Engine jumps 100 rpms for a couple seconds

[allan5oh]

I just recently had the clutch done, and now occasionally going up hills it will gain 100 rpms for about 1-2 seconds then come back down to normal. I do have the "low to high" grinding problem occasionally (maybe once a month?) and I was going to do that but didn't have the time. Maybe I should've. Could that be it?

I always thought it would "jump" if it was the range. Not just feel like something is slipping.

I did have an air leak problem, the blue line rubbed through but I fixed that(happened shortly after the clutch job).

[mbozz282]

I had to put a clutch in last year and the first couple hundred miles the truck did the same thing.My mechanic called the manufacture of the clutch and said sliping was normal for breaking in.After that it never did it again.maybe thats whats happening to your truck.

[Kranky]

I just recently had the clutch done, and now occasionally going up hills it will gain 100 rpms for about 1-2 seconds then come back down to normal. I do have the "low to high" grinding problem occasionally (maybe once a month?) and I was going to do that but didn't have the time. Maybe I should've. Could that be it?

I always thought it would "jump" if it was the range. Not just feel like something is slipping.

I did have an air leak problem, the blue line rubbed through but I fixed that(happened shortly after the clutch job).

If the range shift was jumping out on a hard pull you'd know it! There would be one hell of a bang and tremendous shock unloading/loading of the entire driveline, probably with secondary damage to other parts from the shock.

So, that suggests clutch slippage.

Better pull the bottom cover off the bellhousing and check for proper clearance between the release fork and throwout bearing when the clutch pedal is all the way up. If not correct, adjust the pressure plate until it is.

No sense burning up a brand new clutch.

.

[allan5oh]

Thanks guys, I'll check it out today. There is nowhere near a bang, if I was distracted I'd probably totally miss it. It feels like wheel slippage, but normally when I have wheel slippage there's no increase in rpms or speed on a hill, but the cruise actually throttles back. This was totally different, no throttle back, no change in speed (actual or gauge) but just a small increase in engine rpm.

[bob h]

I just recently had the clutch done, and now occasionally going up hills it will gain 100 rpms for about 1-2 seconds then come back down to normal. I do have the "low to high" grinding problem occasionally (maybe once a month?) and I was going to do that but didn't have the time. Maybe I should've. Could that be it?

I always thought it would "jump" if it was the range. Not just feel like something is slipping.

I did have an air leak problem, the blue line rubbed through but I fixed that(happened shortly after the clutch job).

Clutch replacement questions; Was flywheel re-surfaced? Is clutch adjustment correct? Was the clutch burnished after installation?

Who was clutch manufacturer (or rebuilder)? ...I just can't imagine why a clutch manufacturer would tell anyone that break-in slippage is normal? Oooops, wait a minute; their warranty doesn't cover blue clutches, that would fall into the category of driver abuse.

[allan5oh]

Well it turns out the slave cylinder wasn't adjusted correctly, it was putting pressure on the throwout bearing. On Volvo's the fingers that push the throwout bearing like to wear out, and you need to slowly lengthen the rod that touches the slave cylinder so that the clutch brake will work. I guess when you rebuild everything you need to shorten that rod as short as it will go. As we were shortening it the 1/2" clutch adjustment started to open up more. So then we went all the way, and adjusted the clutch back. We believe the adjustment is correct now, but now I'm worried that they put the slave cylinder rod assembly on the wrong tooth, and no amount of adjustment will compensate for that. I believe that is called being out of time, or being out of phase.

Unfortunately I don't know of any way of checking if the fingers are putting pressure on the t/o bearing. Does anyone know how?

Bob to answer your questions the clutch is a rebuild, the flywheel was machined (they said it was quite worn), and I do not know about clutch burnishing, please explain so I have a more informed conversation monday morning with the installer and Volvo.

The slave cylinder was replaced on installation, as well as the cross shaft/bushings, input shaft, tranny mounts, etc..

[allan5oh]

Oooops, wait a minute; their warranty doesn't cover blue clutches, that would fall into the category of driver abuse.

Heh, if they think it falls under that, they'll have a serious fight on their hands! I'm probably the easiest driver in the world on clutches. I don't sit at a light wearing out the t/o bearing by having the clutch to the floor, and every time I start it's in low gear even with an empty trailer. Only bobtail do I start in 1st.

