Is my mechanic right? My 4Runner is nearing 100 thousand miles and the mechanic syas I need to replace the timing belt. He says they’re prone to fail without warning and that if it fails the engine will be ruined. Is he right?
Yep!
If the mystery engine on your 4Runner is a 4 cylinder, it has a timing chain, rather than a belt. Chains are usually a lifetime component.
If it has the V-6, it does have a timing belt, and the belt does need to be replaced by 100k, or on the basis of elapsed time (most likely 7 years). However, the V-6 engine is not an “interference” engine. This means that when the belt snaps, no engine damage will occur, but you could wind up being very much inconvenienced or possibly endangered, depending upon where you are driving when it snaps.
Your question indicates that you have not looked at the maintenance schedule that is sitting in your glove compartment. I would strongly suggest that you start using that schedule as your guide to maintenance, even if it is a bit late to begin this process.