1985 300D (turbo), many many miles (odo stuck at 229k, estimated more like 400k+). runs good, not perfect but gets from place to place.

consistently heats up to 100 degrees and stays there even at speed. if i idle for a couple minutes it will start creeping up, but under no conditions will it dip more than half a needle-width below the 100 degree mark.

the coolant level is correct, the water system is not leaking at all, at least not visibly (no drips, no apparent loss of coolant).

how do i figure this out? i'm assuming it is either:

-water pump worn, not pumping enough water
-something wrong with radiator, so its not getting rid of the heat
-the actual channels that water flow through are clogged somehow, so the flow is not cutting it

any help in diagnosing this issue would be much appreciated!

I just had this same problem with my 116 300sd. It was the thermostat. I had a cheap Advance auto one made in China in it. Put German one back and all is cool in the world.

Mercedes says " If the coolant does not puke out of the bottle overflow, this is acceptable."

Measure the coolant at the thermostat neck with an infra red heat gun. You may have a lying temp gauge. The Behr and Stant 'stats are usually good.

thermostat was my first thought but i didnt think it would be the cause since the car is at 100 when going 75mph down the highway for an hour on flat ground.

short of going and getting an infrared thermometer, is there a way for me to verify my coolant temps to rule out the gauge being incorrect?


and what about the issue where the temp also climbs while sitting still? my fan clutch seems to be working just fine, so i think we can rule that out.

Mine did the same thing. It would creep up in traffic or going 90 mph. It got close to the red a couple of times and it turned out to be the thermostat. It is easy to change 3 10mm bolts and you are done. My symptoms sounded exactly the same as yours

The first thing to check is your thermostat. No point in theorizing about things it could be, just test your thermostat and if that is ok then we can make some other suggestions.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Nothing wrong with thinking cheap. Nothing cheaper than a thermostat or testing the one you have. They do fail or go off calibration with age and use over time. Has to beat needing a new radiator or worse in my opinion.

I experienced a similar overheating issue with my 1985 300D at round 260,000 miles. I found a significant amount of debris trapped between the condenser and the radiator that effectively blocked about 30% of the surface area. In addition, a large number of the radiator fins were blocked with bugs, dirt and straw. I pulled the radiator and power washed it and also power washed the condenser coil in place. After reassembly, my average temperatures dropped from over 100 C to around 85 C.

I subsequently flushed the cooling system, installed a new MB thermostat and fresh MB coolant. Since that additional work, the average temps are now slightly above 80 C with ambient temperatures above 90 F.

__________________
Blair Tyson
1985 300D
300,000 + miles

"aeromechanix I experienced a similar overheating issue with my 1985 300D at round 260,000 miles. I found a significant amount of debris trapped between the condenser and the radiator that effectively blocked about 30% of the surface area.

This is quiet a common occurrence. I find this in many vehicles. You need to pull the radiator clips off and lay the rad' back towards the fan and check down there.

Better yet, remove the radiator and clean it. (Some hardware store coil cleaner will help in that endeavor.) Then you can also do a shadetree "flow check" with the garden hose.