[allan5oh]

Did some reading on hydraulic systems, and they do not have free play which is what I thought. However I think what was happening is the slave cylinder was pulled fully back(clutch pedal not touched) yet there was still a lot of tension on the throwout bearing. I'm going to look at it tomorrow, to see how much tension is on the rod attached to the slave cylinder. I'm assuming there should be absolutely no tension. The rod pushes back on the hydraulic system, but since the pedal is fully up the system is not closed and the rod just pushes fluid back up into the resevoir.

Am I on the right track bob?

[Kranky]

With the usual pull type clutch setups there should be 1/8" clearance between the fork and throwout bearing, however I'm not familiar with the Volvo hyd clutch setup.

In any case even if the fork is supposed to have zero clearance with that setup, you do not want the fork exerting any pressure on the throwout bearing when the pedal is all the way up, or partial disengagement and slippage will result.

.

[allan5oh]

Looked at it today and I'm very happy with it. I can move the outside of the cross shafts with my thumb, there's a little clutch adjustment indicator. The clevis rod goes into the slave cylinder about 3/8". I just have to adjust it well before the fingers move that far. Otherwise it will bottom out again and put pressure on the throwout bearing.

One thing though the little tab indicator was right in between the "clutch needs adjustment" and "normal operation" areas, right on the line. Is that ok? There's a large range of "normal operation" and a small "clutch needs adjustment" area. Do you look at it when the clutch pedal is up or depressed?

I also noticed last trip my fuel mileage increased, probably because the throwout bearing was seized before. That sucker must've been getting really hot with all that friction. You couldn't move it by hand.

[bob h]

Looked at it today and I'm very happy with it. I can move the outside of the cross shafts with my thumb, there's a little clutch adjustment indicator. The clevis rod goes into the slave cylinder about 3/8". I just have to adjust it well before the fingers move that far. Otherwise it will bottom out again and put pressure on the throwout bearing.

One thing though the little tab indicator was right in between the "clutch needs adjustment" and "normal operation" areas, right on the line. Is that ok? There's a large range of "normal operation" and a small "clutch needs adjustment" area. Do you look at it when the clutch pedal is up or depressed?

I also noticed last trip my fuel mileage increased, probably because the throwout bearing was seized before. That sucker must've been getting really hot with all that friction. You couldn't move it by hand.

That "load" left on the hydraulic linkage will wear the yoke fingers and the release bearing contact pads as well.

After new friction lining has been installed, they must be worn-in to match the new surfaces together. Burnishing is a series of aggressive clutch engagements to break in the lining to the friction surface, similar to burnishing brakes. The competent installer generally does this during their road test.

[allan5oh]

Well its slipping again this time a lot worse. First it happened thursday morning, about a mile after firing it up. I was in 10th gear at full power with a full load and it jumped from 1500 to 2000 rpms in an instant. Backed off let it coast for a while then it was fine.

This was in Georgia, by the time I got to Kentucky it was slipping quite often, 200 rpms or so going up hills at full power. So I would back off to half power. I pulled into the Pilot at Oak Grove and took the plate off to have another look. This was after parking for an hour. It was absolutely blazing hot. I touched the pressure plate and if I had left my hand there for more then a few seconds my skin would've melted.

So I left the plate off and made it home at half power. When I got closer to home I started testing it, and at full power just tapping the clutch pedal causes it to slip. Not even pushing it down 1/4" causes it to slip. I checked the clutch adjustment, linkage, everything again. I could still push in the slave cylinder, so it is not loading the release bearing.

Took it back to the shop today that did it, and they're talking about burnishing, but seemed doubtful that was a solution. I think they did do that before.

Talked to a friend of mine that knows a lot about clutches, and he pretty much gauranteed it has to come out.

[allan5oh]

It appears as though the wrong cross shaft was installed resulting in the clevis being 10 degrees out. This causes the slave cylinder to be compressed a little too much resulting in too mcuh pressure on the release bearing and bad geometry. The clutch was also not clamping correctly, something about a sleeve moving?

The truck had an update from a vertical slave cylinder to a horizontal one, and you need a different cross shaft. A vertical style cross shaft is not compatible with a horizontal slave cylinder and vice versa.