96 zj i 6 4.0 i have replaced
distributor (cam sensor)
rotor
cap
plugs/wires
crank sensor
ignition coil

Tested compression around 60 psi running 95-100 in off position
there is no engine knock or ticking cleaned out the engine with sea foam (one can through the throttle body and one in the gas tank) changed the oil ZJ likes to backfire and turn off while running hard to start after it shuts down we are at a loss with this one we want to keep her but no one knows what else to do by jeep failed for NOx gases for emissions

 

NOx is caused by high heat in the combustion chambers, running too lean can cause too much heat. What is the fuel pressure? I don't quite understand your compression numbers, those are really low.

 

Sounds like timing is off. Did you take the distributor out?

 

The PCM and its connectors are prone to issues and can cause all sorts of havoc on the 96-98 ZJs, particularly random stalling and popping. Give em a wiggle and also try tapping on PCM while its running and see if it stumbles or stalls.

 

while the engine is running it has a compression of 60 psi
after the engine is turned off at the tester reads 90 psi before i release the pressure from the reader to disconnect
and yesterday it made a popping sound like popcorn popping before it backfired