Aqua, how have you missed bypassing the switchover valve!?!? I could have sworn I mentioned this to you already! Haha. It is also called Overboost protection switch. There are two. One on firewall in front of Fuse box. And one on the same firewall but around toward engine. The second is the one you need to bypass.

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000