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Slipping out of fourth

I have a Nissan Frontier, 2002, 4 cyl., manual 5 speed transmission. It seem to slip out of fourth gear, and only fourth gear, especially if I try to drive aggressively, or if I try to accelerate from fourth gear. It happens most when I am driving at lower speeds in fourth gear. Also, the colder the morning, the more likely it is to slip. IS this a transmission problem or a clutch problem (or something else), and what needs to be worked on or replaced- i.e. a whole system, or only a select couple of parts?

Thanks everyone

One more thing, if I feel the truck slipping out of fourth (especially while trying to accelerate a little more quickly), I just lift my foot off the gas a little, and then apply it more gently, and it seems to fully reengage the gear and drive normally.

Are you having issues engaging that particular gear it drops out of?

With the transmission in neutral and the engine idling, release the clutch and listen for a whirring sound that stops when the clutch is depressed. If you hear such a noise the input to output idler is worn… Check the oil level.

I am wondering if the input shaft is moving around radially or axially.

The input bearing could be worn allowing the direct gear dog teeth to walk out of the clutch sleeve. Usually a bad input bearing would make noise in all the other gears.

Another possiblity is that the pilot bearing in the crankshaft is allowing the nose of the input shaft to be off center causing the input shaft and dog teeth to wobble.

Of course you must have checked the bell housing bolts to make sure they are tight to the engine block. You have to think of any situation that would allow the crankshaft center line to be offset from the transmission centerline.

Hope this helps.

The input to output pilot bearing is loose needles and a low oil level seems to take that bearing out first.

Any one of a number of things could cause this and it’s probably internal to the transmission. Too much end play in the main shaft, worn shift fork, wear in the synchronizer hub/sleeve assembly, etc, etc.

Depending on your driving habits, if you spend a lot of time in 3rd and 4th gear and have a tendency to drive with your hand resting on the gearshift lever, then this could cause problems such as fork and/or synchronizer assembly wear.