Mia, It may be the mono heat valve, you can buy the valve for about 65 to 80 dollars four screws removes the valve without draining the coolant. An easy fix and one that I have done twice in the 235,000 miles on my 300sdl, and twice on my 81 300sd in 400,000 miles. The 84 300sd has the same type valve located next to the battery. Jo
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18 years ago