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10.85 pass, then current record for a stock Corvette on drag
Take Care of Your Clutch--The Ranger Protocol The
driven aggressively, GM cars with manual transmissions and the LS1,
LS2, LS6, and LS7 engines sometimes fall prey to clutch pedal
issues during or following high-rpm shifts. For clarity sake,
driven “aggressively” means cars that are launched or see high-rpm
up-shifts or down-shifts. And consequential clutch problems affect
many brands of car besides GM and motorcycles too.
The general symptoms include a clutch pedal that
• Slow to return or lazy
• Hung mid-way up or
• Stuck to the floor
These misbehaviors are commonly called clutch pedal “woes.”
is a cliff-notes version of the written commentary that follows
After you launch the video, you can switch to HD and full-screen by
clicking the proper buttons in the embedded
the years various theories have been advanced on the root cause of
pedal woes. There is no broad agreement, but one fact does seem to
with pedal woes, replacing all clutch and hydraulic components with
new stock parts does not prevent a quick return of the same
symptoms in a car driven aggressively. And upon tear-down
inspection, none of the parts shows obvious failure.
After GM’s “replace all parts” approach failed to solve pedal woes
on my 2001 Z06, I sought an effective remedy on my own. Through
experimentation, I found that
keeping the clutch fluid fresh and clean prevented a recurrence of
pedal woes entirely.
Three Z06s and 700 passes at the drag strip later, my clutches have
behaved normally due to my keeping the clutch fluid clean, via to
the protocol I recommend.
I particularly want to thank George Westby, Director of the
Advanced Process Laboratory of Unovis Solutions, and his lead
engineer, Martin Anselm. They collaborated in the analysis of
clutch fluid and clutch dust samples, which I sent them from 2006
Click this link to open a copy of the Unovis Lab
That report forms an important basis for concluding that clutch
dust is infusing the clutch fluid of aggressively driven cars. And
it is clutch dust that, if allowed to accumulate, constitutes an
abrasive that damages integrity of seals in the hydraulics.
The good news is that we have figured out the root cause. That
means the preventative maintenance (frequent swaps of the fluid via
the master cylinder reservoir) definitely works. It also explains
why, if [B]the seals are already damaged[/B] by clutch dust, no
amount of clean fluid will heal them. The crucial element is
prevention, never allowing the clutch dust to accumulate in the
fluid. The tell of its presence is [I]murkiness or cloudiness[/I].
So don’t allow the fluid to stay murky.
I think water entering the fluid via heat-cool cycles plays a roll
in clutch pedal issues. So frequent changes of the clutch fluid
ought to be part of routine maintenance for owners who don’t do
aggressive launch or shifts.
Hope the video helps more owners avoid the trouble and expense of
clutch pedal issues…and helps GM further reduce warranty claims for
clutch issues easily avoided by preventative maintenance. The
routine I personally follow costs about $20 and two hours of my
time per year. That’s a small investment with a big return in
reliability and driving pleasure.
Does the Clutch Fluid Have to Do with Pedal
simple answer is the ravage of heat and water and clutch dust.
Clutch fluid is actually brake fluid. It degrades when subjected
(1) high heat produced during aggressive driving
(2) water absorbed via condensation from heat-cool cycles
(3) infusion by abrasive clutch dust that circumvents the main
heated enough, degraded fluid will boil its water content and emit
a gas (steam). In the hydraulic system gas, unlike fluid,
compresses under pressure and causes malfunction of parts that
depend on an unachieved level of hydraulic pressure. Meanwhile
accumulated clutch dust will abrade the seals and make them
malfunction over time. Think of what saw dust does to the operation
of your eyelid. The wiping action of the lid can snag the wood
shard and lacerates the cornea. Likewise, shards of copper and iron
in the dust from the clutch disc adhere to and abrade the surface
of rubber seals in the hydraulics. These abrasions cause wear spots
and leakage where none would otherwise occur.
