4th gear popout

[fB2k]

Hi, I have a 91 240sx 5 speed. I changed the transmission fluid today with redline mt90 and now the ****fting feels much improved, but now a new problem has arised. When I'm cruising at say 40mph and leave it in 4th, it'll stay in gear but popout like 10-15 seconds later. I've tried all the gears and seems only the 4th gear is having this problem. What could be the problem? Could it be the synchro? :help

[virus77]

what about when you slam your brakes in gear, does it pop out then, cuz it could be your transmission mount. A broken mount will cause tension and it will do what your saying, just something to check.

[fB2k]

No it doesn't popout when I slam the brakes. Just does it when driving normal.

[xekushnr]

hm, it could be many things. check the transmission linkage and shift lever arms, clutch pilot bearing, synchro inserts and springs, and play in the countershaft and mainshaft.