Infrequently sticking clutch in 08 VW GTI

Question

I have owned my 2008 GTI MKV FSI for the past 4 years. For the first ~8k miles (bought at 42k), everything behaved as expected and performed nicely. At a certain point around 50k, I noticed that the clutch pedals bite point would sometimes change dramatically. This would usually happen after engine braking down a long hill. After awhile, the clutch pedal would occasionally bite much sooner than I was used to, and raise to about half the clutch depth. Putting a toe under the clutch could make it pop back up, which point it tends to behave normally for another random interval before repeating the problem.

I noticed that this was more frequent after spending any time at high RPMs. Also I found that by staying lower in the RPM band, the clutch pedal engage point would raise itself nearly up to the top of the pedal. At first I thought the only thing that might vary between low and high RPM driving would be the use of the dual-mass flywheel. But there's more to this adventure! I found that some days, my brake pedal would remain slightly squishy even when fully depressed, and that by pumping the brake pedal (wait for it...) the clutch engagement point would drop!

So far I've spent more than a little bit of money trying to find the problem, and here's what I've done/found:

• One-way restrictor in bleed valve removed. The clutch has more 'feeling' now but didn't affect the frequency of encountering this issue.
• Clutch master cylinder replacement (and clutch lines bled), no effect.
• Brake lines bled and refilled (did not affect 'squishy' feeling)
• At some point, I had a bad motor mount that broke my intake free. After replacing the motor mount and putting in a trans mount insert, no improvement.
• I've had the mechanic look at the pedal assembly and the position/fill state of the shared hydraulic reservoir, nothing appears out of the ordinary.

The one thing I've avoided so far has been slave cylinder replacement, because it costs so much to access the slave cylinder in these cars (inside bell housing) that if you take it apart to that point to find the problem, you might as well replace the clutch while you're at it. Now you're looking at at least $1300 with no guarantee of fixing the issue.

Here are my list of plausible hypothesis. I would love to hear anyone else's input on any one of these, or things I could try to demonstrate it is one particular problem instead of another:

• Dual mass flywheel failure leading to the clutch backing-out, occasionally, reseating itself after high RPMs. Don't know how to test this besides dropping the bell housing.
• Cracked pedal assembly affecting each-others master cylinders.
• Bad slave cylinder that sometimes catches when releasing the clutch, or whose auto-adjustment operation is messed up.
• Output shaft end-play. More than one post on VW forums from people showing this same problem ended up reporting end-play that exceeded tolerance, leading to full engine/transmission replacement (under warrantee for them, I'm not so lucky).
• Token "there's nothing wrong with the car, it's the driver" answer.

The car is at about 80k miles now, and I've spent a bit more money on it than it probably deserves. Should I quit now and trade it for something else? I love the way it handles, but I just can't get over the annoying clutch behavior.

Answer 1

The symptoms you are describing exactly match what I experienced with clutch slave cylinder failure on a car. Even a tiny leak causes issues, but in general the clutch still functions as needed for day to day driving. In my case the vehicle used an external slave cylinder, so diagnosis was easy. We could manually actuate the lever the slave cylinder used in order to verify it was the problem.

I was hoping I could locate some images of the bellhousing of the transmission. But apparently your car comes with dozens of different engine and transmission combinations. But the "easy" way to diagnose this is to wait until the leak gets so bad it just spews fluid everywhere inside the bellhousing. Many cars have an inspection port, so it is usually pretty easy to open that up and see clutch fluid getting flung everywhere. I don't actually recommend this as you'll wind up stranded and with a damaged clutch.

I don't understand what you mean by "Output shaft end-play". An output shaft would be on the other side of the transmission, not the side with the clutch.

That being said, here is what can cause the symptoms you are describing

1. Slave cylinder failure, with a slight leak
2. Crankwalk, when the crankshaft moves in the cylinder block along its axis
3. Failure of the flywheel, extremely common in dual mass flywheels
4. Failure of the pressure plate, less likely but possible
5. Cracks in the bellhousing

The last item, a cracked bellhousing would take a competent mechanic less than 5 minutes to diagnosis on most vehicles with a lift. It is also extremely unlikely unless you've struck a hard object at high speed.

The remaining items all require the removal of the transmission for proper diagnosis. If it is crankwalk, this is almost certainly the end of the line for the vehicle. Crankwalk usually requires entire engine replacement to fix.

Is the repair worth it? Well you must consider if you like the car, if you need the car, and if you own the car. This question is more properly answered by the personal finance stackoverflow site in my opinion. I have no way of knowing what that car is worth where you live. For all I know, there are 10 brand new cars just like this leftover from 2008 sitting at a nearby car dealer lot with deep discounts available.

That being said, if you decide to repair it then I recommend the following be done

1. Install a "single mass" flywheel, also called a flywheel. Dual mass flywheels are inherently a failure item. A regular flywheel can't fail under any normal circumstance. I am assuming this car is driven on the road and not used for closed course competition.
2. Install a brand new clutch and pressure plate.
3. Have any worn or damaged transmission mounts replaced at the same time.
4. Install a new throwout bearing.
5. Install a brand new OEM clutch slave cylinder, not a rebuilt one.
6. If the clutch slave uses a flexible hose (many do), make sure that is a brand new OEM unit as well.