nitrojunky

Figured I'd throw this out as a new thread. Not sure if I'm putting out any new information, but this is what I came up with after thinking about the issue some, and so far, I haven't seen the actual process of what could cause the sticking adequately described. So the fluid boils, how does this make the pedal stick? And how to test if the fluid actually boils?

So I put this out for you guys tell me if/how this is wrong.

MadDogZ06 emailed me tonight regarding a theory on the 'sticky clutch'. Caught me in a problem solving mood as I was studying physics-type stuff... so I naturally went off on an entirely different tangent . Anyhow, here's where I went with it:

so... clutch + flywheel + output shaft gets hot if clutch slips a little under WOT acceleration.

(i) In any normal car, the slave cylinder is attached to the throwout bearing with a long forked lever, which is attached via a metal fulcrum to the bell housing. So if the throwout bearing gets hot, it transmits this heat to the forked lever, which will then transmit much of its heat to the bellhousing instead of the slave cylinder.

In the 'vette... there is no such conduit for the heat to flow; instead, immediately opposite the throwout bearing is... hydraulic fluid.

Perhaps the clutch boils it for this reason?

(ii) When the pedal is back at its equilibrium point, the slave cylinder is free to move fluid from and to the reservoir. This is how it 'self adjusts'.

Given these two, I suggest the following process occurs:

step on it in first, a little clutch slippage occurs (not really noticable), and things get hot. Meanwhile, pedal is at top of travel, gas is expanding, pushing fluid back into reservoir. Bubble hangs around the master cylinder area (maybe never even leaves it), but excess fluid flows to reservoir. Wouldn't take much, maybe a half cc or so would be sufficient? Bang second, things feel mushy because you're compressing some gas before you hit hard fluid. Let go of clutch, step on gas... pedal returns to top of travel. Clutch slips more (again, not really noticeable, SOTP), things get hot, gas continues to expand and is free to push still more fluid into reservoir because of self-adjusting mechanism; bubble remains in vicinity of slave cylinder, probably at the front. Next gear is worse... Repeat thru fourth, by which time the trapped bubble is big enough to allow the pedal go almost to the floor before you hit solid fluid. Things continue to get worse until you stop pumping heat into pressure plate.

Cool things down for a few seconds, bubble condenses, one pedal pump, and you're fine.

The point is, under this regime, the gas is not under any kind of high pressure (because it can freely push fluid back to the reservoir when the pedal is hanging out at the top (just like you can push in your brakes by hand when you're changing your brake pads)), so it doesn't have to push in the clutch to do this, and because it's at a 'low' pressure, when you step on the clutch after WOT in a given gear, the gas has a long way to compress before it'll have enough pressure to start moving the pressure plate fingers.

Don't have much of a reason why this happens with clutch and not brakes except perhaps that (a) maybe the bearing itself produces a lot of heat, which is what maddog is suggesting (b) perhaps they dissipate heat better (c) they aren't producing the most heat they can while your foot is -off- the brakes, whereas the clutch is heating the fastest while your foot is off the clutch, and this is when it can freely dump excess fluid back into the reservoir.

edit:

credit for this part goes to maddog...

so maybe in fourth or fifth, you let out... push pedal to near top of travel, and the gas continues to expand until heat source is no longer present (because you let out of the gas)... then things begin to cool, and the bubble condenses 'sucking' pedal to floor.

all of this would support the hypothesis that keeping fresh fluid in the system will reduce the problem... especially if the system gets hot enough to boil water-saturated brake fluid, but not fresh fluid. If at higher power levels (cammed, heads, etc.), it slips the clutch enough to get hot enough to boil new fluid... you're screwed.

Possible solutions:

a) cool the slave cylinder
b) keep fluid very fresh
c) very good clutch which slips very little
d) introduce a non-conductive throw-out bearing
e) design the ******* thing properly to begin with!!

If this hypothesis is correct, GM left no margin for additional HP, old hydraulic fluid, and no way to flush the hydraulic fluid so as to alleviate the problem when the car is in stock form. Not cool.

nitrojunky

method to test theory:

(do not try at home or on public roads, yadda, yadda)

know volume of one clutch-stroke. if it's large enough to be -very- noticeable on reservoir...

a) mark reservoir level
b) do WOT run thru a few gears, enough to make the problem very, very noticable
c) at last gear, put car in neutral (still with mushy clutch)
d) hopefully, pedal is super mushy and goes to floor after coasting
e) coast to stop, measure fluid level

if it's high, then I'm pretty sure it's a clear indication that boiling fluid is the culprit. If there was no boilage, then there should be no change in fluid level.

This won't work unless one pedal stroke worth of fluid is very noticeable because the issue (at least in the case of my car) is anywhere from 1/4 pedal stroke to 3/4 pedal strokes' worth, and the car will likely not be very level at either the start or the finish.

and now I should sleep.

aggiez28

fixed

nitrojunky

For me, cars and option (f) never appear in the same paragraph.

aggiez28

then you are doing somthign wrong

Bill Curlee

I think you have this problem NAILED! Great work!

BC

nitrojunky

Thanks! Hope so...

so far, the theory works for every symptom I've encountered... but it's only correct until a counter example is presented

couple reasons why I don't think it's binding/mechanical in nature (in my case, anyway)

- after a good WOT run through third, subsequent part throttle shifts to fourth, fifth, and sixth (getting to pretty low RPM by sixth) exhibit pronounced 'sticky clutch' symptoms.

- if it were binding, shifts after experiencing 'sticky clutch' would be followed by massive clutch slip when I went WOT

- taking the warm car up to ~6k RPM for about 20 seconds in neutral (while coasting on the highway) and then depressing the clutch 4 times produced no sticky clutch symptoms

aggiez28

im going to try and take the time to actually read your whole post this weekend lol

looks interesting

nitrojunky

couple updates (getting ripped up on cz28.com lounge, heh)

• this is also common on LSx f-bods. Which means it isn't unique to our weird slave cylinders. Headers also doing it?? Perhaps sometimes it's the headers and sometimes it's the clutch boiling it in the case of C5's??
• drill mod is common (haven't seen it much here, but is common on ls1tech.com)

The drill mod kinda emphasizes my THY about what the boiling fluid does and how it can cause symptoms:

Boiling fluid can make pressures easily exceeding 100 PSI, moving the fluid up to the reservoir. Remove the heat source, and the fluid condenses. However the maximum pressure trying to return the fluid to the clutch line is only 14 PSIG. This is why the pedal drops to the floor. Any restriction (e.g. the material the drill mod removes) will emphasize this effect.