Help - Car pops out of gear Options

[Stu37363]

I have a very limited knowledge of transmissions and was hoping someone here might be able to diagnose my problem or tell me what to check.

Lately when accelerating in 4th gear, the shifter will pop into neutral.
3rd gear has always pushed itself towards neutral while accelerating, but never actually went into neutral.

In 3rd or 4th gear i can hold pressure on the shift knob and feel it pull towards neutral when accelerating, then if i release the throttle, it goes back fully into gear.

Recent car changes:
Teaching daughter to drive a manual. there have been some rough clutch engagements lately.
I recently bottomed out on a speed hump
my front motor mount insert has deteriorated (haven't checked the rear mount, i had installed kirkasaurus mounts years ago)

Im hoping maybe i just need to adjust the slack on a cable somewhere.

Any thoughts?

This post has been edited by Stu37363: May 23, 2016 - 6:49 AM

[MScME]

So the hypothesis is that the drivetrain has dropped causing the gear linkage to pull towards neutral? Sounds like you would experience resistance at the end of the motion in the shifter when engaging 4th in that case?
Also, if the same phenomenon is present with 3rd gear it's contradictive of this as 3rd and 4th are in the same gate, that is if the gear linkage pulls towards neutral from 4th, it would pull into 3rd from neutral as well.

Synchronizers are self-locking when fully engaged and torque is applied due to the back-taper of the synchronizer teeth, so a well functioning gearbox will pull into gear upon acceleration, not out. Sounds like there might be extensive wear on the coupling disc and sleeve back-taper.
This might occur if the driver pulls the shifter out of gear without fully disengaging the clutch, grinding the two back-tapers and wearing them down, for example when pulling very rapid gearchanges during "enthusiastic" driving so to say. Or an inexperienced driver who has an off timing between clutch end gear..?
It will take abuse of this kind over a period of time though, the taper is small, typically 4°, but the teeth are hardened.

You could try parking your car facing up a small gradient with 4th gear engaged. When you have turned your Engine off, and come off the brake, your car will roll an inch or so backwards, this is the drivetrains play. Now if you've parked at too much of a gradient, gravity will pull your car further, turning the engine over, if this happens find a new hill.
While stationary, try pulling the shifter into neutral without depressing the clutch. You should experience a lot of resistance and probably not be able to pull it into neutral without force. If it comes out easily though (be prepared with the brake) the back-taper on the coupling cone and sleeve are worn and the synchronizers 3rd-4th will need replacing.

Is it just during acceleration, or during engine braking as well? If so, then the back-taper on the opposite side of the coupling teeth are worn as well, you can test this the same way but facing down instead of up the hill.
In any case, it's a matter of exchanging the synchronizers.