1998 3.9l crank no start
it has gas and spark but no compression.
I have tried compressed air comes out exhaust when cyclinders on compression stroke .
is this a timing issue or bent valves?

 

Sounds like the cam isn't turning or the timing chain is worn and jumped a few teeth. First pop the distributor cap off and crank the engine to see if the rotor is turning. If it's not then neither is the cam. if it is then line up the timing mark on the front pulley with the TDC mark and then see if the rotor is pointing at the cap tower for the #1 cylinder spark plug wire. If it's not then the timing chain is worn and jumped a few teeth. Also if the rotor seems to be 180 degrees off, then rotate the engine one full turn, line up the timing marks again and see if the rotor is now pointing at the cap #1 tower.

 

Thanks for the reply.
yes rotor moves and points to #1
the rockers move .