Guys, can anyone share your expertise on what may be causing the following issue.

997Gt2 stock gearbox with Sachs 764, 890NM clutch, GT3 4.0 RS disc and LWFW, engine, lots of torque, more than standard amount of heat generated, current clutch has done 7000miles.

So after an hour or so of hard driving, lots of full throttle up to 7000rpm in second and third gear everything piping hot the gears start to get a bit sticky. Sitting stationary with the hot engine idling trying to get into reverse is VERY difficult, takes a few de clutching and re clutching and some force to get it in. First gear also doesn't want to go in only slips in after a bit of force.

Turn hot engine OFF and then try the gears, absolutely perfect, snick through all the ratios beautifully including reverse. Start engine up again, exact same, reverse, first, and even second sometimes, extremely sticky and don't want to engage.

Ideas welcome

 

This sounds like linkage binding or sticky and not a transmission issue, since you did not mention grinding.

Are your engine mounts OK or move causing linkage/cable bind? Or something may have shifted or linkage/cable issue?

 

No grinding. If it was linkage binding or engine mounts, or linkage cables then surely it would still bind when the engine is off ?

The fact it only does it when everything is HOT has to be a clue but of what ?

Thanks for the response

 

Which gearbox fluid are you using, how much, and how long has it been in there?

 

I think cables would still bind when hot. They're mechanical cables. They'd still be hot.

 

Clutch...

 

But wouldn't they still bind when the engine is switched off, they won't have cooled down at all ? goes from sticky when engine on to perfect when engine off - still hot....

 

I'm pretty sure my shop changed it to a better fluid fairly recently in an attempt to help, didn't make any difference.

 

but which parts are causing this, any idea ?

 

I'd try bleeding the clutch to start, perhaps replacing the slave cylinder too

 

I have had the isolator fail in the engine mount causing misalignment and very tough shifts. I have also switched to solid mounts and significantly improved shift quality (linkage part) and that's why I thought it maybe the cause.
Please expand as I would like to learn something here as well. If clutch was bad, you would grind going into first or reverse.

 

OK, will do, is there any way the heat would cause the slave cylinder to malfunction, only when hot ?

 

Been there. Bleed it... I’m betting if you shut the engine off it shifts fine. When it’s cold it shifts fine. When hot, no bueno...

 

Sorry, I misread the OP.

 

not necessarily a slave malfunction but there could potentially be some stiction happening internally from localized heat, also a chance a slightly bad seal pulling in air over time, may not leak when seeing positive pressure but could be on the edge of weaping/pulling air...but I'll bet it's a fluid issue