What is the knocking sound of your 1991 explorer you are wondering if it is the lifters or valves and what can you do to remedy this?

An engine that has had long intervals between oil changes and lots of miles, can easily develop stuck hydraulic lifters. This can be a loud clicking/ticking noise at the top of the motor.Try this: Change the oil and subsitute one can of transmission fluid for engine oil then change it at 1000 miles or less . It make take a few changes but if it is a lifter ( most common problem) it should improve. I hope this helps you. Mark