After the engine has fully warmed up, the coolant has expanded and raised the pressure in the closed system. Loosen the exp tank cap slowly (careful you don't burn yourself) to release the pressure. Tighten the cap and let it run a few minutes then loosen the cap again. There should be no pressure. If you have pressure there is likely combustion gas leaking into the cooling system.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked