i think my timing is messed up. my 99 tahoe wont start it cranks strong but its either my spark plugs or my timing. any suggestions

 

I had the same problem last week. It turned out to be the crankshaft sensor (Just behind the harmonic ballancer). There is also a camshaft sensor (under rotar)they both work to controle the spark timing to the ECM (next to coil) any of those things will cause the problem. I would start at the coil if you are getting spark out of the coil then do a tune up if not go to the sensors and ECM (the coil is the halifway point for making the spark happen so you can narrow down witch way to go from there)

Good luck