Annoying Driveline Rubber Band Effect?

[Cinnabar325is]

I'm not sure the best term or description for this issue. I'd say it's always been present in this car, but I feel it's become worse lately.

My best description is that when letting off the throttle with the car in gear, the driveline will start slapping forward and backward, with a sort of rhythm. It is EXTREMELY annoying. I can't get it to happen 100% of the time, but it is quite frequent.

For example, say I'm cruising at 35mph in 4th gear and approach a stop sign. As I let off the throttle (no matter how gently I let off) and let the car coast in gear as the RPMs slowly drop, the driveline will start bucking. This bucking motion builds upon itself and can get pretty violent. I can stop this by de-clutching.

I'm wondering if my engine/trans mounts have something to do with this. They're probably 5-6 years old at this point, the engine mounts are stock M20 parts, the trans mounts are the green painted OEM M5 mounts I believe, installed by the PO.

Perhaps its the combo of those softer front of driveline mounts with the stiff rear mounts I have? I have AKG 80A rear subframe mounts and an OEM M Roadster/Coupe diff bushing.

Any thoughts please let me know, this is starting to really bother me.

Thanks.

[ky0u]

Any clunking sound accompanying the jerking?

[Cinnabar325is]

Any clunking sound accompanying the jerking?

Nope

[pantelones]

I'd say it's always been present in this car,


For example, say I'm cruising at 35mph in 4th gear and approach a stop sign. As I let off the throttle (no matter how gently I let off) and let the car coast in gear as the RPMs slowly drop, the driveline will start bucking. This bucking motion builds upon itself and can get pretty violent.

My guess is driver error... What is your final drive ratio? And a g260 right? If you are in 4th gear slowing down from 35 and stay in 4th you are out of the acceptable range for that gear ratio. Does this happen if you slow from 60 to 40 in 4th? or 45 to 25 in 3rd? I have expirenced this in my last e30, but it was strictly in a scenario like you described (very rare...), or occasionally in 1st in a parking lot when I violently let off the gas at low speed. I had brand new motor/trans mounts tightened down properly, new drive shaft and associated parts, mint transmission, and a decent diff that made next to no noise (was running AKG 80a sub frame/diff mount). Not sure what to tell you, but I know of the "bucking" you speak of and it only happened due to my driving error.

[Cinnabar325is]

My guess is driver error... What is your final drive ratio? And a g260 right? If you are in 4th gear slowing down from 35 and stay in 4th you are out of the acceptable range for that gear ratio. Does this happen if you slow from 60 to 40 in 4th? or 45 to 25 in 3rd? I have expirenced this in my last e30, but it was strictly in a scenario like you described (very rare...), or occasionally in 1st in a parking lot when I violently let off the gas at low speed. I had brand new motor/trans mounts tightened down properly, new drive shaft and associated parts, mint transmission, and a decent diff that made next to no noise (was running AKG 80a sub frame/diff mount). Not sure what to tell you, but I know of the "bucking" you speak of and it only happened due to my driving error.

I see what you're saying, maybe I didn't give the best example. Normally I would downshift from 4th gear before 35 mph. But whatever the speed and gear may be, I don't think the car should behave this way.

I understand that there is an acceptable "speed range" for each gear, particularly upon acceleration, but just the same, from a strictly scientific or mechanical point of view, I don't see why coasting down in gear would suddenly cause a car to start bucking because of the gear ratio.

Two other things that crossed my mind, maybe this could be a symptom of a lightweight flywheel not damping the drivetrain as much as the OEM unit? I have an 8lb aluminum JB racing flywheel. Also, the guibo is about 6 years old, I had it out 2 years ago and it was in good condition then.

I should add that I've never experienced these symptoms in another car.

[jlevie]

If there is no sound associated with this I'd say the bucking is from an engine management problem.

[AndrewBird]

Does the shift knob move around when this happens?

[Cinnabar325is]

If there is no sound associated with this I'd say the bucking is from an engine management problem.

