Rep Power: 0

2004 Honda Civic, Standard 5-speed transmission, 142K miles

After 20 to 30 min of being at 60 - 70 mph on the highway in 5th gear, the shifter pops back into neutral when accelerating. As the drive continues, it happens with increasing frequency. If you pull over for 15-20 min, it will happen less frequency for a period. This leads me to believe it only happens when the car/transmission is warmed up. It also only happens when the throttle is being pressed. If you hold you hand on the shifter and push in the throttle, you can feel the shifter pushing back on your hand with proportional force. Accordingly, you can prevent it by bracing your arm against it and not allowing the shifter to pop out.

Any ideas on what the cause of this would be?

 
Rep Power: 221

Check fluid level, check engine/trans mounts, check the shifter ball under the stick and see if it's split

 
Rep Power: 0

I had this exact same problem a couple of months ago with my 2004 Civic EX 5 speed. The transmission was remanufactured and installed 5 years ago when I rebuild the engine. The VIN was removed from the transmission so I don't know what the year of manufacture was. I tried all the typical advise ( check/change transmission fluid, check engine mounts, check shifter and shifter cables, check lubrication of shift cable attachments at transmission, replaced clutch friction disk, pressure plate, pilot bearing, though out bearing and had flywheel reground) nothing worked. Problem was clearly inside transmission. My final and satisfactory solution was to pickup a used 2005 Civic EX transmission with 135k miles, replaced the input shaft bearing and seal as well as the axle seals and installed with fresh honda MT fluid. It now works great, the kick back I could feel in the shifter when accelerating is totally gone. Since I did all the work my self the cost came to about $600 - $700. There was a honda service bulletin for a similar problem https://www.tsbsearch.com/Honda/01-081 don't know if it applied to model years other than 2001 but to complete the service bulletin would have been $1300 in parts alone discounted.