Strange drivetrain problem. Need advice!

[tekdeus]

Hey guys,

Today while driving around town a problem developed and got progressively worse. When accelerating at low rpm in the lower gears, the car would bounce/surge forwards and backwards, similar to the quick back and forth movement of a car when in 1st gear and the throttle is pressed and released quickly. Higher rpms and boosting seemed fine, it was just this annoying fore and back surging bouncing when slowly accelerating, I guess you could almost call it a major drivetrain shimmy, but about 5-7 surges per second.

I noticed a couple of moderately loud clunks from underneath me, sounded like the tranny or driveshaft. I have an R154, fidanza flywheel, act 6-puck unsprung clutch, aluminum driveshaft and a torsen diff. The engine revs smoothly when not in gear, so I'm guessing it's not engine related.

I had a look underneath the car today and didn't see anything that looked loose or broken. Subframe looked ok. I tried to forcibly move the driveshaft near the tranny and it flexed a little bit against the rubber mount there, but it was not loose. When driving, it felt like something in the drivetrain was loose and flexing, causing a surging, vibrating feedback loop.

Can someone tell me what that big flat bolt is right on the bottom of the rear tranny mount? It looks like an adjustment bolt or something. Is it supposed to be flush with the rubber?

Any advice would be greatly appreciated. I couldn't find any specific info in the TSRM or search.

Thanks

 

[thedave925]

That flat nut back there is the drain. i get "the surge" at around 1500-2000rpm in first and second. kind of annoying, I'd like to know if it was designed in or if there is actually a problem. I pacify my paranoia with the idea that it's just the power stroke of the individual cylinders...

 

[tekdeus]

The bolt appears to extend through the mount and I can see the other end of the bolt hitting the upper metal of the bracket. Perhaps I should take a pic. The surge was never there before, so something is definately wrong!

That flat nut back there is the drain. i get "the surge" at around 1500-2000rpm in first and second. kind of annoying, I'd like to know if it was designed in or if there is actually a problem. I pacify my paranoia with the idea that it's just the power stroke of the individual cylinders...

 

[thedave925]

I thought you were talking about the drain plug when you said rubber. that large one in the center of the rear tranny mount facing down, mine isn't touching any rubber, instead it sticks down from the mount like a half inch. I'm waiting on the picture if you'd like to see...

 

[tekdeus]

I don't think this is a drain plug. It looks to be some kind of mount adjustment.

 

[Facime]

To stick down like that is normal I think, but it also looks like you have quite a bit of "hole" around it. from memory I dont think mine looks like that (both mine stick down a bit like that though)

regardless, I dont think thats your problem. Is your rear end a LSD? maybe the clutches are bad in your rear end? Also it could just be your engine is not running right as you describe it as a surging like letting on and off the gas. Vacuum leak? bad fuel? plugged filter, failing pump? bad pressure regulator? All kinds off possibilities. Ya really need to kinda narrow it downa bit.

 

[RacerXJ220]

Once you start "the surge," stopping it required you to let off the gas/clutch, or accellerate right?

Mine does that too.

I think it's my lead/insensitive foot that wants to give it gas when my brain told it to just chill out and go a steady speed.

 

[IJ.]

The "bolt" is a travel stop so you don't tear the mount in half under acceleration.

 

[tekdeus]

Once you start "the surge," stopping it required you to let off the gas/clutch, or accellerate right?

That's right. It's the load of the engine on the drivetrain under acceleration that causes the bouncing effect. I wonder if it's worn engine mounts. Is this a symptom of that?

 

[thedave925]

When I get my ecu hooked back in I'll check vacuum at said rpm level. Maybe that will reveal pointers...

 

[tekdeus]

Found the problem. My Torsen diff was on it's way out and had lots of slack, causing the drivetrain bounce. Good news is I know what the problem is and I can fix it. Bad news is my Torsen's gone!!!

 

[thedave925]

ugghhhh... I hope not me too, but i don't have the jza70 diff either, so we'll see. Good luck with repairs

 

[tekdeus]

Try rotating your driveshaft by hand. You'll know if your diff has lots of slack. Mine turned about 3/4" in the slack