No power, dumping has, rough idle
 

02 s80 2.9 non turbo
Serpentine tensioner wheel failed making the belt snap. Fixed it and now engine idles rough, has no power, is dumping gas (using too much) according to the on board computer. No codes yet either. Also the exhaust sounds different. Almost like it's plugged

check your coils, all 6 of mine went out within 200 miles and not all at the same time. i had a rough idle, and loss of power (no power with 2 bad coils, like the engine went into limp mode) without spark the gas will not be burned off..mine was so bad my cats glowed red as if they were clogged, i replaced 3 coils and all of a sudden no more glowing cats.

How do I go about testing them myself to see if they are bad?

search on here or there is a lot of video's on youtube. basically take the covers off and start the car, unplug/remove 1 coil at a time. if the coil is bad the car will not make a change in rpm, if the coil is good and you unplug/remove it the car with change in rpm (run rougher/possibility stall out.)

I doubt it's your coils. This post doesn't contain the full story of what happened with the timing...

I've been re tracing everything that happened. My buddy who works on American cars, not metric, jumped in and was trying to get the serpentine out from behind the camshaft pulley, loosened the nut on the crank pulley, and saw what it was and put it back on. The cog wasn't taken out but I don't know if that mattered. From what I've been looking at loosening that nut was a mistake and allowed for the motor to then become out of time.

Loosening the nut for the balancer won't cause any problems. The problem was caused when part of the serpentine belt got sucked in the timing cover. That caused it to jump time.

No, not the balancer, which was taken off, but I know that wouldn't take the timing off. The center nut to the pulley was taken off. The cog was never touched

Your post says "...loosened the nut on the crank pulley, and saw what it was and put it back on. The cog wasn't taken out but I don't know if that mattered. From what I've been looking at loosening that nut was a mistake and allowed for the motor to then become out of time"

Either way, you probably have some engine damage.