MODS, please "sticky" this thread... I have searched this site and have included the pertinent 4.0 harmonic balancer threads into this one, to make it easier for future reference and to help those who are seeking the proper information regarding the 4.0 balancer issues. This is a very problematic issue w/ the 4.0 Explorers (as well as the 4.0 Sport Tracs). The first signs of a 4.0 balancer starting to fail - distinct "chirping or squealing" of which resembles a serpentine belt squeal. At idle, the noise will resemble a "chirp", however, when driving, the noise will progressively get louder as RPM's increase and will turn into a loud pitched squealing, slowing down again when coming to a stop. The noise will be coming directly from the balancer if positioned under the vehicle while listening at idle. IF the serpentine belt is removed, the balancer will not squeal or chirp, it will only do it w/ the belt on and under tension. If all other front end accessory pulleys do not make ANY noises when manually spun (w/ engine & belt OFF), the harmonic balancer is the problem. Besides the obvious chirping/squealing, other ways to detect a failing 4.0 harmonic balancer: 1) watch the serpentine belt & belt tensioner @ idle - if the belt is not following a smooth path while running and the belt tensioner is sporadically "jumping or bobbing", it's a bad balancer 2) if while watching the serpentine belt @ idle and looking down at the balancer, if you see ANY slight wobbling of the balancer, any run-out or that it's not tracking straight, it's a bad balancer 3) with the serpentine belt off, if you tug on the front of the balancer pulley and the OUTER ribbed pulley moves even slightly, can be wobbled back and forth (side to side) or in some instances will pull/slide forward (towards radiator) or can be pushed backwards (towards engine), it's a bad balancer 4) if the bonded rubber elastomeric center that secures the outer ribbed crank pulley to the center of the balancer is either protruding (not flush w/ the face of the metal pulley) OR there are visible signs of dry rotting/cracking/deterioration, it's a bad balancer 5) IF the serpentine belt is NOT on the RIBBED section of the crank/balancer pulley and is in FRONT of it on what appears to be a smooth rubber surface, then the balancer has already separated and you should not be driving the vehicle. Too many of us have had to replace this part - luckily some of us did not have extensive repair bills, while unfortunately other failures have resulted in more damages & extensive repair bills. There is a revised 4.0 balancer (crank pulley) through Ford that is better and not like the original design. The revised 4.0 harmonic balancer Ford part # is: 8L2Z-6B321-A IF you take your vehicle to a Ford Dealer (or other shops) - do NOT let them tell you it's just a bad belt or other misc. pulley causing the chirp or squeal, especially after you have confirmed via steps 1-5 above that the source of your problem IS the balancer. You do not want to continue to drive a vehicle w/ a bad harmonic balancer as you will and can only cause more damages to other surrounding components or even worse, end up with possible engine damages - fix a bad balancer ASAP. PICS & DISCUSSION OF FAILED BALANCER: http://www.explorerforum.com/forums/showthread.php?t=296853 MORE PICS & DISCUSSION OF FAILED BALANCER: http://www.explorerforum.com/forums/showthread.php?t=267709 HOW TO R&R THE BALANCER: http://www.explorerforum.com/forums/showpost.php?p=2509603&postcount=4 MISC. BALANCER DISCUSSION: http://www.explorerforum.com/forums/showthread.php?t=299522&highlight=harmonic+balancer MISC. BALANCER DISCUSSION: http://www.explorerforum.com/forums/showthread.php?p=2589015#post2589015 ~~~~ If others have more to add to this thread, feel free to do so. It's important that other/all 4.0 Explorer and Sport Trac owners are aware of this common problem.