Check the thread I posted for a way to route coolant hoses back to the tank with no drilling of the body. It is not too hard to do. I think you may need more heat in the system. I have a coolant wrap around my filter, 16 plate heat exchanger, in tank heat exchanger and 10 feet of HIH and am getting in the 150 F range. 160 is Ideal.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine