My mustang gt 96 ive changed cables,coils,plugs,mass flow sens,tps,iac,fuel pump,o2 sensors,fuel pressure reg,cleaned egr and throttle body and still having rough idles and back firing....

Yes check engine is on and its backfiring from tail pipe...

Trouble codes are po171 and po174 showing that its running lean.

Ok thanks will check that out first thing in the morning..

Also consider a possible burnt exhaust valve

It seem that when i spray some throttle body cleaner it runs ok for a few hrs but then it starts again

But if thats the case on a burnt valve what should i do?

You need to do a compression test first, if it is a burnt valve the only fix is to remove the cylinder heads and have them done and replace the valve.

Hey thanks for the advise jim i noticed that my car had cloged solenoid that comes from the fuel tank that connects directly to the throttle body what i did i just bypassed the solenoid and now runs like a champion ill replaced later anyways but thank alot for your time and advice!!!!