hey guys, i just got home from a drive, and on the way i noticed,
as i roasted a 300zx, that when i shifted into fourth
it crunched slightly. perplexed i tried it numerous times on the way
and found it does it every time i try to change gears quickly.
but when i slowed down the gear change from 3rd to 4th it doesnt crunch
wen i say crunch i mean that it grinds... as if i had dropped the clutch too
early....
any ideas on what my problem could be ?
cheers fellaz

 

your syncro's are going probably

 

hmmms so maybe its the syncro's ? geez... would that mean a box rebuild ?
i think my car is a problem child.... cos i also hear knocking sounds coming from the
rear 1/4 when i drive slowly.... either something is loose.... or something is broken...

 

the sounds from the back is prob the suspension. The bushings always get bad and start making all kinds of sounds, thats not to big of a fix. The tranny thing on the other hand, you wouldnt have to rebuild it right away but its obvious that its going south and needs a rebuild eventually.

 

the symptoms you are describing are those of worn blocker rings...most likely the friction coating on them has worn off but before you jump into a rebuild I would reccomend you play around with some other oils. you can get friction modifiers which can sometimes make the blocker rings bite a little harder and thusly speed up the syncronization. also if your running redline this could be a big problem. redline is a very "slippery" oil which means it will cause less intearnal drag which is good for dog boxes but lessens the bite of syncro's.

 

DEfinitely sounds like damaged synchros and/or gear dogs. Gearbox rebuild. Or shift slower.

It's worth draining the oil and putting in some good synthetic like Redline MT-90. That can help mask some shifting problems, and what kind of metal flakes/shavings you find can point you toward the problem. But it may not fix them.

I've never tried modifying the friction characterstics of the oil, but I will say that GL-5 is more 'slippery' for the synchros than a straight GL-4. MT-90 and Amsoil AGT are the major straight GL-4s out there. You could also try something like using a thinner oil, such as pennzoil synchromesh or bg synchroshift. Or by mixing some in with your regular gear oil. However, thinner oils protect the gear teeth less, and can foam inside the gearbox and cause overheating problems. This kind of stuff won't work miracles. I personally wouldn't stray far from the stock recommendations.

Dave

 

Before you go buying bushings or anything you might want to check the compartment in your trunk where the scissor jack and stuff is. Sometimes that can be the culprit for the knocking if there is something loose back there.

 

yeh, i recently did a complete oil change in the car, so it cant be the jack etc. cos i have removed and replaced them back into the compartment securely....
hmmmz looks likes new bushings and tranny oil for me.