Chattering Clutch/Driveline Shudder

[Scott Ruffner <jpr9c@xxxxxxxxxxxxxxx>]

Hello collective,

I have an annoying problem with my E30eta which has persisted through a
number of "fixes".  I put fixes in quotes, because none of them seem to
directly address the problem, but were plausible sources of the problem.

The problem:

At low RPM takeoff (~1k-1.5k RPM) in first, the clutch seems to chatter or
is "grabby" without engaging smoothly.  This causes the entire driveline
to shudder.  Not a problem in any gear but first.  I can come off the
clutch very slowly (not quite riding it), and avoid the problem.  I can
force the clutch to slip by revving above 1.5k, and really stepping on the
gas as I let the clutch up (at what I consider a normal speed), and avoid
the problem.

Possible Problems/Fixes:

1. The driver - I could just be lousy with the clutch, but I've been
driving clutches of one kind or another since I was twelve and could reach
the pedals on the farm tractor.  I've only ever owned manual transmissions
since then.

2. Bad guibo/driveshaft/support bearing - according most of the web
literature I've read, this is where to start for this symptom.  The only
problem is that the PO had all three replaced six months before I bought
the car from him.  Nevertheless...

3. Bad Differential - the PO sold me the car because he was told his diff
was going out, and after the expense of the driveshaft/guibo/bearing
replacement, he was afraid that the car was about to get very expensive
for him.  He doesn't do his own work, and it's an '86 325e w/ 191k on the
clock- he sold it to me for the trade-in value ($1500), and is much
happier in his shiny new 328i. In fact, I had told him a year earlier that
his diff was _really_ loud.  After driving it for a little over a year, I
bought a used 3.25LSD out of an 88 535i, with <50k on it, and now it's
nice and quiet (and the _best_ performance mod).  When I dropped the
driveshaft, I put in a new guibo and center support bearing just in case
(relatively cheap parts, and I didn't want to have to go back in).

***However, I was under time pressure (using a buddy's lift in his
garage), and didn't have access to a press, so I didn't replace the
differential mount in the diff cover.  Since the mounting ears are
different for the 535 and my 325, I re-used my 325 diff cover with the old
mount in it.  Could this be the problem?  ***

Theory:

The offset of the rear mount "ear" suggests to me that this is to help
prevent torsional rotation along the driveshaft and differential.  I
wonder if I'm "wrapping up" the driveline by getting off the clutch to
fast.  When the tension (wound spring effect) on the driveline and
differential (twisting a bit in the subframe) gets high enough and the
clutch is right at the stick/no-stick point the clutch slips enough to
release the tension on the driveline/differential, and then grabs again.
As soon as it grabs, the diff twists again, and the whole process repeats.
This seems consistent with the work-arounds: revving the engine while
taking off just "overpowers" this effect, and getting off the clutch
slowly (sort of riding it) prevents the whole driveline/differential from
twisting so much.

There were no visible cracks in the rubber of the differential mount, but
do you think it's possible that an old mount has enough flex in it to
allow this? I do have the new mount, and could easily enough pull off the
diff cover and get it pressed in.  I have checked the mounting bolts on
the top of the differential and they're tight.

4. In keeping with this theory, is it possible that I need to replace the
subframe bushings?  I suppose it's possible one of these is broken in such
a way as to allow the whole subframe to twist relative to the body of the
car - that would certainly explain the "thunking" sound I get out of the
rear end when the shudder is happening.  I've always assumed the thunking
was from the exhaust moving around.

5. Bad motor mounts?  Same theory as above two.

6. Bad ICM/ICV - Both of these have demonstrated questionable behavior.
I've replaced the ICV, and most of the time the surging idle went away.
Sometimes I still get it in extreme hot weather, but generally the idle is
pretty good.  However, I'm wondering if a questionable ICM (I think mine
is) or a bad TPS could be the cause - not enough fuel mixture to the
engine at lower take-off RPMs.  I think it's possible that the more open
throttle ((>1.5k RPM) that the Motronic system is providing better "gas"
to the system, and so the engine RPMs are more stable as the load is
applied.  At lower RPMs the motronic system is off and not giving the
system enough "gas".  This seems like a stretch to me, but I'm really out
of ideas.

I'm getting ready to put the car back up on jackstands to go underneath
for a bunch of little stuff - when I did the differential replacement, I
also replaced the leaking rear trans seal, but it didn't take (I need to
reposition a new seal where it will ride on "virgin" metal on the output
flange)...and O2 sensor, and brake pad sensors, etc.  So...while I have
everything off underneath, I want to get this fixed.  Any and all ideas
are very welcome!  I love this car, and plan to get another 100K out of
it.  I don't like high RPM takeoffs (all the time), and I don't like
"riding" the clutch.

Thanks,

Scott

==================================================================
Scott Ruffner
Computer Systems Senior Engineer       Computer Science Department
ruffner@domain.elided                University of Virginia
(434)982-2219                    http://www.cs.virginia.edu/~jpr9c
--
to be removed from bmw, see /bin/digest-subs.cgi
or email "unsubscribe bmw" to majordomo@domain.elided