I have an 1987 260E, 143,000 miles. Since it has gotten warmer, the engine continues to heat up above the normal 80C while in traffic with AC running, once on the open road, temp seems to return to near normal. Aux fan is working, full of coolant, new radiator several months ago due to an accident. My thoughts were the thermostat or the fan clutch. Any suggestions, on how to determine where to go from here.