In the prevailing GM clutch design the clutch actuator cylinder
(slave) is located inside the bell housing. That location puts
clutch fluid in close proximity to high heat and blown clutch dust,
especially during launches and high-rpm shifts that necessarily
entail slippage of the friction surfaces between the pressure
plate, clutch disc, and flywheel. These bursts of friction-induced
high heat are sufficient to boil clutch fluid if its boiling point
has been degraded significantly from the nominal specification on
the can. The friction also expresses clutch dust that pelts the
seal on the actuator where it can be squeegeed into the fluid on
each stroke of the actuator during clutch-in and
classic indication of degraded fluid is a change in color from
nearly clear to dark and murky. When it reaches that state, the
fluid should be cleaned up. If that’s not done, normal operation of
the clutch hydraulics will be disrupted under high demand
circumstances. This can cause incomplete disengagement of the
clutch during high-rpm shifts, which in turn can lead to
shift-refusals and, worse still, missed-shifts that damage the
racers know they must bleed the brake fluid frequently to keep
their brakes from misbehaving during track sessions. Why does
anyone think that the brake fluid in the clutch is any different?
In fact it’s worse in the clutch because of pelting of the main
actuator seal by blown clutch dust.
the fluid in your clutch hydraulics free of accumulated clutch
dust. And Keep the fluid near the nominal dry boiling point on the
can (450F to 500F, depending on the brand), and keep the fluid dry,
e.g. without contamination by infusion of water.
If that is accomplished by frequent changes of the reservoir
content, the clutch hydraulics will never see accumulated clutch
dust nor a temperature high enough to make the fluid boil, the dual
origins of pedal woes. And the seals will enjoy a long
service-life, if kept free of clutch dust ravages.
Murkiness is “the tell” that fluid is degrading. The darker the
fluid is when agitated, the more urgent it is for it to be cleaned
up before pedal woes take hold.
the fluid in the clutch master cylinder reservoir. If it’s not
clear and clean move to step (2).
the fluid in the clutch master cylinder reservoir.
(a) Draw out the discolored fluid with a syringe, keeping the
corrosive fluid off your paint.
(b) Wipe down the reservoir and the diaphragm on the cap, using a
clean, lint-free towel.
(c) Locate the fill-line in or on the reservoir.
(d) Refill the reservoir to just below the fill-line with fresh
fluid specified in your owner’s manual. Do not over-fill. If the
reservoir has both minimum and maximum marks, suggest filling to
(e) Clean the diaphragm if dirty and properly seat it on the
(f) Replace the reservoir cap snugly. Step
the engine off, pump the clutch pedal full-top to full-bottom to
full-top 30 times. This action causes fluid in the clutch
hydraulics to circulate, blending the new and old fluid, and
revealing the color of the blend. It also helps scour residue from
the actuator (slave) and displace upward into the reservoir any air
trapped in the hydraulics. Alternatively, you can drive the car
re-inspect the blended fluid in the reservoir. If it is totally
clear and shows zero murkiness, you are finished. If it's not,
restart at (2). Continue to change the fluid as many times as
needed, with the full, slow top-to-bottom-to-top pedal pumps
between changes, until it remains absolutely clear and clean.
Getting the fluid clean the first time is the hard part.
come the easy part, keeping it clean. Re-inspect the clutch fluid
each time you add gasoline or prepare for spirited driving,
including the track. If it is not still clear and clean, restart at
Step-2. Usually will take only two or three swaps to get the fluid
Even badly degraded, dirty clutch fluid can be substantially
cleaned up in 10-20 reservoir changes. This takes less than an
hour, costs less than $10 for fluid and a syringe, and doesn’t
require a service visit. I recommend this protocol for clutches on
cars with LS-X engines that see any form of aggressive driving. If
you follow this protocol, you very likely will avoid clutch pedal
issues. And that is a very important assurance, because the car’s
performance depends on a properly operating clutch.
Clutch Fluid to Use? You
can’t go wrong by using the clutch fluid specified in your owner’s
manual. That’s a GM-branded fluid. But note that GM now recommend
Super DOT4 brake fluid for all Corvette clutches.
When considering alternative brands, keep in mind there is no magic
fluid that prevents pedal woes. All fluids will degrade under
aggressive driving and need changing regularly.
If you want an alternative to the GM-branded fluid, I’ve used
Prestone brand with good results for eight years in three Z06s.
That means, following the fluid change protocol with Prestone, I’ve
had no clutch pedal issue. I am currently using Prestone DOT4
Synthetic in my 2006 Z06. But I have no reason to believe that my
results would be different with any other good brand, so long as
the protocol is followed.
If you’re running DOT3 fluid in the clutch, you ought to consider
upgrading to DOT4, which is usually compatible with DOT3. But be
sure to read the label on cans you’re considering. DOT4 brake fluid
generally has higher dry and wet boiling points than DOT3. Given
the minimal price difference between the two, it makes sense to use
an excellent chart of alternative high-quality fluids, linked by
permission of Seine Systems. In this chart, note that
Pentosin-brand Super DOT4 brake fluid is repackaged and distributed
under the GM-brand and is specified for C4, C5, and C6 Corvettes
removing the dirty clutch fluid, I use a syringe that’s made for
mixing oil and gasoline used in two-stroke motors. It has given
good service for eight years without leaking. Some prefer medical
syringes; others still like a turkey baster. I prefer a syringe
because it’s reliable, compact and doesn’t leak. Mine was purchased
at Walmart from a pegboard near the gas cans.
cleaning the reservoir and rubber diaphragm, I use blue paper
towels intended for car detailing. They are thick, absorbent and
Clutch Fluid Change Kit
my car I carry a clutch fluid changing kit with the following
Can of fresh brake fluid
• Can for storing spent fluid
• Syringe for drawing fluid from the reservoir
• Small funnel to get a precise pour in dim light
• Cup to hold the syringe and some lint-free paper towels
• Two one-gallon zip-lock freezer bags for storing the entire kit.
One bag is inside the other gives a little extra protection against
the Clutch Hydraulics Work
It’s always a good idea to go to the source document for a
definitive description of the operation of the clutch hydraulics.
In this case the Corvette Service Manual 2006, Volume 3, page
The clutch hydraulic system consists of a master cylinder and an
When pressure is applied to the clutch pedal (pedal depressed), the
pushrod contacts the plunger and pushes it down the bore of the
In the first 0.8 mm (0.031 in) of movement, the recuperation seal
closes the port to the fluid reservoir tank, and as the plunger
continues to move down the bore of the cylinder, the
[COLOR="Navy"][B]fluid is forced through the outlet line to the
actuator cylinder[/B][/COLOR] mounted to the driveline support
As fluid is pushed down the pipe from the master cylinder, this in
turn forces the piston in the actuator cylinder outward.
As the actuator cylinder moves forward, it forces the release
bearing to disengage the clutch pressure plate from the clutch
On the return stroke (pedal released), the plunger moves back as a
result of the return pressure of the clutch.
Fluid returns to the master cylinder and the final movement of the
plunger opens the port to the fluid reservoir, allowing an
unrestricted flow between system and reservoir.
There you have if from the source.
Clutch fluid circulates between the master and actuator
The volume of fluid in the entire system is a very few ounces. One
ounce is in the master cylinder reservoir until diminished as you
depress the pedal and create the flow described above.
Clutch Dust Enter the Fluid I
spent time at the workbench with two Chevy master technicians (one
who’s worked on Corvettes since the C3 was first released). We
examined [B]the operation of the Corvette clutch actuator[/B] and
1. The bell housing is fouled with blown clutch dust during
2. Clutch dust is penetrating the accordion shield on the actuator
main shaft. That is obvious from visual inspection.
3. The piston slides along the shaft sealed by an O-ring. The shaft
has a film of lubricant or clutch fluid on it. During aggressive
driving, this film gets coated on each stroke with a fine layer of
blow clutch dust. That is obvious from visual inspection.
4. The O-ring slides along the shaft and squeegees some of the
clutch dust down the shaft where it contacts the clutch fluid and
5. The conclusions were unanimous and seemed obvious from a
physical exam of the surfaces involved.
Plus, keep in mind that it is confirmed that clutch dust is getting
into the fluid. The question is how. We believe the answer to that
is in points 1-5 above.
Water Infiltration to the Clutch Fluid
I've been following the protocol for eight years now and never had
shift-related pedal issues. That's includes a lot of burnouts,
launches, and red-line shifts. So what I'm doing is definitely
Brake fluid is indeed hygroscopic. By my approach the cap is off
the reservoir for very brief periods and, while that occurs, each
time you are swapping about 35-50 percent of the used fluid with
new. Do that one-to-three times and the impact of water absorption
from the atmosphere is essentially nil. In fact the more times you
make the swap in succession, the less the impact of transitory
The industry standard for "wet" boiling point is the fluid
containing 3% water. That's amount of water infusion is not going
to happen in fluid that's kept clear and fresh via the reservoir. I
suppose water infusion can be an issue if you remove the cap from
the reservoir outdoors on a very humid day (or in the rain) and
then take a lunch break. But few of us would do that. And a couple
flushes by the protocol and the water is gone anyway.
Another aspect of risk is using brake/clutch fluid from a can
that's been open for months or left with the cap off for an
extended period. That's ill-advised. But I routinely keep a can
going for 30-60 days by....
(1) cutting a slot in the membrane at the neck vice removing the
(2) replacing the cap when not pouring from it
(3) keeping the can in double layer of double-seal zip-lock bags
Those steps minimize moisture infiltration to a can that's been
are five alternatives to adopting this
Refrain from aggressive driving and high-rpm shifts. This is the
automotive equivalent of abstinence. It’s also an explanation of
why many owners never suffer pedal woes…they don’t drive
Complain to the dealer about your clutch pedal issue. If your car
is under warranty, the GM remedy may be invoked, which is swapping
all the clutch parts. That may sound great. But keep in mind a
clutch swap entails risk, unless the servicing tech is experienced
and meticulous in following correct procedures. An additional risk
is the new rotating parts may not be properly balanced. I've lost
track of how many owners I've counseled through vibration issues
after botched clutch installs.
Have the clutch hydraulics bled. This takes a competent technician
about an hour and requires a lift. Cost is around $125. But the
procedure includes some inherent difficulties:
• A hard, constricted reach with a tool to loosen the bleeder valve
in its barely accessible position.
• A messy job because fluid dribbles into the bell-housing and all
over adjacent surfaces.
• Need for two people, one at the bleeder valve, the other at the
clutch pedal; or, if using a pump tool, the other keeping the
clutch reservoir filled.
• if not done right, residual air will remain in the hydraulics,
causing continued pedal issues.
• if crud has built up in the actuator (slave), a simple bleed
won't dislodge it.
• Under the best of circumstances, the clutch will need bleeding
each time the clutch fluid deteriorates. Not a cheap
Some owner install a remote bleeder valve in the engine bay to
simplify a full bleed of the clutch hydraulics. This is a good
modification to make when replacing a clutch. At the same time,
insulating the clutch hydraulic lines thoroughly can partially
mitigate the adverse impact of heat.
More details on a Remote Bleeder are
Change to an aftermarket clutch and hope for relief. Unfortunately,
some aftermarket clutches suffer pedal woes too, particularly those
using stock GM clutch hydraulic components (clutch master cylinder
and clutch actuator (the slave). This is especially true if the
clutch fluid is maintained.
Do nothing and suffer continued clutch issues, which often give
rise to transmission damage. Other
Clutch Issues Pedal
issues on high-rpm shifts are largely the result of deteriorated
clutch fluid. But they can also result from poorly timed clutch
engagement (clutch-out) on the shift, causing excessive
Pedal issues on launch
are generally caused by driver technique error--too slow a pedal
release while the throttle is engaged. The
C6Z06's LS7 clutch,
even with clean fluid, will glaze on launch or a sloppy shift if
the clutch is slipped, intentionally or not, by a slow release at
elevated rpm, say above 2500. In such an event, the friction
surfaces overheat from the slip to such an extent that the nominal
clamping power of the clutch is lost; and the clutch pedal usually
hangs mid-way up. There is no widely accepted explanation of this
behavior’s exact cause. But it is an annoying flaw in the LS7
personality. If you want to ensure an LS7 launch without clutch
issues, keep the launch rpm at 3600 or less and make a very fast
clutch release, with the clutch foot coming up in a single motion.
Since learning that lesson, I've not glazed the clutch in my
WARNING on Clutches Slipping under Load As
you add horsepower/torque to a motor, the load may eventually
surpass the rated clamping-force of the existing clutch. Once that
happens, the friction surfaces will slip excessively in high-demand
circumstances, producing even more heat than normal. These heat
loads can cause malfunction of the slave.
A slipping clutch placed under the demands of launch and strong
shifts may suffer a catastrophic failure. You may have seen
pictures of those events. Quite ugly and not warrantied by anyone.
Last one I saw cost the car owner $13K in out-of-pocket repairs.
Pressure plate exploded; clutch fluid sprayed the headers causing a
fire. Not a happy ending.
Moral of the story. Know the limits of your clutch and don't exceed
them. If the clutch is slipping, get a new one with appropriate