The design of the belt tensioner is that the spring is allowed to push the idler pulley against the backside(outer) of the belt. If the tensioner got loose, the spring would still push, and it would do so until the bolt holding the tensioner to the front of the block fell out. At least I think it would continue to push. When you install the belt, you allow the tensioner to spring the idler against the belt, and then tighten it in place. You do not apply any more force than that, so it is all spring.
Compression numbers look decent, and you'd likely get different numbers if you ran the test again, so I'd rule out valve, ring and gasket problems. So far.
I would be looking at the ignition and the injectors. How old are the plugs, what is fuel pressure, and what condition are the plug wires are all questions I would be looking to answer. If the engine will idle smoothly, then the parts mentioned are functioning okay, so look for fuel delivery problems. If the engine idle is wobbly, or has definite intermittent misses, check the parts. What is the plug gap? Electrode condition?
tom
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