Jeep: 1998,F-150,4.2engine,automatic, timing chain, nash

Question
engine make pinging noise when accelerate,but running OK when get normal speed. THANKS NASH!

Answer
I guess the noise you are hearing is spark knocking.  Try running premium gas, if the problem goes away or is not as noticable you have a timing related problem, maybe incorrect distributor timing or just a worn out timing chain or motor. Hope this helps you!