Hi all.

My '87 300sdl is having cooling system issues that I can't seem to track down. Hopefully someone has a bright idea that I haven't thought of...

Ambient temps have been between 90 and 100F during the day, but symptoms persist even in 70-75 degree temps at night.

Symptoms:

-On flat ground on the highway it stays fairly steady at around 95
-Downhill on the hwy it cools off to ~85
-Any incline on the hwy it heats up well past 100, often as high as 110 and sometimes as high as 115
-It town it runs between 90 and 100 depending on traffic and terrain
-Temps stay fairly similar whether AC is on or not
-Turning heater on high has some cooling effect on engine temp, but not drastic


What I've done so far:

-Replaced thermostat twice (OEM from Mercedes)
-Replaced radiator cap (OEM from Mercedes)
-Replaced sending units for temperature gauge and AUX fan switch
-Citrus flush, twice
-Coolant change, at least 3 times, always 50/50 Zerex G05 or Mercedes antifreeze
-Replaced radiator with new Behr/Hella unit
-New primary/secondary fuel filters

System is fully burped as far as I can tell. No air bubbles from top radiator hose when squeezed, almost no air in reservoir (I know this isn't proper but I wanted to be sure there wasn't an air bubble). I spent probably two hours trying every method of burping I could find on the forums until I was satisfied that there was no air left.

Viscous clutch fan was replaced by PO with MBZ OEM, new style (plastic). Fan engages and disengages properly ("roars" when running, then stops as car cools down).

AUX fan functions properly with AC running as well as when engine temp is >105 (turns on high).

HVAC functions properly (cold AC, hot heater - no monovalve problems as far as I can tell).

Temp at top radiator hose and at head are within a few degrees as indicated by temperature gauge. Tested this with non-contact IR thermometer.

Temp difference between upper and lower radiator hoses is within a few degrees. Tested this with non-contact IR thermometer.

When thermostat is open and the top radiator hose is removed from radiator, coolant pours out freely (no apparent circulation issue).

No oil in drained coolant as far as I can tell, no milky oil on filler cap or dipstick, no milky oil at last oil change. No white smoke from exhaust.

No hesitation on acceleration, rough idle, etc.


That's all I can think of at the moment.


The only real anomaly with the car that I can think of is that, when given fuel, it smokes at the joint in the exhaust manifold circled here:



Far as I know it has the "can" thing replaced (name eludes me at the moment dang it) with the new setup under the recall:



Also, I run it on B100.

Any ideas? This is driving me nuts. I'd appreciate any suggestions. Thanks!