Clutch Drag When Hot

[Barefoothb]

First time on the forum! (On another as well.)



I'm stumped. Myself and muliple mechanics have been working on my 98 Wrangler (TJ with manual trans) to sort out a clutch drag issue that only shows up when the engine is hot (30+ minutes of driving).


Symptoms:
1. When cold: All gears shift into gear easily and smoothly.
2. When hot: It gets increasingly difficult to put it into gear when at a stop.
3. When hot: It is difficult, if not impossible, to get it into reverse. Even using the "shift from first to reverse" trick, by the time the stick gets from first to reverse, the trans starts turning too fast to get it into gear.
4. When hot: After the engine is hot and you turn off the Jeep for an hour then crank it up again, the clutch drag is MUCH worse than when it was turned off. If you leave the engine off for 3+ hours then crank again, the clutch drag is gone.


To date, we've done the following:
1. Installed a new clutch kit (pressure plate, disc, throwout bearing, pilot bearing)
2. Machined the flywheel
3. Installed a new Master and Slave Cylinder with new oil and bled
4. Inspected and checked all fork/clutch mechanical components
5. Installed new bearings on the input shaft from the engine (this has helped a little bit but not much)




At this point, I'm not sure what else to attack aside from just starting to replace things until the problem is fixed. Obviously, this could be very costly and time consuming. I'm hoping that someone out there has come across something that might help me out of this situation or have some helpful insight.

[Classic Boy]

Have you considered the hydraulic hose between the clutch master and the slave may be bad?
The rubber is going mushy But when it is cold it expands only a bit because the rubber is stiffer and the clutch can be still engaged but when the rubber is hot it expands (balloons) more easily therefore you lose motion in the slave cylinder.

A good hydraulic hose should not expand perceptibly when under operating pressure.

I have seen brake hoses go mushy and balloon under pressure and clutch hoses are made the same way.

[ren0vator]

Heat build up would point to flywheel & pressure place
But you've already replaced these
Somethings bent or out of alignment

Only additional items I can think of are;
Pilot bearing size compared to input shaft
(shaft starts grabbing as heats up)
Throw out fork pivot is loose or bent
(free play gradually builds up as heats up)

Damn tricky situation
Best of luck

[Barefoothb]

When I changed the Master and Slave cylinders, I also changed out the hose.

I was really hoping that this would fix it but no luck.

[ren0vator]

You've really only got extreme stuff left,
for instance;

bell housing off square, but symptoms would be consistent

flywheel clearances all wrong, pressure plate surface too close to flywheel

Never seen a Wrangler flywheel
Do they have a step in them, or are they flat?

[Barefoothb]

Perhaps the flywheel is warping when its gets hot? Its been removed and machined but it might by still doing that.

The flywheel is along the lines of:
https://www.quadratec.com/products/52027_0006_07.htm

[layback40]

You didnt say what engine it is but I assume a 4.0 6 petrol.
Could there be an issue with the thrust bearings (plates) on the sides of the engine main bearings? Just a thought.

If you pump the clutch peddle does it help?

I would have thought the travel of the MC would have been enough to allow complete disengagement of the clutch. what is the difference in the feel of the peddle between cold & hot?