2.0 Tdi concentric clutch slave cylinder problem at high revs.

[J.R.]

My Yeti was remapped today, it goes like stink now but unlike the PD engine remap where there was a big rush of mid range torque this is progressive and the engine continues to build up power at higher revs whereas it would have run out of steam before.

 

So on the way back I was testing it as you do, after a couple of 3rd & 4th gear high rev acceleration runs the clutch pedal became soft, no resistance on the first half of travel & disengaging close to the floor.

 

When I got back I removed the plastic bleed block from the concentric slave cylinder, I drilled out the restrictor in the poppet valve thinking that the problem may have been the master cylinder pushrod not returning as quick as I was bringing up the clutch (it had caused slip problems on quick getaways before), I pressure bled the system and the pedal returned to normal.

 

I did a 15 mile test drive at normal speeds and everything was fine, then on an empty stretch of road I repeated the 3rd & 4th gear runs holding onto the revs until the power fell off before shifting at normal speeds, the pedal once again got some free play after the first run, one more run and it was once again half way to the floor, driving normally home it got no worse and a couple of times it appeared to have become firmer.

 

I have left it overnight with the pedal wedged down to see if it backbleeds, I will remove the undertray to see if there are any signs of fluid leakage.

 

Is this a known problem, has anyone else experienced it? Is the slave cylinder leaking or does the vibration of high revs somehow allow air into the system?

 

I did notice that the design of the bleed block means that it can wobble around and perhaps allow the O ring seals (which I didn't replace) to leak.

 

Being a 4 wheel drive its a big work up to remove the gearbox & if it is not going to fail big time but simply become aerated on high speed runs then its easy to avoid them or to bleed the system afterwards, I would naturally prefer to effect a permanent repair.

 

Thanks in advance!

[J.R.]

Well I am closer to understanding what happens but in doing so have rendered the car undriveable for a while.

 

You dont need to use the clutch for the problem to occur, just driving at 4k revs for any length of time will create an effect exactly like brake pad knock off with warped discs, in my case either resonance, vibration or perhaps crank end float (to be checked) at higher revs is pushing fluid back to the master cylinder reservoir, it could be that my modification to drill out the bleeder block which some call the "clutch peak torque limiter" has made the situation worse.

 

The actual fault however is within the master cylinder which is either sticking or has a broken internal spring, if you lift the clutch pedal up manually the piston does not return so that it cannot draw another slug of fluid.

 

If the master cylinder was functioning properly then at worst the pedal would be low on the first application but would get higher with a couple of pumps just like after fitting new brake pads, at one stage I was able to pump the pedal and get it back to how it should be, then it degenerated to me having to lift it with my foot finally I cant get a pedal at all, I could probably get it working again with a pressure bleed but it will have to stay off the road till I can replace the master cylinder and then perhaps the bleed block.

 

Hopefully TPS can have me sorted tomorrow morning otherwise its E-bay and a long wait, deliveries have been really slow recently.

[J.R.]

I have discovered what causes the clutch master cylinders to fail and why mine went after the increase in power.

 

The cylinder body is plastic and it is located less than 10cm away from the DPF which runs hotter than 650°c during active or passive regeneration, there is a tiny bit of aluminised cloth between them as a heatshield.

 

Nice job VAG

 

It looks like an absolute pig to remove, I think the DPF will have to come off.

 

I bet the failure rate on them is much higher on the more powerfull models, more power = more heat generated in the exhaust.

[J.R.]

Just collected the new one from TPS, am half way through the removal of the old one, its a challenge to say the least but I am so glad that it happened while back in the UK with my tools & workshop and TPS on my doorstep.

 

4 times the price of those on Ebay but I have it now and not towards the end of next week, its a later revision from all those on Ebay so they may have made some changes, I did a much higher pressure bleed through this morning and flushed through loads of rubber debris so I think the seal has simply given up the ghost, playing with the new one its much simpler than a normal master cylinder and simply has one seal on the piston, no springs, no check valves, I think mine is just lacking any compression.

[J.R.]

The bad news is after doing the pig of a job fitting the new slave cylinder in the tiny space behind the DPF that you can get one hand into but not out again I did not realise that the pipe seal had either fallen out while fitting or I had fitted it the wrong way round, the union was leaking but not in view and the fluid was going behind the bulkhead insulation.

 

I spent most of the day trying to pressure bleed the system wondering where all the air was coming from and where all the fluid was going to. Much later I saw the lake forming on the block paving under the vehicle

 

The good news is that an autopsy showed that the old cylinder seal had broken up into tiny hard & brittle pieces, it would not have lasted much longer and the extra heat had put it out of its misery, I'm glad it has happened now and not later in France, I am confident that I wont have to remove the gearbox to do the concentric bearing which for a while it was looking like I may have to (I pressure tested it earlier). I will add some extra thermal insulation around the cylinder to hopefully prolong its life but will know the early symptoms now if it does fail again.

 

Snow is forecast for tomorrow so its going to be hors service for some time yet.

[J.R.]

Job finally done, having learned the hard way and had to take it all apart and start again today in a snowstorm please dont make the mistakes that I did!

 

A new cylinder is cheap, £26 on Ebay or £112 retail from VAG (minus 8% TPS discount) which has the latest revision number so maybe they have improved the seals? _ who knows, its a pig of a job initially (but I could do another one tomorrow with ease) but preferable to being stranded with a clutch that will not disengage.

 

When working on it you can only just squeeze one hand down behind the DPF and have to work blind by feel, as your forearm gets scraped by the heatshield fixing studs it and the hand swells up and you cannot get it out again .

