. My thoughts are maybe the thermostat is bad, or the water pump might be bad. My plans are to replace both, but I don't think I can duplicate the same conditions here to see if I fixed the problem. Anyone have some thoughts on what could cause the overheating?

Did you pick up a fresh Coors in Golden?

In case it's either of the two, take a look at the links below for procedural reference if needed. Sounds like an awesome trip - best of luck!

https://www.pelicanparts.com/techarticles/Mercedes-W123/08-WATER-Thermostat_Replacement/08-WATER-Thermostat_Replacement.htm

https://www.pelicanparts.com/techarticles/Mercedes-W123/21-WATER-Water_Pump_Replacement/21-WATER-Water_Pump_Replacement.htm

Boiling point is reduced at high altitude because of lower air pressure (about 92C at 9000 ft) Diesels also build a lot of heat when they are laboring uphill. If you’re going to drive in those conditions often, the best defense is a higher pressure radiator cap. This is yet another example of water NOT cooling better. The lower vapor pressure of water makes boil over more likely. So stick with 50-50, or you can even increase the glycol to 60 %.

Only problem I've ever had with mine getting too hot over the passes was when the space between the radiator and condenser was half-full of compost, so you might check that.

Of course now I have no condenser so that solved that problem for good.

-Rog

Stay out of the mountains.

Which is of no real real relevance if the rated cooling system pressure is being maintained.

Where is the evidence that only water was being used?

Discharging coolant from the reservoir is not necessarily synonymous with overheating. It could just be a faulty radiator cap that only manifests at reduced atmospheric pressure. What was the temp indication on the gauge?

You should change the t-stat regardless, once they overheat the wax pellet may fail sooner rather than later, so cheap insurance is a new T-stat.

Is the auxiliary cooling fan working? On a 124 car, simply jumper the temperature sensor on the head to put that fan on high, not sure if the same procedure works for a 123 car.

Other ideas: Pull the radiator to have a shop flow-test it. Check the function of the hydraulic fan clutch.

If you replace the water pump, try to get a replacement with a cast impeller, the pumps with stamped steel impellers have a bad reputation for not moving enough water.

Looks like a fun trip ! .

I spent three weeks in that area in my aging 240D, the AC failed the first day out but I loved every (slow) minute of those mountains .

What was the indicated coolant temperature when you were boiling?
What is the coolant %mix?- You might mix for a higher boiling temp at altitude.
Was the pressure cap tight and in good condition?
Is the expansion tank OK?

A good question is does the pressure cap vent at an absolute pressure (30psia would be 15psig - gage pressure at sea level and 20 psig at 9000 ft.) or a relative pressure (at 9000 feet, it would then vent at about 15 psig which is 30 psia at sea level, and 25psia at altitude)



If this is right, absolute pressure at that altitude is about .7 atmospheres.
https://www.engineeringtoolbox.com/d...r_pressure.png

If you are boosting to develop normal power, the working fluid is at normal density, and probably high temperature because of adiabatic heating. Using the this equation for, this gives an air temp rise of 30C just to boost the intake air to sea level pressure. I used 270K for T1 and 1.4 for Gamma, the specific heat ratio of air. I used P2/P1 of 1.5

https://wikimedia.org/api/rest_v1/me...a8cdbb7f6a4c7b

Your cooling air is at ambient temp and pressure, so low density, lower heat capacity, about a third less than at sea level.

If you're climbing, then power (and heat generation) is high and the speed of the cooling air is lower.

Long story short, being a little aquainted with you, I don't think anything is neglected or broke, and this is a symptom of the use and the circumstance. If power at altitude is critical, find a 20psig pressure cap and watch the temperature gage, letting up on the throttle if gets in the don't go there zone.

This is the zerex G05 data sheet, boiling is at 15psig, presumably over sea level, so 2 atmospheres:
https://sharena21.springcm.com/Public/Document/18452/f93a8057-fe75-e711-9c10-ac162d889bd3/c264d227-0dbd-e711-9c12-ac162d889bd1

If you top up with with undiluted zerex or mix to say 60% glycol and watch your temp, that might help.

Temp gauge was just about into the red, Had a 50/50 mix in the cooling system when I started out. Never had a problem with it over heating before or since. Drove home the whole way in +90º temps with the AC cranked and gauge read about 90ºc, with less than the 50/50 mix I started out with after adding just water after it cooled down. Radiator cap on the tank is new and the correct one. My thoughts are bad thermostat and or water pump, so I am going to replace both and hope that solves the issues I had while on the trip.

While the pump is removed, examine the scroll area of the pump housing (the area of the housing that matches the angled, back edges of the impeller blades) for corrosion/erosion. If significant, that loss of material from the housing leads to a reduction of pump efficiency, and consequently to a reduction of coolant flow.

Check your hydraulic fan clutch by rotating it by hand (engine off).
2 things: steady resistance, if not the silicone oil amount is low
And noticeable resistance. In case it rotates easily the silicone oil might be worn or whatever happens to it.
Years ago I bought a small bottle of this oil from Toyota. They sell different viscosities to adapt to different climate zones.
You can fill or refill using a syringe through the activation pin after removing the bi-metal.
Helped on my SD and recently on my 124.

Tom