You will not want to hear this, but here goes. I have seen this on more than one occassion, each time it has been one thing, one or more of the connecting rods would be bent. This will not show up on compression or any other test. Pull the head and it will be seen. The reason is the EGR will clog up with carbon and the engine will suck it in. When this happens, something will have to give, usually it is the rod(s).
1992 500E (very soon I hope
1981 280GE SWB
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