On the 616s and 617s I have done, I cut the hoses off and bring the cooler to the bench. Then I make a cut across the steel nut, being careful to only go as far as I must to weaken the nut. I stick a large screwdriver in the cut and twist to open up the nut, then soak the wound in penetrating oil. So far, haven't lost a cooler in 3 tries.

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)