 

The pipe will only come out of the cylinder after a big fight, the pedal assembly needs to be unbolted from inside after removing the knee airbag, airbag support bracket, heater duct and lower dash panel, it then need to be tilted upwards and held in that position while you forcibly spring the pipe to remove it, with the fill hose and the sensor wiring removed the assembly can be withdrawn from inside the cabin.

 

The old seal will have remained in the cylinder, logic says that you fit the new one into the new cylinder in the same way do not do this, it will leak, the new seal needs to be pushed onto the pipe but even though its a tight fit it is not in the correct position and will leak, it needs to be pushed with a lot of force and for this you need to get a second arm & hand down through the gap only big enough for one so that you can hold the pipe and push the seal until it snaps into place.

 

You will have as much trouble gettin the pipe back into the new cylinder, it needs to be angled again and with a lot of force you can get it started, then the pedal box needs to be bolted up and the pipe & union pushed home & the retaining clip reinserted.

 

At this stage I advise you to go straight to the clutch bleeding because if the seal is leaking it will all need to come apart again so you dont want to struggle with the other connections before being sure.

 

I fitted extra thermal insulation around the master cylinder, the DPF already has a non asbestos type matting under a metal shield and there is a seperate reflective foil cloth type barrier between it and the master cylinder but there is just not the space needed.

[SuperbTWM]

Good work @J.R. , you did a good job of diagnosing that one, I bet many garages would have fitted a new clutch kit and slave ylinder first before looking at the Master.

[J.R.]

I have read loads of reports on various forums thanks to google, all of them were people who relied on their dealer, they all had pretty much the same story, not sure what it was as the dealer ended up replacing the bleed block, master cylinder and then the slave cylinder often insisting on replacing the whole clutch & dual mass flywheel as well.

 

The cynic in me sees a different scenario given that the bleed block and the slave cylinder can be hydraulicly or pneumatically tested via the bleed nipple with the flexible supply hose clamped off (thats exactly what I did), also you cannot visually verify that the slave cylinder has been replaced. Also many had initially charged for bleeding the system and said that if the pedal dropped again it would be a gearbox out job to replace the slave, this is absolutely not true because it would point to the master cylinder being faulty.

 

I reckon they started with the easy job of the bleed block, then when the problem was not resolved decided correctly to replace the master cylinder, they got their hands literally and physically burned by the job being much harder and taking far longer than they planned, they probably made the mistake with the seal like I did (and like the VAG dealer & independant garage did with my MK1 Octavia) and so to recover their loss the customer was invoiced for the long and expensive job of the slave cylinder which was never done.

 

Failure of the slave cylinder can only be through a leaking seal which would result in fluid leaking from the bellhousing and also the reservoir level falling and the low fluid warning light being activated.

 

For information the take off from the shared brake & clutch reservoir for the clutch system is at 3/4 height and corresponds to the level at which the warning light comes on, it is impossible for a clutch hydraulic failure to affect the braking system, conversely a leak in the brake system will manifest itself first (if the driver does not feel the imbalance) by clutch failure.

Edited 8 February, 2021 by J.R.

[J.R.]

10 days of testing, bleeding, going back to a standard unmodified bleed block or max torque limiter to use its other name and also reading just about every Yeti clutch pedal thread going with the benefit of hindsight I can conclude that the problem is 100% one of the heat generated by the DPF when giving the engine some beans.

 

Its much better with the new cylinder and some glasswool insulation between the cylinder and the refractory shield but its still apparent, if I do a wide open throttle run through 3rd & 4th gears changing up at 4.5k between roundabouts on a dual carriageway I can feel the pedal sink slightly, if I come back the other way and do the same immediately then there will be maybe 1/4 of the pedal stroke as lost travel, it will pump up and driving normally for a couple of minutes its back to normal again.

 

I have not mastered the live data logging in VCDS but had the number 1 exhaust temp sensor value showing on the computer screen while I was driving, I could only get the occasional glimpse when safe to do so but I saw 700°c on one short acceleration run and am sure that it goes much higher when I get the pedal problems.

 

It has definitely been worsened by the extra heat generated if I use the extra power from the remap but the problem has always been there (probably from al the regens) as witnessed by the decomposition of the original master cylinder seal and the black sediment in the fluid when bleeding which others have reported, its a design problem and if you look at the premature failures it tends to be the 170hp variants, if anyone has one perhaps they could advise if there is any additional heat shielding, a photo would help.

 

Tomorrow I am going to try to see if any part of the metal fluid line is exposed to heat from the turbo or downpipe and see if I can do anything about it, it will probably be a case of working by feel again.

[Breezy_Pete]

A radiator fan over-ride switch might be a workaround, so you could pre-empt engine bay heat rise with an increase in airflow if you knew you were about to get heavy with the right foot. 

LHD versions presumably don't suffer from this problem? 

[J.R.]

I dont think that at speed the radiator fan will have much effect on the exhaust components sandwiched behind the rear of the engine block & the bulkhead, especially after blowing through the intercooler, aircon condensor and engine radiator, at a standstill while a regen is happening I can see the validity.

 

Good point regarding LHD/RHD, I dont think the fluid is boiling but perhaps expanding and pushing back to the reservoir, the brake master cylinder is metal, exposed and not affected, the clutch master is different to any other as it lacks the second internal seal and spring, when the pedal is at the top of the stroke there is no restriction to fluid being returned to the reservoir.

[J.R.]

After another bleeding session and a good look at the pipe runs etc I reluctantly conclude that it is not a heat soak problem, it occurs too quickly, I'm sure I could do some more tests to prove it but each time I end up bleeding the thing again although no air comes out.

 

Reluctantly conclude because that means the problem has to be a mechanical one acting on the concentric slave cylinder which has been both vacuum & pressure tested & does not leak, a mechanical problem points scarily to the possibility of a Sachs pressure plate

Edited 18 February, 2021 by J.R.