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Discussion Starter #1
Not exactly sure where this fits, so apologies if it needs to get put in a different forum topic. I've been dealing with something on my GT that's been driving me nuts for well over a year. Basically it's that if I start from a stop and give it enough gas (doesn't need to be floored), it sounds/feels like I have a stuttering sound coming from the rear of the car. I can feel it under my seat as well. It's not a clunk like suspension, but more of a repetitive sound (similar to the rhythm of a woodpecker...like 4-5 thuds per second). The more throttle you give it, the louder and more pronounced it is. I've had it diagnosed numerous times and they can never find anything. I've had my struts replaced, bushings, axle seals, etc, which is why I don't think it's suspension. It stutters too quickly to be related to the wheel turning as the wheel isn't rotating that quickly from a stop, so I'm thinking it's more like a gear/cog slipping somewhere, something related to RPM rather than wheel rotation. If you roll into the acceleration it doesn't do it, nor does it do it if you hammer the gas while driving (I get a periodic power drop/surge when floored, but I think that's something different). I can sort of get it to do it in a second gear start, but primarily it only does it on a first gear start from a stop. After you accelerate enough, it'll just fade out and stop doing it all on its own. Drive/sport/manual shift doesn't matter. Does it with both my summer and winter wheel/tire combos. Could it be something like the ABS or some other traction control thing kicking in to limit wheel spin when it's not needed? I turned off the stability control and it still did it. Appreciate any thoughts that I can take back into my mechanic to look at.
 

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Sounds like wheel hop to me. Sort of a consequence of overall suspension stiffness and tire grip and probably other voodoo that makes the tire jump off the pavement under enough torque. G8s have historically done it when brand new, so it can't be blamed on weak shocks or bald tires. For me, wheel hop was reduced when I put a spare tire in the trunk well, but still happens from time to time.
 

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Thinking of drive shaft support bearing. Is it worst under load and less coasting?
 

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I second the carrier bearing.
 
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Discussion Starter #5
Thinking of drive shaft support bearing. Is it worst under load and less coasting?
It only happens under initial load from a dead stop (or close enough to be considered one). If you get rolling up to a couple mph and then hit it, it won't do it. Any load after you're moving (up to and including WOT) and it doesn't do it.
 

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I'd definetly be looking at the driveshaft carrier bearing.
 
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Discussion Starter #7
Sounds like wheel hop to me. Sort of a consequence of overall suspension stiffness and tire grip and probably other voodoo that makes the tire jump off the pavement under enough torque. G8s have historically done it when brand new, so it can't be blamed on weak shocks or bald tires. For me, wheel hop was reduced when I put a spare tire in the trunk well, but still happens from time to time.
I'll keep that in mind. I just wouldn't think I would get that so consistently regardless of conditions (I can't get it to do it much unless it's dry, but that generally because I will actually get the stability control to kick in as I get legitimate wheel spin) and with such a metered sound/feel to go with it. I would think wheel hop would be more random as things try to get traction. Also, it does it with very little throttle. Think normal acceleration to edge out a Toyota Corolla from a stop, not drag strip launching. ;)
 

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Current mileage?
 

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I am assuming the OP knows when his tires are spinning/hop.
 

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From the initial description, it sounded like it needed a good goosing to happen, and it was like a quick repetitive hammering. A lot like wheel hop. The hop isn't a random thing, it's rhythmic.

Now it sounds like any conditions (as long as it's dry) and not a whole lot of gas. That does not sound like wheel hop.
 

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Best test you can do for yourself. Take everything out of the console. Pul your hand on the bottom of the console and pull away so it make the thumping. If it’s the bearing, you will feel it in the bottom of the console.
 

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Carrier bearing. If nothing else, it's pretty cheap and somewhat simple to replace. And at 150k miles, I'm willing to bet it's torn pretty bad.
 
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Does it happen with traction control off? Sounds like a combo of traction control and wheel hop. Had same issue on my GXP. Turned off traction control and issue is gone.
 

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Discussion Starter #18
Does it happen with traction control off? Sounds like a combo of traction control and wheel hop. Had same issue on my GXP. Turned off traction control and issue is gone.
It still does it with traction control off. I've got an appointment next Monday to have them check the carrier bearing.
 

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I’m sure the OP knows what wheel spin and hop feels like. Can always have a friend watch
 

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I would seriously look at a transmission clutch pressure issue.
It will give you a stutter that is not present at lower throttle and under real heavy throttle may not be there also due to pressures are maxed out when computer see's certain load requirements.
Generally between 30 and 60% throttle.
I have seen some GM trans that have a metal line that actually splits, and due to slightly lowered pressure you don't notice unless you are just at the right spot then the whole car shudders and it actually thumps through the whole car like wheel hop.
First time it happens many people panic for a second thinking they hit something.
Just the right throttle and it shudders, a little or allot feels fine.
Can be hard for tech's to find because of the sweet spot that you know from being your car and they just seem to miss the right amount of throttle it takes to replicate.
A good trans shop should be able to look at PSI and even take a snapshot of when it happens and look for slippage.
You may have to drive to show them the issue while they capture it on computer.
 
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