Ideas? Sticking clutch pedal only when braking heavily

[Oldcarguy]

I know a sticking (on return) clutch pedal can be a symptom for slave going bad, air in lines, clutch bearing, etc. but I’ve got a slightly different manifestation.

Under normal and spirited (heel and toe) driving I have no clutch pedal issues. Smooth engagement, proper pedal travel, etc. Clutch has ~4,000 miles on it since rebuild by PO’s independent shop.

However, when braking extremely hard with the clutch disengaged simultaneously (classic ‘panic stop’) the clutch pedal returns halfway and sticks. If I depress it again, it returns normally. I discovered this because I occasionally engage the abs on purpose to exercise it. But sometimes the pedal will also stick under very heavy braking without getting into the abs. Brake and clutch pedals are firm.

I’m going to purge the brake/clutch fluid (with Motul 600) and see where I’m at. If nothing changes regarding the sticking pedal, looks like I may be headed for a clutch slave cylinder replacement next. I need this fixed before I put the car on the track for HPDE events.

Any thoughts?

[Gilles]

Why don't you try to cleaning (and lube) the shaft where the pedal hangs from and the return spring as well?

[Oldcarguy]

Originally Posted by Gilles

Why don't you try to cleaning the shaft where the pedal hangs from and the return spring as well?

Will do, it’s creaking a little. I’m also going to see what the status of my helper / return spring is. It’s still there but the pedal is noticeably more firm than my friend’s 986. The spring may be broken.

[Blackroot]

Originally Posted by Oldcarguy

I know a sticking (on return) clutch pedal can be a symptom for slave going bad, air in lines, clutch bearing, etc. but I’ve got a slightly different manifestation.

Under normal and spirited (heel and toe) driving I have no clutch pedal issues. Smooth engagement, proper pedal travel, etc. Clutch has ~4,000 miles on it since rebuild by PO’s independent shop.

However, when braking extremely hard with the clutch disengaged simultaneously (classic ‘panic stop’) the clutch pedal returns halfway and sticks. If I depress it again, it returns normally. I discovered this because I occasionally engage the abs on purpose to exercise it. But sometimes the pedal will also stick under very heavy braking without getting into the abs. Brake and clutch pedals are firm.

I’m going to purge the brake/clutch fluid (with Motul 600) and see where I’m at. If nothing changes regarding the sticking pedal, looks like I may be headed for a clutch slave cylinder replacement next. I need this fixed before I put the car on the track for HPDE events.

Any thoughts?

The brake fluid brand/type would be interesting if that changes.
DOT 4+ is DOT4 +
The volume isn't going to change in the master or slave with new fluid.
Might want to see if the slave cylinder hard line (12mm flare wrench used only) is tight. The slave bleeder is 11mm.
Could be many things...
1. pressure plate springs are weak or failing.
2. slave cylinder is not quite right with air bubbles.
3. Bad throw out bearing or in pieces
4. Master cylinder return spring (mentioned)

Also, if you think about what you just said "when braking extremely hard with the clutch disengaged simultaneously (classic ‘panic stop’) the clutch pedal returns halfway and sticks", remember these are shared fluid systems, so the caliper might not be returning enough fluid to pull up the clutch.
Seems to point to air in the system

[Oldcarguy]

Originally Posted by Blackroot

The brake fluid brand/type would be interesting if that changes.
DOT 4+ is DOT4 +
The volume isn't going to change in the master or slave with new fluid.
Might want to see if the hard line (12mm flare wrench used only) is tight. The bleeder is 11mm.
Could be many things...
1. pressure plate springs are weak or failing.
2. slave cylinder is not quite right with air bubbles.
3. Bad throw out bearing or in pieces
4. Master cylinder return spring (mentioned)

Thanks BR. Hopefully it’s air bubbles that the fluid change will take care of, but one or more of the points you mention are definitely in the running.

The thing that gets me is that it only does it in conjunction with extreme braking.

According to the paperwork I received from the PO, the clutch only has ~4,000 miles on it. But, I do have a heavy clutch pedal even though the helper spring *appears* to be in good shape & working properly, the engagement point is good, no chattering, no leakage, no gear grinding etc. If a check of the helper spring and the fluid purge with my Motive doesn’t fix the issue I may kick it over to my indie. Too many other irons in the fire and no time to pull the transaxle and redo the clutch before first HPDE in April.

Frankly I am leaning toward a hinky throw out bearing or pressure plate but I don’t get why it only happens in conjunction with extreme braking. Front motor mount is relatively new (987), but maybe a transaxle mount?? Clutch is hydraulic and the transaxle/clutch assembly is a pretty solid unit so I just don’t see a possibility there.

[Blackroot]

Originally Posted by Oldcarguy

Thanks BR. Hopefully it’s air bubbles that the fluid change will take care of, but one or more of the points you mention are definitely in the running.

The thing that gets me is that it only does it in conjunction with extreme braking.

According to the paperwork I received from the PO, the clutch only has ~4,000 miles on it. But, I do have a heavy clutch pedal even though the helper spring *appears* to be in good shape & working properly, the engagement point is good, no chattering, no leakage, no gear grinding etc. If a check of the helper spring and the fluid purge with my Motive doesn’t fix the issue I may kick it over to my indie. Too many other irons in the fire and no time to pull the transaxle and redo the clutch before first HPDE in April.

