Date registered: Jun 2007
Vehicle: '95 E300 DIESEL, '91 600SEL, '92 600SEL
Location: San Diego/USA & Belgium/Europe
Mentioned: 20 Post(s)
Quoted: 900 Post(s)
Since you have a '87 diesel, do you have the dreaded head#14 or some later revision???
Usually running hot at speed indicates a bad WP, but in your case it sounds like either clogged coolant passages, or lack of proper pressure. The fan clutch does very little when the vehicle is at speed.
IF you do replace the fan clutch, ONLY use a genuine MB fan clutch. All the others are junk, including the BEHR fan clutches purchased from places other than MB. BTDT
Also, does your radiator cap have the proper rating??? I believe for your year it should be 1.4 bar. If you use a 1.2 bar it will prematurely overheat. Only use genuine MB radiator caps. Water pumps should only be Graf, Laso or rebuilt by MB. The water pump impellers have a tendency to spin loose (on old or substandard pumps) also causing hot running conditions.
Can we assume you are running genuine MB coolant (a.k.a. Zerex G5) mixed 50/50 with distilled water and nothing else???