So I already made an appointment with the dealer to see the car in 2 days. But I still want to see what you guys think the problem might be. Perhaps someone had the same problem in the past and wants to share his experience.

Anyway, here is the info:
Car is a 2009 G37S 6speed manual at 40k miles. It is still under extended warranty purchased from the dealer on day 1.

The problem appears when I switch from 4th to 5th and when I downshift from 6th to 5th. It will grind the gear change if I do it quick. Like in 1 sec or less, which is not that quick in my book. If i give it time, like a couple of secs, the gear change will happen normally.

It started happening 1-2 weeks ago and even though I drive manuals my whole life(I'm fast approaching 50) I initially thought i messed up somehow.
But it wasn't me.

I can replicate it at will:
Get the car up to speed(anything over 40 really) where it's time to switch from 4th to 5th.
Depress clutch all the way to the floor and keep it there.
Go to neutral.
Then get it to 5th.
If you flick it or do it with any sense of urgency, it will grind. (clutch still on the floor)
If you casually, sort of push it very lazy like to 5th, the gear change will happen fine.

All other gear changes happen fine no mater what I do. It's just when i need to get it to 5th that I have to be mindful now.

Any thoughts will be appreciated.

Stratos.-

 

Symptoms suggest worn synchro's on the gears that are grinding. You'll likely be getting a replacement trans.

 

What are manual transmission synchronizers? | HowStuffWorks

Pretty sure it's less economical to rebuild our transmission than it is to replace it. That approach may have more to do with contemporary auto service rates than it does with the actual work involved... IDK. It's just an opinion.

Anyway, here's what I think about your problem: it's just 5th and 6th gear. These are cruising gears, and don't affect the majority of your daily driving. So baby those gears and live with it, even though I'm sure it's annoying. It would annoy the heck out of me, too.

Dammit, jsolo! Let me finish typing already!

 

^^If there's a warranty, why not take advantage of it?

Cute

 

I skipped right over where he said he had an extended warranty. Figured being an '09 he's on his own. My bad. Absolutely, get that new transmission while the getting's good.

I wonder if a new tranny means a new CSC? That would be nice.

 

Should be a new csc. At least according to the fsm that's required to be changed if the trans is dropped.

Probably not needed but also a good time to get the clutch replaced as the overlap in labor will be extremely minimal.

 

At 40k I'd say keeping the original clutch is OK. If funds aren't an issue, then Yea but aside from saving on the labor, I don't see a point in replacing the clutch unless it's an upgrade or free within the warranty which I doubt. The CSC is a whole different story, I'd definitely get that even if it's not part of the warranty. I'd also see if they'd be willing to install an upgraded csc. Take it from someone who had the csc go bad out of warranty, not fun at all.
As far the grind, it's the syncros for sure. Good thing you're covered.

 

That's a ongoing issue with these transmissions, I've had the 5th gear synchros grind on my G35 when it was about 80-90k miles and on my g37 it started around 50-60k. Bad design if you ask me.

 

similar issue to my G35 gearbox. it was replaced due to worn synchros and it also came with a fresh clutch friction disc. the G37 gearbox should come with a new CSC at least. i had the CSC replaced under warranty on the G37, and did the MC a the same time. the dealer flushed and refilled hydraulic fluid with 'GTR Spec' fluid.

i would advise the OP to do the MC at the same time. not that much extra and it is wise to do. the FSM strongly recommends doing the MC whenever the CSC is replaced, so try to get the dealer to do both under warranty. have a look at the pressure plate and friction disc, if the disc looks worn then i agree with Jsolo and throw on a fresh disc while they have the gearbox out unless the new gearbox comes with a new disc at which point you're all set. this will leave you with essentially a brand new gearbox assembly, aside from the FW and PP themselves.

if this wasn't under warranty i'd say that Rochester's advice is on point - you could baby these gears and not affect overall driveability too much.

 

First, I'd like to thank you guys for all the great info. It helped me greatly.

Second, I just came back from the appointment with the dealer to look at the car.

New transmission under warranty is the verdict.

He needs to ask for authorization to do the repair from infiniti and then order the "parts". He will then call me to schedule the repair. I didn't ask what they are replacing exactly because he gave me the impression that was kinda early to tell? I'll ask him when he calls back and I'll post an update.

Some other thoughts:
Boy did I lucked out with agreeing to buy the extended warranty. It was a bit over 1k if I remember correctly and it was a no brainer back then since I knew I will be keeping the car for awhile. Today they offered to extend it even more but we didn't talk specifics. Depending on the numbers I will probably do it. The car has been great to me, still looks immaculate in and out and I still look forward to get in and drive it.
It gives me pause though that the transmission needs to be replaced at 40k miles. If I didn't have the extended warranty I'd be more than upset right about now.

 

Did you purchase your vehicle new? Factory extended warranty is usually a good choice; as you have found they usually pay for themselves.

If you purchased it used, perhaps the previous owner put some abuse on the transmission? The manual transmissions are pretty stout but I've experienced that the synchros will eventually give out with repeated diagonal shift transitions between 4th and 5th.

 

^Car was new with 12 miles on the odometer when I bought it

Update:
The dealer called and said that he got the authorization and he is ordering the parts. When I asked him what are 'we' ordering exactly his answer was to the tune of "..we don't repair manuals that often so we don't know until we pull the transmission off". In other words he didn't know. We'll see where that leads us.
He also offered me options for extending the warranty.
3 more years for 1.6k sounds appealing at the moment.
I have time to decide until everything is done.

 

^ great news for you. nice to hear of a member coming out on the + side of an extended warranty. interested to hear exactly which parts get re+re'd.

from the sounds of it, this repair is not a must-have as the vehicle is still operational in all forms except for quick shifts which is no big deal. by the time you're shifting into 5th speed-shifting is not a priority. but if the dealer is on board for a fresh gearbox that's great!

 

If for whatever reason they say they're going to re-use the Clutch Slave Cylinder (CSC), push back on that hard. The CSC is a common point of failure in these cars.

 

Have you driven a manual with bad syncros? It grinds on normal shifts not even speed shifting. The only way it doesn't grind is if you push on the gate until the rpm is just right for it to click in; pretty similar to changing gears without pressing the clutch pedal down.

Agree that the dealer will probably just get a whole new transmission unit (which has CSC attached already). Might be a good time to opt for a STS though!