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Discussion Starter #1
Hi everyone...

I recently returned from a 5500 mile month long trip out west towing my Airstream. My truck developed a new issue, and it started suddenly when driving solo; NOT pulling the trailer. My truck is a 6spd manual and has Lonnie's LS7 clutch that he installed in 2012 along with 4.56 gears. I would guess the clutch has 25-30k miles on it. Truck itself has just over 60,000 miles on it. Clutch has been 100% trouble-free as has been the transmission. The fluid in the clutch is frequently flushed (before every trip and in the middle of every trip with Motul RBF600) and I last flushed the tranny fluid in 2015 with Dexron III. This problem I'm about to describe ONLY occurs when downshifting to slow the vehicle, never during upshift and never during downshifting to accelerate.

I frequently downshift through the gears to slow down the truck coming down big hills/climbs, especially pulling the trailer. I've been doing this for decades with all my vehicles without issue. I've never rev-matched until I had this problem (read on).

When I was in Ouray, Colorado on Hwy 550 (one of the steepest/curviest switchback roads in the country) driving the truck alone I noticed a grinding vibration in both the clutch pedal and gearshift when I would let out the clutch to downshift to slow down coming down the big climbs. There was no one instance I can point to when it started. The vibration/grinding sound does NOT occur when actually OPERATING the only occurs when letting out the clutch to downshift after said gear is selected. The grinding vibration is accompanied by a slight grinding noise. It only does it when I start to let the clutch out and whatever gear I've chosen begins to engage. It DOES seem to do it more frequently if I'm downshifting to third gear but it will do it downshifting to any gear-though not as prominently. There is a "sweet spot" in the clutch pedal travel where the grinding vibration occurs. It only does it right when the clutch starts to "grab" and ONLY when I try to downshift to SLOW the vehicle. If I downshift to speed up, it NEVER does it. Also, if I rev-match when downshifting to slow the vehicle, it doesn't grind. I push the clutch in, give it a little throttle, and let the clutch out. It works silently like that. But, rev-matching when trying to slow down isn't ideal. Also, the grinding vibration occurs ALOT more when the transmission and clutch are cold (i.e., just starting out). The warmer everything is, the less likely to grind, even if you try to make it do it. never does it on upshift.

I'm not sure if the problem is the clutch or the transmission. The clutch works as smooth as the first day it was installed when upshifting. Smooth as butter. There is no throw-out bearing noise, and I think if it were the throw-out bearing it would do it any time the clutch was engaged or disengaged. But, I'm not sure. There is no burnt clutch smell, no problems with travel or change in release-point. It grabs at the same spot it always did.

I was thinking it also might be a synchronizer failure in the transmission since when I rev-match on downshift it doesn't exhibit the problem . But, I can put the transmission in any gear upshifting or downshifting without the clutch with no problem (the old way where you would nudge the shifter a little and wait for it to "fall" into gear without the clutch). I would think if there were a synchronizer issue it wouldn't slide into any gear without the clutch as easy as it does. But, again I'm not sure. The transmission shifts perfectly smooth and is silent. There is never a problem with it NOT going into any gear. Even though something may vibrate/grind, the tranny will always actually downshift. There is no problem getting it into any gear and it always does so smoothly.

My girlfriend and I flushed the clutch in Colorado with 2 full quarts of RBF-600, probably over 1000 pumps of the clutch through 5 flush cycles to get the old fluid out. Even after just a couple weeks and 2500 miles, the clutch fluid had some sediment (which has always been my experience). We flushed it until what came out looked exactly like what we were putting in. This didn't eliminate the problem.

My plan is to drain the transmission and look for any bits of metal in the fluid. Also a possibility is that the fluid has moisture in it. My truck sat at Eric's for a few years and no telling what happened while it sat. The vent is not too high and I have washed the underside as it was filthy. I could have got some water in it. My rear end had some moisture in it after sitting at Eric's (I flushed that back in April). In fact, before this last trip, I changed all the fluids in the truck except for the tranny (since I just flushed it in 2015). The fact that the problem isn't nearly as prominent when warm lends credence to possibly contaminated fluid (I'm cautiously optimistic but realistic that this might be what it is). I have a 2002 VW Beetle that got some water in the manual tranny from washing the engine (stupid vent design allows water intrusion), and it would barely downshift at all (but it didn't grind). Changing the fluid fixed the problem in that vehicle immediately.

I read an account of a similar vibration/grinding issue in a T-56 transmission on the F-body forums. But, his vibration/grinding was exhibited on upshift as well and turned out to be a badly worn clutch down to the rivets. But, there are lots of similar downshifting grinding/vibrating T-56 tranny issues documented on the LS1 forums that turned out to be worn synchros and friction cones. My problem started suddenly. Hope it's not the tranny.

The LS7 clutch has been great but I have concerns it might not be strong enough for a 5000lb truck pulling a nearly 5000lb trailer. Perhaps the original dual flywheel would be better for the weight, but that setup while competent always felt a bit clunky (and was more easy to smoke/slip upon takeoff pulling the trailer). But again, the LS7 setup has been fine since 2012. But 25-30k miles isn't durable enough (if it turns out to be the clutch). I haven't used the truck since 2015 and when I do use it it is only for a month long 5-6k mile trailer-towing trip once a year. Other than that it sits.

To sum up:
1) Issue started suddenly
2) Only does it on downshift to slow the truck, not when downshifting to accelerate
3) Upshifts are fine
4) Does it when downshifting to any gear but more likely to do it when downshifting to 3rd
5) Does it much, much more when cold
6) Always goes into gear regardless of grinding
7) Does NOT occur when actually OPERATING the only occurs when letting out the clutch upon downshifting
8) Problem doesn't occur if I rev-match on downshift

Any ideas?


Ex SoCal Nut
9,787 Posts
Sounds as if you have a trans problem going. Reverse load on the rotating assemblies opens up some tolerances. Not sure if the trans has full on bearings or bushings involved. Kinda the same load on the main bearings when downshifting.
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