4th gear grinds at high rpm
 

just changed my tranny fluid and now 4th gear grinds when shifting around 7-9k. im going out on the track soon and want this fixed! anyone know what it could be? i used the right fluid and it isnt low or too full... maybe synchro? idk imput plz

Rx8 trannies could have been built stronger. We have a little better then a Miata tranny. You could throw some heavy shock in there but I'm afraid once this happens its only going to get worse and tracking a car like this is going to be bad fast.

My 4th gear grinds period

You mention "right fluid" but exactly what fluid are you using?

^He's right. Live with the grinding and save for a new trans or get it rebuilt now before the costs go up. It's only going to get worse.
I had to replace my trans in Nov.

If I use any other trans fluid other than MT90 my fourth gear grinds, just like the OP has described.

MT90 fluid.

Well if it wasn't grinding with OEM and it is now grinding with MT90...switch back to OEM fluid.

I have had mt90 for the last year and it only started grinding for me in 4th gear for the last month. Obviously this started a month after the warranty expired.

I've had this twice: in fifth gear and second. Both times required a rebuild (under warranty). Both times the sub shop that Wilkins Mazda used screwed up the job and they ended up keeping the car twice as long as needed.

how long dose the warrenty for the trans last? i know the motor is 100k but what is the tranny?

it lasts as long as your car's warranty.
Once the gears start grinding it is time for a rebuild. You can mitigate the problem long enough to save money for the new transmission or the rebuild but an oil change won't solve the problem.
All i have found for our transmission (Aisin AZ6) are straight cut gears and or different ratio gearsets but nothing that addresses the whole synchro\shift forks problem.

What's a rebuild typically cost? Any chance to DIY? I'm a willing mechanic.

Op, before you go about looking into replacement of your tranny, stop and think logically; if you didn't have a single grind until you switched to MT90 it means your trany is not agreeing with your new oil selection. I do not agree with people here stating it means your transmission is nearing the end of its life. If you had the grind when you've used the same oil (if MT90) you have before and it just started grinding on its own then yes, it's likely the transmission.

My car did the same exact thing when I went from stock oil to MT90 this summer. MT90 made it grind only in 4th past 6k RPM and when I tried to remedy it thinking I need to go heavier to 75w-140 lightweight shock proof, it got worst. Grinding then was persistent past 6k rpm in both 3rd and 4th. I went with MTL (70w-80) and the problem went away. This was checked multiple times on the highway screaming it up to 8~9k shifting without a single grind. This told me it was NOT the transmission but was the oil that caused the problem. Unless there is some rare coincidental timing of your transmission wanting to crap on you when you switch to a different oil, if the new oil creates a grind get something else to work. I switched out the trans. oil thrice within a month until I found the right oil to work with it.

Try going to a lighter weight oil such as 75w-85 GL4 (not GL5) or MTL (70w-80) as I did, or perhaps even better yet, stock oil. 75w-85 GL4 wasn't a selection I recall being available during the summer when I was looking online all over for that grade before going lower to MTL.

Not a chance to DIY if you have to ask, sorry.

I'm sorry but I don't agree with your point of view on this matter while I find it to be right in general.
Couldn't it be that your synchro is worn and the stock oil "covers" the problem while the mt90 doesn't? :)
That's what I would think to be the case, not the oil's fault. If it was Royal purple's maxgear or some other proven transmission breaker i would agree though.
I wouldn't use the mt90, the 75w90ns is a better choice imho.
I have used litres of Redline oils as well as the stock ones and it simply makes no difference after a while. I'm waiting for my 3rd transmission to arrive :)

Transmission failure greatly depends on 2 factors in our case. Parts and Heat.
The parts are craptastic and they also can't stand the heat well. So if you just keep shifting it may or may not (although it likely will) fail sooner or later. If you track your car the extra heat will kill it. In this case it is not a matter or IF but rather a matter of WHEN :mad:
My 4th gear synchro on the current transmission only grinds at the track after a couple of hot laps, especially if I use semi slicks. Putting a transmission cooler with an electric pump slightly helped but i'm still going with a 2009 model transmission. Shooting a synchro around the gearbox while tracking is not fun, nor a chance i'd like to take :)
In this folk's case i'd just switch to an oil that minimizes the issue and start saving for a new or good used transmission.

Thank you!

this post. thanks man im actually going to get MTL (70w-80) later tonight and just bite the bullit and change it out and compare. the last thing i wasnt is a grinding trans at a track day with no trailer. :o

it didnt "fix" it but i cant feel the grind as much. its almost completly gone now 4th gear just kinda clicks at high rpm lol

ok so heres an odd update for ya. the above post was true for about 3 days then the grind came back even worse. i was looking around for a new tranny and everything. well this past saturday i had a track day that i pre registered for two months ago so i wasnt gonna let a grinding gear stop ma and i said fuck it. there was only one 3rd-4th gear change on this track and it was really rough for about 5-6 laps. then suddenly it stopped f=grinding and went into gear perfectly. now it dosent grind at all! so my advice is if you have this issue... go to the track? idk how that works anyone have any idea how this is possable? lol

its very likely that there were some metal shavings stuck inside the syncros not letting the gears to mesh properly. but I wonder why didnt the blocker pin block the gear out?

idk man, either way im happy because the whole tranny actually feels smoother and cleaner.

I had the same problem when I was using royal purple then I switched to Red Line and problem solved just my 2 cents.

You were just lucky, when a transmission starts grinding 80% of the time it's a mechanical problem, 20% a fluid one. You fall in the 0% category of the lucky assholes :)