Okay heres a thought,on a sprintcar to clear out the cylinders of fuel,you put it in gear and rock it backwards,this makes the engine push everything out the exhaust (could it be a fluid lock?) ,it may be a stretch but that seems where you are,I would put it in your highest gear and try to rock the car backwards,Johnny After reread you would have to make the valves open to remove any fluid if it was trapped and indeed the culprit,this is indeed a puzzle.

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85 300sd SOLD ,85 745i,95 740i,1972 Suburban,1938 International