I have a Toyota AE91 that has been running well for a few years. Lately, however, the fourth gear slips out while driving and I am forced to bypass it and engage fifth. What could the issue be with the gearbox? Is it repairable or do I need a new gearbox.
Your gearbox is aging. The engagement of the operating sleeve with the dog ring on the gear has been affected. “Cold pop-out” and dog ring/operating sleeve damage are the most likely causes.
Cold transmission pop-out (“cold pop out”) is an oil contamination problem that can lead to operating sleeve and dog ring damage, so it could be tied in with the other cause.
Operating sleeve/dog ring damage is when the mating teeth of both parts are damaged to the point where they will not engage each other or will not stay engaged.
This damage can occur when the synchroniser units wear out, could be from snap-shifting, shifting while the clutch is not fully disengaged, and from popping out of gear. The operating sleeve can be modified to increase its engagement with the dog ring. However, this must be done before any damage occurs. By now it is too late.
You need to rebuild the gearbox, or just buy a new one.