On my 89 I have been experiencing popping in the exhaust on a decel. I was told that it was most likely air in the exhaust. The cars still retains all emissions components, most of which were replaced with new pieces when I recently installed the engine, the only pieces which weren't were the down stream check valve from the smog pump to the cat., the diverter valves and rubber hoses. All the valves appear to be working fine. I double checked the vac. diagram to make sure I didn't mix up the valves. I was told the down stream check valve was most likely bad. I was able to blow through it in one direction, and in the other it holds pressure I don't see how it could be bad. I have Bassani cats on the car and I found a leak were the air pump tube joins together (the rubber hoses they give you burned against the exhaust). After fixing the leak the popping lessened and seems to happen at higher rpms as opposed to all the time on decel. I was wondering if my cam could be causing the problem ( E 303 cam) , I thought that maybe the overlap of the cam was causing the check valve to open falsely allowing air into the exhaust. I am also worried this my wreck the cats. I would greatly appreciate any help you guys could give me. Mike