If the crank bolt loosens up, it allows the harmonic balancer and crank sprocket to shake back and forth. First, this unwanted movement usually causes an rpm-sensitive noise and vibration in the front of the engine. You can watch the harmonic balancer wobbling while the engine is idling. Beyond a point, those drive belts are the only things keeping the balancer on the crank snout. It's anyone's guess as to how long a driver will ignore this noise and vibration.

Second, although the loose crank sprocket causes an erratic CKP signal, experience shows that it rarely triggers a crank sensor trouble code. However, the unstable signal creates an assortment of driveability problems. For instance, the engine often idles okay but misfires during acceleration and the ECM sets one or more misfire codes. Eventually, the engine may misfire all the time and become hard to start. Before you dive into a diagnosis, always get the vehicle history and watch for a wobbling harmonic balancer.

When in doubt, thoroughly test the crank sensor with your oscilloscope. Look for the familiar, cyclical, sine wave-like pattern at idle and higher rpm. (Note that a flat spot among the reluctor teeth creates a crisp, repeatable pattern “blip” representing piston position.) Sprocket movement causes an ugly, erratic crank sensor pattern.

A driver may ignore the noise, vibration and misfiring symptoms until it's way too late. By that point, the loose parts may allow the woodruff key to wear or wallow out the keyways in the crankshaft and/or crank sprocket! The unwanted movement also may distort the woodruff key itself. Sometimes the wear and looseness allows the crank sprocket to twist or shift far enough to alter timing; then overall performance suffers. So if you discover a wobbling harmonic balancer and a loose crank bolt on one of these Toyota engines, a tear-down and thorough visual inspection is the best way to know what's damaged and how badly.