Our 2002 Altima was recently repaired for white smoke from exhaust - replaced the valve guides(1800$). After getting the car back we found the radiator was cracked at the top, but not before our daughter let it overheat. Repaired and cleaned the radiator. And realized we were 2 quarts low on oil after only 150 miles from the initial repair and oil change. Original mechanic put in a valve cleaner and said to run it about 500 miles. The car used 8 quarts of oil on that trip and black smoke started from exhaust. Now the mechanic says he gives up, can't figure what is wrong. Any ideas.