LJ78 Clutch Issues - I'm Stumped! (1 Viewer)

[AirheadNut]

Here's the picture: about 10k km ago I swapped the 2L-TE for a VW TDI engine and in the process installed a brand new clutch, pressure plate and throwout bearing. Everything from the back of the engine to the front of the transmission case was swapped to 3.4L V6 parts from a 4Runner (bell housing, input shaft, clutch fork, new clutch, pressure plate and throwout bearing). I retained the original LJ78 clutch master cylinder but swapped the slave to a 4Runner one and removed the original clutch accumulator. I drove it this way for about 6 months and 8-9k km, but within the last 1000 or so km the clutch has been intermittently failing (push pedal down, clutch doesn't disengage). Here's what I've done to try and solve the problem:
- Bleed clutch system
- Replace used slave cylinder with new
- Replace original master cylinder with new
- Remove, bench bleed and reinstall master cylinder
- Rebuild original accumulator and reinstall
- Bleed system extensively

All this has helped briefly, but the problem keeps coming back. Sometimes the clutch will work fine, and without warning utterly fail, leaving me to have to rev match if driving and start in gear in order to get going from a standstill. Sometimes the clutch will slowly get worse with each shift.

A couple of things to note:
- I measured the bore and stroke on the original Prado and 4Runner slave cylinders and they match
- The piston, seals and pushrod from a T100 master cylinder fit perfectly in the Prado master cylinder (the T100 and 4Runner share the same slave cylinder)
- There seems to be too much dead travel at the top of the clutch pedal stoke, but I've adjusted the pedal as far up as possible to maximize stroke length
- Sometimes failure will occur after a period of driving, sometimes first thing on a cold start. Shutting off and restarting the engine "seems" to help some of the time

At this point I have run out of ideas as to what could be causing this problem. I have a new set of master cylinder seals coming just in case there is an issue with the current seals holding enough preload against the clutch fork. Is there any way some part of the throwout bearing/pressure plate setup could be causing this issue?

 

[pithicus]

That's a tough one given all the mismatched clutch parts. Maybe a bigger slave not from a 4runner could be something to try.
I assume the braided flex pipe is in good nick? Not swelling?
Interesting conversion you have! What size VW TDI engine is it?

 

[AirheadNut]

From what I can tell, the 4Runner slave cylinder should work fine with the rest of the hydraulic system, since it shares the same internal dimensions with the original slave. I also forgot to mention that the flex pipe from the fender well to the accumulator was replaced when the engine was swapped.

Another thing I've done is measure the bore and stroke of both the master and slave cylinders and found that a full stroke of the master cylinder would make about 0.59” of stroke on the slave. Currently I'm getting about 0.5", but I can't find any specs for minimum clutch fork travel. I haven’t been able to measure slave cylinder stroke when it is malfunctioning.

One further thought: is there a way that the pressure plate could be failing/broken that would cause intermittent lack of clutch disengagement? Broken springs, bent fingers, etc? At this point I feel like I have pretty much ruled out the hydraulics, unless the seals in the master are bad.

@pithicus I used a 1.9L ALH from a 2000 Golf. There’s a build thread in my signature with full details. When the clutch works it’s a hoot to drive!

 

[malteser]

Does this issue occur when the truck is warmed up after a drive ? Maybe exhaust running too close to piping or heat shield touching the master and tranferring heat to it and causing fade... I know its a long shot but I know someone who had a similar issue and this was the cause.

 

[pithicus]

ere a way that the pressure plate could be failing/broken

I am guessing you would rather not have to pull it apart to check? Is it possible to see in there with an inspection camera?

 

[AirheadNut]

Does this issue occur when the truck is warmed up after a drive ? Maybe exhaust running too close to piping or heat shield touching the master and tranferring heat to it and causing fade... I know its a long shot but I know someone who had a similar issue and this was the cause.

I have had it occur both after driving for a while and first thing in the morning after a cold start. The clutch lines are definitely close enough to the downpipe to get hot, so it can’t hurt to add some heat shielding, but I don’t think that’s the cause. It may be a contributing factor though.

I am guessing you would rather not have to pull it apart to check? Is it possible to see in there with an inspection camera?

You guess correctly! The only opening in the bellhousing is where the clutch fork goes through, so it’s pretty tight, but an inspection camera would probably fit. I may have a friend who has one that i could borrow.

 

[AirheadNut]

This evening when I started the Prado to pull it into the shop after sitting outside all day it wouldn't shift into gear again. I shut the engine off, pumped the clutch and shifted gears a couple of times, and restarted; still no luck. Repeated the above procedure and it worked perfectly, shifted easily into 1st and reverse. No discernible change in pedal feel. I'm becoming convinced that it has to do with the throwout bearing or pressure plate, or both. If I can't get my hands on a borescope within the next couple of days I will probably just drop the transmission and have a look.

 

[AirheadNut]

I pulled the rubber boot off the clutch fork to see if I could get a glimpse inside the bell housing and found that the resting position of the clutch fork is nearly at the rear edge of the opening in the bell housing.

When the pedal is depressed the fork is almost in contact with the edge of the opening, and there are even marks in the aluminum where it looks like there has been contact.

When I looked at the rubber boot I noticed that the clutch fork has chewed through it in one place. I kept the boot off and haven't had another issue in over a week of driving.

I plan to grind a little off the edge of the opening in the bell housing to allow the clutch fork room to travel back. That said, it seems like the clutch fork should be more centered in the opening; unfortunately I don't have any photos of when I first put it together to compare it to but I'm pretty sure it wasn't that bad. I have been noticing that the throwout bearing rattles a lot in neutral, is there any reason that some component in the bell housing could have worn and changed the position of the clutch fork?

 

[pithicus]

could be just fatigue on the tines in the pressure plate [re.changed the position of the clutch fork?] I little grinding might be enough to keep you going for years.

Also the boot pinching could explain your intermittent problems.

 

[AirheadNut]

could be just fatigue on the tines in the pressure plate [re.changed the position of the clutch fork?] I little grinding might be enough to keep you going for years.

Also the boot pinching could explain your intermittent problems.

Yes, I wondered about the pressure plate tines wearing or bending, but wasn't sure if that was something to be concerned about or if its relatively common. I do think the boot was occasionally shifting and causing the intermittent failure.