Strange engine cooling issue on a 300D

[PreferVintage]

Hello all...I've read through previous posts and can't seem to find my issue, so I'm hoping you gentlemen (and ladies) will be kind enough to offer a suggestion or two. Over the past month, my '82 300D is running a bit hot...which in turn thins the oil and has oil pressure drop a bit...it normally is rock solid at 80C but now creeps up at highway speeds to a 100...slowing down it comes back to 80. The engine doesn't have a fan clutch...solid mounting...cooling system pressurizes OK. I've only seen this before with a bad fan clutch; a stuck thermostat wouldn't vary like that...no leaks from the water pump weep hole, and a loose pump impeller would probably manifest overheating all the time...A/C was not on. Car has 188K and runs beautifully aside from this issue. Any thoughts anyone? Many thanks in advance!

[Sugar Bear]

Sounds like either a thermostat starting to fail by not opening completely or a radiator beginning to clog. I'd try a new high quality thermostat first.

Good luck!!!

[gottarollwithit]

Might consider flushing the cooling system, especially the rad. If you see a buncha crap in there, flush the heck outta the rad. If it were me, I’d look at the coolant in the res, if it’s the cheap green stuff, switch to Zerex G05, flush the block and rad with a garden hose. Dump a thermostat in for good measure.

Water pumps on these cars are pretty robust. Possible, but lower on the list of causes

[PreferVintage]

Hi guys...it's 11PM here and I was just checking a few things on the car that were nagging at me. First thing, I haven't driven the car for a day, yet the system was pressurized! So now I'm thinking I have air trapped in the system. I've had W126's that were gas, and this W123 diesel, never had this happen before...but I remember reading somewhere this is why there was a bolt on top of the thermostat housing....Kent at MB Source once had a thread about removing the upper radiator hose from the thermostat housing and filling it up, and then quickly reattaching it...then squeezing the hose until bubbles at the overflow stopped. So I tried that....system started to pressurize, temp gauge was OK, but no movement of fluid in overflow reservoir, and water remained cool, even though upper hose was getting warm. So now I think I need to do two things...remove and check that thermostat you guys suggested, and pressure test my cooling system cap to see if the vent works to relieve excess pressure. I'll keep you guys posted....

[Sugar Bear]

Using a n infrared thermometer you could shoot various points on the radiator for cooler spots indicating a blockage or restriction. Look very closely at the external radiator fins particularly toward the bottom for corrosion or restriction. Try to inspect the fins by shining a light through them.

Good luck!!!

[barry12345]

You put or have coolant in the system. When you run the engine the coolant heats and expands. Creating coolant system pressure. When engine cools down the coolant returns to the original volume. . This should not allow full system pressure after it does.

I might test the coolant for exhaust components. I have not played around with enough 123 engines yet it is logical to me. Pressure testing the system may show nothing. Cracks in heads are not totally unknown on the 123 engines.

Once again I am not saying this is the issue but perhaps it should be checked. I probably would before trying to shotgun for the issue. Also look for bubbling in the system with the cap off. Careful though and perhaps just starting the engine cold and let it reach operational temperature. Or since the engine may not be reaching atmospheric boiling temperature it should not blow coolant out. People can get scalded if not careful remember.

Also unequal cooling can be created with the by products of combustion getting into the water passage and forming a type of blockage or bubble in the fluid. Again I would test the coolant for exhaust components. One reason is it probably is the cheapest thing to do. Plus it clears or condems easily what the cause may be.

I also have no issue with if you post it was something else later. As it certainly could be. I just did not want you to ignore this as a possibility.

[Diseasel300]

If you haven't driven the car in a day and you had a pressurized cooling system, it's time to consider shifting into "blown head gasket" or "cracked head" thought mode. Air in the system will not remain pressurized after the engine shuts off. Combustion gas in the cooling system will, and it will also impede coolant flow resulting in high temps.