Frankly I am leaning toward a hinky throw out bearing or pressure plate but I don’t get why it only happens in conjunction with extreme braking. Front motor mount is relatively new (987), but maybe a transaxle mount?? Clutch is hydraulic and the transaxle/clutch assembly is a pretty solid unit so I just don’t see a possibility there.

Yup and as I said on the bottom here-
"Also, if you think about what you just said "when braking extremely hard with the clutch disengaged simultaneously (classic ‘panic stop’) the clutch pedal returns halfway and sticks", remember these are shared fluid systems, so the caliper might not be returning enough fluid to pull up the clutch.
Seems to point to air in the system"
If I was to bet, I would say air

[Oldcarguy]

Originally Posted by Blackroot

Yup and as I said on the bottom here-
"Also, if you think about what you just said "when braking extremely hard with the clutch disengaged simultaneously (classic ‘panic stop’) the clutch pedal returns halfway and sticks", remember these are shared fluid systems, so the caliper might not be returning enough fluid to pull up the clutch.
Seems to point to air in the system"
If I was to bet, I would say air

I’m on my iPad and I thought I read your whole post, but I missed your very pertinent point at the bottom. Those 4 pot calipers are holding a lot of fluid under extreme braking. Also, I have no records indicating that the fluid has been changed in the last few years so there may be who knows what in the system.

I’ll try to change the fluid and purge the brake and clutch systems (also ensuring to engage PSAM and ABS) next week. I’ll update as soon as I have the result.

[Blackroot]

Originally Posted by Oldcarguy

I’m on my iPad and I thought I read your whole post, but I missed your very pertinent point at the bottom. Those 4 pot calipers are holding a lot of fluid under extreme braking. Also, I have no records indicating that the fluid has been changed in the last few years so there may be who knows what in the system.

I’ll try to change the fluid and purge the brake and clutch systems next week. I’ll update as soon as I have the result.

I just used one of these with fantastic results. I did it dry @ 22PSI
https://www.amazon.com/Motive-Products-European-Bleeder-Pressure/dp/B0002KM5L0

Be quick on the bleeder or you will drain the brake fluid tank.
I'm a weekend warrior, like you

[Geof3]

Originally Posted by Oldcarguy

I’m on my iPad and I thought I read your whole post, but I missed your very pertinent point at the bottom. Those 4 pot calipers are holding a lot of fluid under extreme braking. Also, I have no records indicating that the fluid has been changed in the last few years so there may be who knows what in the system.

I’ll try to change the fluid and purge the brake and clutch systems (also ensuring to engage PSAM and ABS) next week. I’ll update as soon as I have the result.

HPDE events in my region require brake fluid replacement annually. Still seems like an odd symptom overall. Don’t forget to bleed your clutch slave as well...

[JayG]

Unless I am mistaken ( I certainly could be) you must bleed the clutch with the pedal down
see: Clutch Bleeding With a Power Bleeder

[Blackroot]

Originally Posted by JayG

Unless I am mistaken ( I certainly could be) you must bleed the clutch with the pedal down
see: Clutch Bleeding With a Power Bleeder

You are not mistaken, but I tried the same thing 2x.
Both times failed miserably and I lost 16$ worth of brake fluid.
I found success with pedal up.
I have driven the car 400 miles since all the work and it shifts like it just came out of the factory.
Butter.
Again, pedal up worked for me, and doesn't mean the best way.
Just to be clear, I have a 2004 S special, and I had a new slave cylinder that I installed on the gear box before it went back in.
Also, be sure to drain and re-fill your gearbox while you have it out. 75W Mobile 1

BTW, there is nothing more rewarding than driving your car after all that hard work.
Faster/better or not, it was worth every minute of work.
In fact, next time I can do it in half the time from this experience.

[Oldcarguy]

UPDATE: I believe I have found the problem. My clutch power spring was broken and wound back into itself. Under certain conditions the broken part of the spring would bind on the inner guide of the spring cover, sticking the pedal about half way through its travel. Just looking at the spring action when depressing the clutch was deceiving - the spring appeared to be intact and expanding/compressing correctly because the longer broken piece was long enough to keep tension on the pedal in both the released and fully depressed positions.





I’ll be rebuilding the power spring assembly using the lighter McMaster-Carr spring as specified in the long Rennlist thread which discusses the 986/996 power spring in great detail ( //********.com/forums/996-forum/343297-improved-clutch-action-dramatically )

One quick point regarding removal of the power spring - I did not have to remove the clevis pin from the power spring assembly to remove the power spring assembly from the car. The spring assembly simply sets/snaps in the clutch pedal receiver with the pin in place.

Be careful if you elect to work with the power spring assembly - the spring is under ~90lbs of compression pressure. You can seriously injure yourself and/or your car!


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[Anker]

A lot of Boxsters are driving around with broken clutch assist springs. They cause several perplexing problems. One is that the cruise control becomes unreliable, or simply doesn't work because the clutch lockout switch never gets depressed enough. Another is the one you describe, and finally heavy clutch action.

[Oldcarguy]

McMaster-Carr spring arrived and is installed. ~1/2” shorter, smaller diameter, lighter spring rate. Haven’t had the chance to do anything other than dry test it, but I already know I’m going to like it. I’ll be really surprised if it doesn’t address my binding clutch pedal problem. Against Porsche install instructions I did apply a light coat of lithium grease to everything. I’ll post final drive test results as soon as the car is back on the road.





Thought I took a pic of it compressed and pinned but I guess not.


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