Diesel Power,

Your problem is most likely a water or air flow problem as when you drive in 5th gear, the load on the engine is highest and the water flow rate is lowest. Any air flow restrictions and you may well be unable to remove the heat at the rate the engine is making it.

You might consider examining the air conditioning condensor which is in front of the radiator to see if it is clogged with twenty years of bugs being sacrificed to getting you from here to there, or other road junk, and corrosion. These units are usually painted and when they get real old sometimes the metal underneath is exposed by abrasion, leading to oxidation of the material. Eventually the paint can start to flake off and block the air passages.

Another common problem is a thermostat that is not set or built correctly. Even new ones come out of the box and do not work correctly. Try boiling it in a pot of water with a thermometer to see if it opens when it should. Remember, these thermostats have two functions, one is regulating flow to the radiator, and one is regulating the flow that bypasses the radiator and mixes with the cold water from the radiator before it goes into the head. In the overheating condition the thermostat should be blocking all bypass flow and sending all the water pump output to the radiator.

There is one thing to consider with the water pump. The 5 speed unit may have a different water pump than the 4 speeds and automatics. You may want to check to see, if the water pump has ever been replaced, if the water pumps for these cars are all the same part number. Given the 5 speed unit has an overdrive ratio, it may require a higher flow rate to handle the heat load. I am just guessing here though.

Good luck, and I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)