What'* the code?

It was something P1507 or P507 something like that. I forgot to rite that one down. But it clearly said low voltage something something o2 sensor #2 (something along those lines)

Those don't work. If it was for O2 sensor I would start there.

Something doesn't sound right when cranking,Something sounds...off like mechanical. It almost sounds like it'* way out of time. Reason I say is I had an 87 chevy 4x4 1 ton with a 350 (carburated) and Something very similar happened,ended up timing chain broke. Does this 5.7 have the cam gears with plastic teeth and belt or steel teeth and chain?

Time for a compression test.

Between 80-120 psi if memory serves correct?

Do a compression test.

Please update your signature to reflect the vehicle in question. It sucks having to backtrack through 3 pages of posts to see what exactly it is you are referencing.

A 99 Vortec 5.7 has a metal timing chain. No plastic coated gears either.

It'* a 99 K1500 suburban 5.7 will crank not fire. Has fuel has spark. Tried starting with ether but would only backfire through throttle body. Only code the OBDII gave was for o2 sensor (I knew was going bad anyway,have 4 new o2 sensors to put in)
something sounds different when cranking. I know some GM models they tried saving money by using plastic gears and a belt instead of steel gears and chain. Truck has over 200k miles but motor has just shy of 100k miles. Guy I bought it from cut the fuel filter completely out of the fuel line. Motor is bone stock from what I can tell.

Like the others said, verify compression by using a test gauge on each cylinder with all the spark plugs out.