I got this Toyota Corolla 1.6 GLS 2001 that pops out of 4th gear and 5th gear. It drives fine, it has no sounds, for all intents and purposes it feels like a great car, right up until I start accelerating in 4th or 5th gear it just pops back into neutral.
I started having a look at the gear linkages, they seemed fine except one had what could accurately be described as a scatch gear linkage pin. I replaced the pin, needless to say that had no effect. I then realised that the right side mount was completely shot. I replaced the mount, and surprise surprise no effect. The last issue was to check the oil level. I had been treading this because someone had strapped the gearbox filler plug.
I had some experience with this problem because I had strapped a gearbox filler plug on a different Corolla myself last year and it turned out to be a complete nightmare. I had already drained the gearbox so I couldn't drive the car to my mechanic to get help. I had to go fetch him and even he struggled to get it off. So I knew what was in store for me with this one and I was determined that I was gonna get it off myself.
Armed with my prior experience I didn't drain the gearbox before getting the filler plug off. This lived up to my expectations. It took 3 days of struggle, I even bought a Butane torch to try and use heat to expand it and that still didn't work. Eventually I had to go buy an extractor set. I drilled a whole in the middle of the plug and put an extractor in. It took 3 hours to gently drive the extractor while trying to not create iron fillings and I ended up using back of a socket to drive it. When it finally came off I thought I broke something.
I could then confidently drain the gearbox knowing I would be able to fill it. To my utter shock, only half a cup of oil came out. This gear box on a similar car last year took about four 500ml bottles. I think the most likely scenario is that someone made the same mistake I made last year. They drained the gearbox before opening the filler plug. They then strapped the filler plug trying to remove it so they couldn't refill the gearbox. So they pulled the dick move of just closing the drain plug and giving the car back to the owner and not saying anything.
I bought a new filler plug with a much bigger head and refilled the gearbox which took like I expected four and a half 500ml bottles of oil. I took the car out and it stayed in 4th and 5th gear under acceleration even pushing it all the way to 120kph. So this was the issue, someone didn't fill oil in the gearbox.
The last issue I'm still having with this car is the ABS light. It throws error 13 and 14 which I will still have to figure out.