Hmm... maybe you're on to something? My throttle body is a bit funky because the plastic sleeve inside the spring on the main pivot is broken and rigged together... maybe that's inhibiting its motion. Granted it doesn't seem to when I move it by hand, but who knows? I just bought a good, used one to clean up and install.

Does the shift knob move around when this happens?

Nope

[kronus]

I had this happen and then it turned out my giubo was in 3 pieces. Check your giubo.

[AndrewBird]

If the shifter doesn't move around when it happens, then the engine is likely not moving around either. My vote is for engine running problems.

[pantelones]

I understand that there is an acceptable "speed range" for each gear, particularly upon acceleration, but just the same, from a strictly scientific or mechanical point of view, I don't see why coasting down in gear would suddenly cause a car to start bucking because of the gear ratio.

Under deceleration load the gear ratio can become "out of sync" with the engine speed parameters, and in-turn causes this "bucking" we are trying to diagnose. You can do this in any car, essentially (theoretically) in any gear, but 4th and 5th are usually 1:1 or less than 1:1. Add in your tire size and diff ratio and you can find out exactly how many RPM's your drive-line is turning your motor. The motor may be forced to turn at a speed less than it was ever designed to run, and the engine management may or may not be able to adapt to the circumstances. Hence, a hesitation or "bucking". This is not entirely your problem, or may not be the issue at all, but more so a combination of things like...

Two other things that crossed my mind, maybe this could be a symptom of a lightweight flywheel not damping the drivetrain as much as the OEM unit? I have an 8lb aluminum JB racing flywheel. Also, the guibo is about 6 years old, I had it out 2 years ago and it was in good condition then.

I should add that I've never experienced these symptoms in another car.

Is this the only car you have owned/driven that had a lightweight flywheel? I have heard that lightweight flywheels and solid clutch disks can cause this "bucking" under normal driving conditions like the scenario you have stated.

If there is no sound associated with this I'd say the bucking is from an engine management problem.

jlevie is probably right, and the flywheel may be exaggerating the symptom.

[Cinnabar325is]

Under deceleration load the gear ratio can become "out of sync" with the engine speed parametersAdd in your tire size and diff ratio and you can find out exactly how many RPM's your drive-line is turning your motor. The motor may be forced to turn at a speed less than it was ever designed to run, and the engine management may or may not be able to adapt to the circumstances.

The tach will tell me the RPM at which the motor is spinning. If I were dropping below 1000 RPM or even 1500, then maybe. But we're talking a normal range of operation here, anywhere between 3500 and 1500 RPM.

Is this the only car you have owned/driven that had a lightweight flywheel? I have heard that lightweight flywheels and solid clutch disks can cause this "bucking" under normal driving conditions like the scenario you have stated.

The car does not have a solid clutch disc. It is not the only car I own with a lightweight flywheel.

Lightweight flywheels can sometimes cause transmission chatter, which I occasionally get, but this issue is entirely different.

[Cinnabar325is]

I had this happen and then it turned out my giubo was in 3 pieces. Check your giubo.

Checked it this evening, it is in good condition.

[pantelones]

The tach will tell me the RPM at which the motor is spinning. If I were dropping below 1000 RPM or even 1500, then maybe. But we're talking a normal range of operation here, anywhere between 3500 and 1500 RPM.

The car does not have a solid clutch disc. It is not the only car I own with a lightweight flywheel.

Lightweight flywheels can sometimes cause transmission chatter, which I occasionally get, but this issue is entirely different.

Gotcha, again the more details the better. I experienced this around the 800-1500RPM range in 3rd/4th/5th in my eta and that might equate out to a higher RPM on an I motor, but from the few I's I have driven it wasn't an issue.

Does the car drive fine otherwise? Do you have a noisy trans or diff? (indications of excessive ware, or "lash") Are you running an aftermarket chip? Aside from inspecting the drive-shaft, and taking a good look over the motor I am stumped as to what could be causing this... Good luck?

[NJ2.5rser]

I have been experiencing the same issue in my car after swapping in a built 2.8l stroker and it's driving me nuts!! Ever figure out what the issue was?


Sent from my iPhone using Tapatalk