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G50 gearbox rebuild at 90,000 miles...?


Scumfrog

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It looks like I might have to get this done this winter. 

The symptoms are a slight notchiness when putting in gear: moving the gear lever, say 20mm there's then moderate resistance before it finally goes into gear. So you can't get a quick change out of it. 

Porsche mechanic says yes it's on its way out but could last several thousand miles like that. Possibly better to do it now before some expensive damage is done.

It looks like it only had one gear oil change in the service history before I got it; but I'm still surprised it would be bad after just 90k.

The (superior IMO) 915 gearbox in my other car is still good after 140,000+ miles. 

So the question is: how normal is this for a G50?

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My G50 is at 117,000 miles, when cold slight notchiness going into 1st, when warm all gears are easy to select. I had a gear oil change after my engine rebuild 4 years ago then changed the gear oil again last year. 

Might be sensible to change the gear oil before you go to the expense of a rebuild,  I would expect it to last for a while yet.

SS

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Sorry, I wasn't very clear in 1st post - I did change the gear oil when I bought it, not done a huge amount of miles since, so it is fairly fresh.  But in the 30 years before that, it had one change early in its life so that's probably led to the wear. 

The gears select easy enough, but I'm told the slight resistance half way in shows it's on its way out. 

Yes it'll last a good while, but it's a bit annoying, and could end up in a catastrophic failure leading to far higher expenses down the line.

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Oh I see. If the hesitation is only when going into 1st then I'd leave it alone, if its every gear then that would annoy me as well and I'd say that it is quite unusual for a G50 box to exhibit that behaviour at only 90,000 miles.

If you can live with it then fine, if you can't or it gets progressively worse then get it sorted.

You mention you have not done a lot of miles. Maybe it's worth driving it some to see if it improves?

SS

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915 'boxes are very sensitive to small gear linkage adjustments and are keen on good condition bushes. They benefit from aftermarket gear changers and shift linkages. I don't know about G50s but at only 90,000 miles, oil changes or no oil changes I'd be changing bushes and linkages before I even thought about reconditioning the gearbox. If it still proves to need reconditioning you've lost nothing as you will have a fresh gearbox made even better with new bushes and linkages. 

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Not a lot of miles for the oil, but I've done a few thousand. It ain't going to get any better.

Mostly noticeable on 3rd gear as that's the one that I want a quick change into mostly.

I'm not sure if it's affected by the linkage either, but the mechanic was pretty certain what it was and it's in the box itself. But one never really knows. I'll look into that.

Edited by Scumfrog
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Mileage alone is just one reference point, its how its been used during its life will also be a factor.

Steve Winter at Jaz say's G50 are normally bullet proof, so are you sure there is no wear in the clutch and all its ancillaries - fork, fork shaft, bearing, pressure plate, etc. 

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I would start with the simple low cost things first.

As suggested above, ensure the clutch slave cylinder is fully operational and operates the mechanism fully. Clutch slave cylinders are inexpensive. 
On another car, I once had a similar sort of problem - I changed the oil to a different brand (same viscosity) and the problem disappeared. 
I’m sure others will disagree with me, but I don’t think gearbox oil needs changing that often - it does not get exposed to all the contaminants engine oil does. When was the other oil change?

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When I got mine, at 110k (915 box), you couldn’t get first while moving, neither could the guy from the dealer that sold it me. I changed the oil in the box, that’s all.

Today with over 130k on the clock, in traffic getting out of Cornwall I snuck into first more times than I could count, not a crunch anywhere. I don’t get how machinery can ever make itself better, but it has, and that urgent gearbox rebuild is now something I never think about.

Edited by ChesterJLampwick
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+160k on my unopened G50, same clutch since 80k and 2 oil changes.

I'd be looking at linkages and clutch actuation before even thinking about a gearbox rebuild.  Also gearbox mounts.

Edited by Nige
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I don’t know about these modern new fangled hydraulic clutches on the G50’s, but the symptoms sound just like how my ‘78 VW camper behaves when the clutch cable adjuster wants a couple of turns. 
 

Weirdly I’ve never adjusted my 3.2’s clutch cable, I don’t even know where the adjuster is, but I’ll find it when I have to.

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Thanks for taking the time to comment and make suggestions. 

I took it out for a spin today for a reappraisal. 

What I can say is that the clutch works correctly. There is absolutely no drag when stationary in gear. There is the right amount of pedal movement before the clutch engages. The problem does not occur when stationary, engine on or off, it goes into gear fine. And I have plenty of experiences of clutches not working properly, it's nothing like that.

It's 100% not the oil. It was changed to try to overcome the problem. Possibly some slight improvement.

The issue occurs sometimes when shifting mainly into 3rd and sometimes 4th. I would say over 30 mph. 

So,  you're shifting into gear, it goes in part of the way, there's a bit of resistance, all normal so far. At this point it either goes into gear absolutely fine or will feel like a crash box. (Nowhere near as severe admittedly, and it rarely happens, but it can). Or it can be anywhere between those two extremes.

Change gear slowly, it doesn't happen. Likewise blipping the throttle seems to work, maybe.

To my mind this corresponds with the synchro not doing its job as well as a new one. 

Perhaps I'm being a bit too critical as it's not really that bad, and maybe a rebuild would be over the top at this point. But a fast change into 3rd can feel pretty rough at times. It never grinds, but you can feel that the gears haven't meshed correctly.

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One of my checks for full clutch disengagement…

Put the car on a smooth, flat, low resistance surface. Engine off, handbrake off, put the car into first gear, fully depress the clutch and start the car. If the car twitches the clutch has not fully disengaged.

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IMHO don’t underestimate the oil choice. here’s a post from Pelican - apparently there are only two oils approved by Porsche.

https://forums.pelicanparts.com/porsche-911-technical-forum/1008772-g-50-trans-fluid.html#post10194614

I put the Shell Spirax into my car and it made a noticeable improvement. No idea what was in there before or for how long but it was money well spent.

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Another oil change is certainly worth a try. However, I'm not sure what conclusions to draw from that thread, although Motul Gear 300 75w-90 seems to be highly regarded. (Or any other high viscosity 75 90.)

If the Shell made an improvement, what issues were you having previously?

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A quick Google of G50 problems reveals a few people complaining about the 3rd gear synchromesh issue as far back as the early 2000's.

They're also known for sudden unexpected failure, although no indication either is common.

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3 hours ago, Scumfrog said:

If the Shell made an improvement, what issues were you having previously?

Prefacing what I’m about to write with the fact that the box has 180K miles and has never been apart… I had the symptoms of worn/slow synchros requiring slow shifts while still getting some negative feedback through the lever about what I was asking of the gearbox. I too wondered if a rebuild was in my near future. The change in oil improved things considerably and I’m happy with the box now. I suspect my clutch doesn’t fully disengage (it doesn’t quite pass the test I described in an earlier post) but as I don’t have a standard clutch and flywheel and have an MPL slave cylinder I’m not sure where the issue lies.

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