Engine Mechanical problem
1994 Mazda 323 4 cyl Front Wheel Drive Manual 233000 miles

I have a mazda that runs great but have one of its four noozle not firing.my mechanic have flush the noozle for blockage,test the brain box but yet no result.what do you think could be the fault.
2. The top cylinder was removed and valve sealed replaced inorder to solve the noozle problem but after the engine was coupled back, the engine began to emit white smoke and start eaking into the silenser.please can something still be done to repair the engine or should i buy a new engine.thank as i await your reply.