I drive a 2000 Toyota Corolla Manual 5-speed that intermittently pops out of fourth gear while I'm driving normally (shortly after I shift into fourth), but usually after having driven at high speeds (or while driving at high speeds). If I only drive in town for a few days, the problem goes away.
Do you know what the problem might be? What's the best way to drive it before I get it fixed to minimize damage?
Mileage? How frequently has trans oil been changed over the last 22 years? Your transmission is probably wearing out. About all you can do is avoid using 4th gear to try avoiding further damage. (Wind out 3rd and shift directly into 5th.)
Mileage is 250k, sorry! I'm not sure about how often the fluid has been changed since I just bought the car. The previous owner did generally take care of it, though.
Although internal wear seems likely there's a chance that there's a problem in the shift linkage, like a rubber coupler or bushing that's gone bad after more than 2 decades, so that's something to look into. Maybe also change the transmission oil and see what comes out.
It's also possible that the collar isn't fully engaging on your synchro gear. Manual transmissions use collars that slide over the synchros, and oftentimes the synchros are what wear out first, especially if during a shift, the clutch isn't fully disengaged and the collar grinds it. Do you notice any grinding when you shift into 4th? If so, that is a strong sign that the synchro is worn out. And fortunately, manual transmissions are very simple in their construction and are generally cheaper to rebuild than automatics. Also, replacing the synchros is generally all that's done in a rebuild, as well as changing the oil. I suggest changing the fluid, as Chuck mentioned, and have a sample analyzed to see if there's any metal shavings in it. If the problem doesn't resolve itself after changing the fluid, maybe consider rebuilding it. Hope this helps!
I agree with @bbenjamin85. The reason it's popping out of gear is because it's not going all the way into gear due to worn synchros. When I do a manual trans rebuild, I always replace snychros AND all seals. Another possibility is that the bronze pads that are brazed to the shift forks have worn out and need to be replace too.
I agree with @Doc.
If you want to keep it running, do not use the 4th gear. For upshift, rev the engine higher when you are in the 3rd gear and and shift it directly to the 5th gear. For downshift, try to skip the 4th gear as well.
Mileage is 250k, sorry! I'm not sure about how often the fluid has been changed since I just bought the car. The previous owner did generally take care of it, though.