1998 4runner ltd
At around 200K miles I began experiencing some pulsing/vibration from the rotors while braking, and since they were original and pretty worn, I replaced the front pads, rotors (Pep Pro-Stop) and calipers (NAPA). After about a month, I started getting the same warped rotor vibration. When I returned the rotors, Pep exchanged them for a higher grade 'platinum' type with new pads. These worked fine for almost a year, but then the behavior returned along with a VERY pronounced pulsing/grabbing when coming to a stop. This seemed to be just on the passenger side, and the behavior seemed to get worse as the brakes heated up. I drove like this for awhile, thinking that maybe it was road contamination that would clear up but it didn't.
I realized that the behavior seemed to appear after I had a flat repair done (discount tire) during my last road trip. On another vehicle I had owned previously, I had experienced 'warped rotor syndrome' after having some shop over-tighten the lug nuts. It seemed like this might apply to my recent worries, since I didn't immediately re-tighten the lug nuts like I usually do when I use a tire shop. Also after a very recent tire rotation at my local discount tire, I checked and found over-tightened and unevenly tightened lugs. My take is Toyota gives you a 10" lug wrench. If you can't use that tool to loosen those lugs, they were overtightened. They claim to use a torque wrench, but I can show you at least three ways to use it incorrectly.
On this car, it is a bit hard to believe that uneven/over torque could warp a rotor, since the rotor is floating but sandwiched between a very sturdy spindle surface and the aluminum wheel. This differs from many cars where the spindle/rotor is a single piece with directly mounted lugs, so lug stress might be conveyed more directly to the rotor.
When I pulled it apart, I could see the rotor surface had a slight bluish discoloration from heat, particularly on the passenger (problem) side. I don't tow or carry heavy loads or drive like Mario, so why the heat? The pads looked fine and were wearing normally on each side. The caliper pistons moved freely. Measuring the rotor thickness showed equal wear on both sides (less than 0.010"). I put a mag-mount dial indicator on the mounted rotors and they both had equal run-out of about 0.002" which is about max, but not outrageous or different.
I set the rotors side by side and noticed a few differences (see pic).
On one of my rotors, you can see the metal plates that form each surface are very unequal in thickness. This rotor is not worn, it is just made poorly. I could believe this difference would cause a problem, but this isn't the side that is grabbing! The two rotors appear to be from different casting lots. One rotor has an embossed lot marking, and other features showing it was part of a different manufacturing process than the other one. When I took these down to Pep, their tech pointed out some diagonal scratches on the disk surface and claimed they were stress cracks. If so, it would certainly explain the bad brake behavior. I actually think these are not cracks, but the remnants of the cross-hatching I remember being present on the disk surface when new.
There is a good write up of disk brakes on wikipedia which describes some problem behaviors and causes I don't remember reading before. In short, incorrect or inadequate break-in can result in 'pad imprinting', and uneven pad deposits leading to further overheating and vibration, similar to warped rotor. Proper break-in involves several hard stops from 40mph with a cool down period between each one. Avoid a complete stop with hot brakes during this period. Oh, and don't get into an accident.
With several suspect causes but nothing definite, I decided to pass on the offer of a third set of identical rotors, and went with a slightly more expensive Bosch brand and a higher end set of pads. I also replaced the original flex lines which were rusty and cracking. BTW the short steel lines on the calipers can get frozen to the flex lines but are only $10ea at toyota. Brakes are perfect again but only time will tell.
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