3

'97 Jeep Wrangler 4.0L, though I doubt the phenomenon is that specific.

Sometimes, when I accelerate normally from a light or a stop sign (at the same rate as everyone around me) while keeping the tach around idle-ish to low-cruise, it feels like the clutch alternately grabs and releases on its own about 2-4 times per second until my ground speed catches up to the engine. As soon as it locks, it's fine, or if I push it loose again, it's fine and may or may not chatter when I try again immediately. Usually, though, I get a completely smooth start the first time.

All subsequent shifts are smooth (except for the occasional operator error); it's only the start from zero speed that's noticeably a problem.

I've also noticed that both the brakes and the clutch are affected by humidity. Just after a rain, they're extra grabby in general and often take me by surprise when I first use them, even though I should know to expect it by now. Don't know if it's related or not, but I can handle that part.

  • 1
    Does this sound similar: At the first grab, right as my foot has moved back enough to slightly engage the clutch, it grabs a little too hard which causes the car to lurch forward a little bit. My foot's "instinct" is to stop pushing on the clutch and maybe even slightly press it back in. When I press it in that tiny amount the car's suspension rebounds and my foot's "instinct" is to push less on the clutch. It repeats and basically causes the vehicle to buck 3-8 times. My foot reacts like that because I'm aiming for a smooth engagement. This all happens before I can consciously react. – Zach Mierzejewski Oct 22 '15 at 23:42
4

It's a standard side effect of the combination of:

  • Not quite enough gas/rpm on the engine and
  • Lifting the clutch to biting point a little bit too quickly and
  • A little bit of play in the rest of the drivetrain components

Essentially it's a bit like bouncing a ball, and then the tiny bit of continuous effort required to keep the ball bouncing.. Or it's a bit like shutting off a tap but not all the way, and then hearing the water pipes carry on making a hammering noise. Like any resonating system that doesn't really feature anything that will dampen a vibration, hitting the right frequency at the start and maintaining a little bit of motion input will keep things bouncing..

If you're finding you can do this every time, try raising the engine speed a little, and bring the clutch up not quite as far, then don't begin bringing the clutch out (further out than biting) until the vehicle has picked up a little more speed. You're looking to make an alteration to the resonant properties of the system - a different engine speed is a different base frequency to begin with, and changing clutch behaviour is "creating less of a bang" when "bouncing the ball"/"shutting off the tap"/starting off the resonance

My partner can do this reliably in our 190,000 mile audi; nearly every set-off she makes has some drivetrain judder, whereas it seldom happens to me. We have somewhat different clutch usage habits; I tend to blip the engine speed and then catch the clutch on it's biting point as the engine is revving down, using the throttle to "catch" or lift the engine speed slightly at the set-off speed when the clutch has started biting. She lifts the clutch further/more into biting, with the engine idling or just above tickover..

Ensure your heel is on the floor and your micro clutch adjustments come from flexing your ankle rather than bending your knee, as the precision is much better from your ankle

There is, additionally, the possibility others have mooted re worn or warped clutch components meaning there's an unevenness to the friction power transfer offered by the plates, but trying a few different driving habits is cheaper than launching into a mechanical investigation. You're right that the friction components are more grabby after damp weather; this is sometimes due to surface corrosion of the friction components - a light dusting of rust that increases friction and makes it more snatchy

  • Hmm, that actually makes sense. The clutch has springs in it to lessen the hard shock of a neutral drop or something like that (never tried it), but no viscous shock absorbers or equivalent. If I hold the throttle just barely off the idle stop with the clutch all the way out (grabbed hard), I can make it buck on its own with just the springs and the inertia(s) of the system. (That sometimes happens by accident while cruising a parking lot.) So there really aren't any damping components in there. – AaronD Nov 30 '16 at 18:10
  • And good point about the friction components changing characteristics due to surface corrosion. I knew that they corroded slightly, but I thought that the surface rust would get scraped off so fast that I wouldn't notice. Maybe that stuff is tougher than I thought. – AaronD Nov 30 '16 at 18:13
2

It sounds as though your clutch is on its way out. The way you explain it, your clutch is slipping on take-off.

To test my theory, you can do the following:

  • Run the vehicle up to about 20mph
  • Put your transmission in the highest gear (probably 5th gear IIRC)
  • Try to accelerate hard (1/2 to 3/4 throttle pedal)
  • If the engine speeds up readily but the Jeep doesn't, the clutch is going out.
  • If the engine lugs down and slowly speeds up as the vehicle speeds up, your clutch is still working as it's supposed to

If it turns out to be good, it could just be how you are driving and are lugging the engine at low speed (fully engaging the clutch before the vehicle speed has caught up to the engine speed). Your description sounds more like the first options, but it might be lacking in a few small details which would lead me to the second conclusion.

  • I recently replaced the clutch because the old one was warped and didn't release completely, and I frequently use full throttle coming off entrance ramps (don't like merging at 45mph into 75mph traffic). If it was going to slip, I'd think that would be the place to do it, but it doesn't. – AaronD Oct 22 '15 at 22:40
  • @AaronD - Which leads us back to driving habits. – Pᴀᴜʟsᴛᴇʀ2 Oct 23 '15 at 0:48

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