Okay, I'm at my wit's end with this one. My '84 300SD with 331K miles is always running at 98C at highway speeds (75 MPH and 3000 RPM), and at about 95C in stop-and-go traffic.

Within the last week, I have replaced the thermostat, engine temperature sensor on the driver's side of the block, a brand-new Nissens radiator, brand-new Meyle water pump, and brand-new Meyle fan clutch. Additionally, I have done a complete flush (not a citric acid flush) and am using Zerex G-05 antifreeze with a 50/50 mix. The coolant that came out when I replaced the radiator and did the flush was extremely clean, with no scale or particulates, even from the engine block drain plug.

The system holds pressure quite well, though I'm not sure the exact pressure it is maintaining. Even an hour after running the engine, removing the cap on the coolant reservoir yields an audible venting sound of the system pressure.

Outside temperature does not have any affect on the engine temperature. Freeway engine temperatures are a consistent 98C, whether the outside temperature is 23F or 78F. I have verified that my cluster temperature gauge is accurate with an infrared non-contact thermometer shot at various locations on the head (about 1C difference between gauge and IR thermometer). A spare cluster I have lying around shows the exact same temperature as well.

The only consolation is that the engine temperature NEVER gets hotter than dead-even 100C on the gauge, even while driving up moderate grades. While driving DOWN a grade (minimal/no load on engine), the temperature drops to about 90C, but then levels off at around 98C once the terrain does. Turning on the heat will cause an immediate drop in engine temperature down to about 80C, but it then slowly rises back to 98C and maintains there.

Aside from a Shout/citric acid flush, I don't know what else to do to get the engine temperature down to where it is supposed to be, and I'm not convinced that the acid flush would cause such a substantial drop in temperature, but correct me if I'm wrong. I'm open to just about anything at this point. Please help!

Sorry for the long post.

Thanks in advance.

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1984 300SD
351,000 miles