You are between a rock and a hard place. I wouldn't want to drop the engine in and see what happens either.

You can do a ballpark compression test by just rotating the engine, While the compression will be different than the engine at operating temp you can see if there is a significant difference, more than 10%, from cylinder to cylinder. That is primarily the telltale information you need to know to start troubleshooting for a problem.

Unless you can borrow a fully functional motor stand it is probably easier to drop the engine in and hook up as little as possible to get it running or take the engine to an independent shop that specializes in BMWs to do a post purchase inspection.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas