• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Why is the Clutch Slipping?

Share this page

RAC68

RAC68

Darth Vader
Offline
Hi All,

Today I attended activities at AHSTC’s Encounter in Princeton’s Forestall Village and on my mainly back road drive home across the NJ experienced some clutch slippage. Although the ambient temperature was only 81F and my engine temperature running at 185-190F, the many Stop-&-Go traffic condition were not unusually high to expect the clutch to stop grabbing immediately and normally.

As I approach the Eastern half of the state, all stops were smooth but the delay in the clutch’s grab seemed to get more pronounced with every start-off. Once under way, however, changing gears seemed normal and any clutch release seemed close, or even, normal with very little or no application delay.

Some Background Facts:

I am using DOT 5 (Silicone) Brake Fluid but see no loss in fluid at the reservoir. The clutch has been performing for the past 20 years and only around 20K miles…if that. I am also using Mobile 1 Synthetic 15W50 and rely on the slinger to keep the oil in the engine (no rear main oil seal).

Last, my transmission is filled with Red Line MTL and the OD is performing perfectly. However originally (20 years ago) the initial MTL fill seeped into and filled the closed solenoid housing of my BJ8 P1 and hydraulically locked the unit. The fluid was drained and all has been functioning perfectly since this remediation

Help:
I would like your thoughts and do not want to over-react as this condition has not happened before and could be an anomaly (disappearing on its own). However, I would appreciate knowing if this condition is recognized by a Forum member and any suggestions on remediation.

Thanks All,
Ray(64BJ8P1)
 
Ray--

Can you be a bit more precise regarding the problem?

For example, will the rpm's change dramatically if you were to step on the throttle hard when driving in 4th? Also what happens when you put the car in first and with your right foot hard on the brake let your left foot off the clutch.

Report back....
 
Hi Ray,
If I'm reading it right it sounds like the clutch doesn't slip once fully engaged, just slow to engage. If it doesn't slip once fully engaged that suggests to me the clutch disc and pressure plate may be just fine and I would look to the linkage - pedal, clutch master & slave, and the arm the throw-out bearing is on. If any of these have any drag/resistance to movement that may explain a slow engaging of the clutch.

Could the flex hose on the slave be the problem? I have no experience with DOT 5 fluids but I wonder if they can cause swelling in rubber parts like DOT 3 or 4 ? If so, I suggest checking the flex hose on the slave? It can sure be a source of problems on brake systems with DOT 3 & 4 and if it is over 20 years old.....

Dave
 
Hi Michael,

This will be difficult as the car does initially take off but then shortly after seems to lock on. There is no free revving and the clutch is definitely connecting the transmission and engine. Additionally, once in motion, slipping when changing gears up or down seems even less noticeable or dramatic but still may exist. My Healey-knowledgeable passenger did not notice anything wrong during our 1-hour driving across the state and was surprised when I mentioned what I was experiencing.

Michael, I will try the Brake-locked take-off you mentioned tomorrow and report.

Thanks, and all the best,
Ray(64BJ8P1)
 
Hi Dave,

I will check the areas you suggest and report back. Your comments have triggered a past experience on a friends car when his clutch line began to swell and would expand when the clutch was applied.

Thanks, I really appreciate the direction.

Ray(64BJ8P1)
 
Ray--

It's difficult to diagnose the problem based on what you write--and I am not sure what you mean when you say the car "locks on" after takeoff. In any case I tend to agree with Dave and it sounds like the problem has to do with the clutch's actuation. Perhaps if you put the car up on stands and--being VERY carefuill--got underneath it to observe while someone was operating the clutch with it running I have a feeling you might be able to find the problem.
 
red57 said:
Hi Ray,
If I'm reading it right it sounds like the clutch doesn't slip once fully engaged, just slow to engage. If it doesn't slip once fully engaged that suggests to me the clutch disc and pressure plate may be just fine and I would look to the linkage - pedal, clutch master & slave, and the arm the throw-out bearing is on. If any of these have any drag/resistance to movement that may explain a slow engaging of the clutch.

Could the flex hose on the slave be the problem? I have no experience with DOT 5 fluids but I wonder if they can cause swelling in rubber parts like DOT 3 or 4 ? If so, I suggest checking the flex hose on the slave? It can sure be a source of problems on brake systems with DOT 3 & 4 and if it is over 20 years old.....

Dave
Click to expand...
I've seen a clutch hose that was so worn out internally that it took a while for it to fully engage the clutch once you let off the clutch pedal.
 
FWIW, I just replaced mine--and overhauled the engine while I was at it--at about 120K miles. My BJ8 had the usual leakage of dino oil out the rear main but the disk and plate were dry (the inside of the bellhousing had a thick coating of oily dirt, though). If it's not the linkage or flex line, I'm afraid the disk may have failed prematurely.
 
Ray, I have been using Silicone Brake fluid (DOT 5) for over 3 years now with no problems. It pulled the steep inclines of the Smoky Mtns just a few weeks ago with no problems.
 
I have been using DOT 5 silicone fluid for more than two decades. I've replaced the rubber seals in all cylinders as there was developing a slight leak after 20 years (losing fluid in the reservoir) I replaced the hoses too as a precaution. Never had any problems with them in terms of swelling.
 
Hi All,

Sorry for the delay in responding.

Not too long ago I converted to 15W50 Mobile 1 Synthetic and although used for over a year, was wondering if this oil could be less resistant to the action of the slinger (no rear main seal) and a contaminating factor in my clutch issue. I appreciate it is unlikely but, for completion, thought I would mention it.

I had converted to DOT 5 Silicone Brake Fluid along with all updated hydraulic and clutch components before returning my Healey to the road in 1989. Although I have experienced issues with DOT 5, it was usually in its price and not its operation. I brought up its presence to be both complete and in case there is an issue others experienced that I am not aware of.

Michael, my meaning for the term "Lock on" relates to when the clutch seeming to stop slipping after a 1st gear start-off. At that time, the clutch seems to apply sufficient support to handle high RPMs and acceleration. My transmission and O/D are standard and when starting off, the engine is commonly revved close or to 5K in 1st and 2nd before shifting. Although I did modify that practice greatly when sensing the clutch slippage, there seemed to be no slippage in upper gear acceleration.

Yesterday, without available help, I decided to bleed the clutch circuit to make sure captured air was not the cause of my issue. However, although I had installed a slave bleed extension outlet in the engine compartment, my initial placement of the bleed tube and bottle was not ideal an I could not tell if any internal air was actually extract from the system. To make matters worse, during the bleed process, I exceeded the capacity of the reservoir and wound up completely renewing the fluid in the system. However, at present, I believe there is no air in the circuit and plan to take a test drive in the car today.

Dave, I am hopping your thoughts are on target and tomorrow will be able to draw on some help to examine the operation of the flexible slave hose. However, either way, the hose has been in place for a while new one is being placed on order.

The funny part of this issue is that I had been attending AH Sports &Turing Encounter 2017 and all went great to and during the event. It was on the return trip home across the state that this all started.

Again, I really appreciate all the thoughts and suggestions as they really are valuable and HELP.

Thanks again to all,
Ray(64BJ8P1)
 
Last edited:
Air in the clutch line will only cause a problem when disengaging the clutch--i.e. pushing the pedal in--and the pedal travel will be excessive before said disengagement. Your problem, IMO, can only be caused by one of two things:

1) something holding pressure on the pressure plate, or
2) clutch disk or pressure plate--not likely, but conceivable--failure

It's a real long shot, and I can't come up with good logic to explain it, but last time I had my gearbox out I noticed the release bearing fork's bushings were sloppy and I replaced them and the shaft. I suppose if the fork got somehow 'cocked' against the release bearing it could put pressure on the pressure plate.
 
Based on your description of the problem, when you release the clutch pedal the clutch slips for a little and then grabs and everything is OK till the next gear shift. If this is the case either the hose is blocked and is holding the slave extended for a little or the guts of the master cylinder are not returning quickly allowing the clutch to engage.
I had a similar problem with a new slave cylinder that needed about 50-100 strokes until it loosened up and the clutch worked properly.
 
Hi All,

After bleeding/exchanging the fluid in the clutch hydraulic circuit (as previously mentioned), I just got back from an aggressive test drive and was pleasantly surprised to have had detected NO issues during the back-roads hour-plus drive. Although I would like to indicate the bleeding of the system was the solution to the issue, I can't even tell if anything had been corrected.

Dave, I am still planning to get under the car tomorrow to see whether the rubber connecting hose is expanding when pressure is applied with every clutch peddle depression. Since the clutch only slipped on initial take-off, the time when friction in the link (pressure-plate, clutch-plate and Flywheel) is most required to propel the car, I am not clear that the loss of hydraulic separation pressure would cause a reduction of clamping pressure to cause slippage AFTER peddle release. I would have more expected this condition to promote difficulty in gear shifting with the potential of crunching during changes.

However, Bob and Joe's suggestion that a blockage in the circuit causing a retarded release of in the application of Clutch Plate pressure would address the symptoms and be consistent with a fluid flush fix.

Again, I noticed no difficulties during my recent test drive and wonder if my Healey was acting up in response to being taken from the party with its friends at AHS&T Encounter 2017.

Thanks all for your help as my mind was blocked by the thought of all the work in undoing the interior, pulling the transmission, and redoing the clutch system...a job that will definitely validate I am growing old.
Ray(64BJ8P1)
 
I had a similar experience when an old flexible hose deteriorated. It acted like a kind of check valve. Once the pressure was released, the fluid wouldn't easily return to the master cylinder. Replacing the hose cured the problem. A friend also had a similar problem with a rubber flex line to the rear brakes on his F-150. The rears locked up, and wouldn't release at all. Again, replacing the hose cured it. Good Luck.

Edit, I just saw you post after I had replied. In both cases, neither hose had swelled or expanded. I guess the rubber inside of the hoses had just deteriorated.
 
Hi Jim,

You present a point that I had overlooked. The inside of the hose could deteriorate and cause it to expand and reduce output pressure when the clutch peddle is depressed but block the release of pressure by blocking the return of fluid. This condition could cause the retention of sufficient pressure as a counter force against the locking force applied by the pressure plate and resulting in much less actual engine-to-transmission locking force. As I see it, this would be a form of the condition that Joe and Bob also presented. Bottom line I will be replacing the hose.

Jim, thanks for your response,
Ray(64BJ8P1)
 
Last edited:
Hi Derek,

In short, the car runs better and performance quickly deteriorates when I don't. However, it is not my practice to rev to 5K every time but I do come close a high percentage. The 1st and 2nd gears in my standard Healey are quite short and my engine has always seemed to run better when driven this way. In the late '60s, as part of a bad original head gasket replacement, the head was shaved 0.060" and not the 0.006" I had intended. Although this change has contributed to improved engine performance, a higher RPM idle and low gear shifts were also part of the outcome.

Ray(64BJ8P1)
 
The rubber hose "check valve" syndrome is pretty common. I had the issue with the flex hose to my LF caliper.
Replaced hose (s) and cured the problem.
On clutch I use SS -3 AN hose
 
Back
Top