This thread is from 2001. It was a cracked head as I recall.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

I went through all of this and replaced everything (raditor, temp sensor, water pump, fan clutch) and still the temp would go up over 100c. Than I ran into a mb tech on the golf course and he had me check the wiring from temp sensor to the cluster. I soldered a new end on the temp sensor AND ran a new ground from the cluster (back of temp guage) to the body. Now the car runs at constant 90 t 95c and I live in south florida. The ground was the last thing that I did and that solved the problem.

1984 300d Turbo 135k
Take The Top Radaitor Hose Off Fill To Top, You Have Air In The System Had The Same After I Change My Radiator And Replace Or Clean The Bypass Small Line On Front Use New Copper Seal Washers, A Kit From Jc Whitney.com Also Rubber O Ring Metic Kit One Fit The Leaking Alda

First, you need to relax. 100 Deg-C on an 84 300D is really not that hot. I have owned several of these models and over 250k miles or so they tend to run around 100 Deg-C. The issue is probably not with the coolant system anyway. In these vehilces, unless you have an obvious problem with the cooling system, running too hot is usually a sympton of poor combustion. I recommend adjusting your valves and see what happens. This is a fairly easy job for a DIY-er, but you will need the special wrenches and a good set of feeler gauges. Replace the valve cover gasket as part of the job. The next thing I would try is running some "Diesel Purge" through the fuel system, or better yet, have your injectors professionally cleaned (this is a lot more $$$). I bet you engine smokes a little on acceleration. That's another indication of poor combustion. But don't worry about 100 Deg-C. Until you start to get up to about 120, you will not damage the engine.

Check out the diesel forum for guys who know more than I about this.

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KCScout
98 S420 My Wife's Baby
97 S320 My road car
83 300D My old road car (love that diesel)
81 300SD Wife's former Baby
81 240D A fun stick shifter
71 220D My first one