1985-1986 MR2s had the C-50 transmission while the 87+ NA models had the C-52, which was an upgraded and strengthened version of the C-50. This included stronger input and output shafts with larger diameter bearings on some of the gears, along with updated syncros. The C-50s are notorious for having what's been described as the 5th gear popout. This problem is a little less common with 1987-89 NAs (using the C52 transmission), but the problem can basically occur on any model/year.
Fifth gear popout can actually occur with any gear, but more frequently with 5th. Under heavy acceleration, abruptly releasing acceleration pressure, and then reapplying it will cause the transmission to jump out of gear.
The cause of the pop-out is not the syncros, hubs, or gears. The problem is frequently the from wear on the main input shaft, and output shaft bearings. The output shaft is almost submerged in gear oil, the input shaft however is partially out in the open, This is the one that shows the most wear.
Do I have it?
To check for fifth gear popout: accelerate and decelerate quickly in each gear; push hard on the throttle, and then remove throttle pressure, letting the car slow, and then hard back on the throttle. The gear lever should move very little during this decelaration. Earlier cars are known for popping out of fifth gear, and occasionally other gears as well.
Can I Fix It?
- Don't suddenly reduce acceleration! Really!
- Don't use fifth gear. Only down side is the noise might be slightly exhausting on long journeys; very slightly lower gas milage.
- Replace the fork and hub. This fix is easy to do (even for a new DIYer), and relatively cheap. Full instructions at IMOC.
- A possible cure is to replace the motor mounts. This method has the advantage of at least being cheap and easy.
- A more permenant but more complex fix is to rebuild the transmission, which is not a job for the faint of heart.