My brother has a 98 F150 with a 4.6 aut. The truck has a engine vibration at 1800 rpm whether you are driving in gear or in neutral parked. It is running good, not skipping or misfiring. We ran it without the belt and it did not make a difference. We changed the harmonic balancer and it did not make any difference. We checked the motor mounts and the exaust and everything is good. We are going to check the flyweel and converter next. The transmission fluid is clean and the trany seems fine. He bought the truck this way. It has 80,000 miles on it. Has anybody ever had this problem before?

 

Could it possibly be dirty injectors?