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Old 07-18-2018, 02:32 PM   #1
Snowphun
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Location: Central CT
Default Need help diagnosing drivetrain slop/jerkiness - STI drivetrain

MY04 STI drivetrain in a 9-2x, I've owned the drivetrain for about five years and did not have this issue initially, so something has worn. It has gotten to the point where I have to be very deliberate with shifts to avoid jerking the car, feels like a rubber band loading up. Anything apart from very easy on and off the gas results in rubberband effect, *unless* I'm already on the gas a bit then it's OK. Which suggests to me there's some play that isn't an issue once the drivetrain is loaded. It's particularly difficult to drive in parking lot or traffic speeds. I've owned Subarus for twenty years so I know how they are in traffic, even with Group N mounts this is excessive and difficult to live with.

This year I've done the following:
New engine
New STI clutch and flywheel (OEM)
DSS CF driveshaft
Outrigger and diff bushings are Kartboy

Car has Group N engine and trans mounts along with an aftermarket pitch stop, they all looked good when engine was swapped. Front axles, r180 and transmission are stock, rear axles are hybrids but I don't feel any play in them. Axle nuts are tight. DCCDPro controller, disengaging does not change behavior.

There is the slightest thunk if I shift a bit more abruptly, often when pulling away from a stop or putting into second after coasting in traffic. I'm thinking it's play in either the trans or the diff, lots of searching around hasn't shown that to be a common issue for either.
With the car off the ground and in neutral, a fair amount of slop in each wheel when turned by hand before the drivetrain starts to resist and turn the other wheels. Same amount of movement from each wheel, nosiest part of the drivetrain is the rear diff though the center is pretty loud too. Turning a wheel quickly back and forth makes quite a racket, unfortunately I don't have another car to compare this to. Here's a video of the noise as a wheel is turned:

Ideas on where to look next?
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Old 07-18-2018, 03:03 PM   #2
mrsaturn7085
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Location: Portland, OR
Vehicle:
2006 Impreza WRX STI
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That looks like normal gear lash to me but if you feel the need to verify, you need to open both the 6MT and R180 and measure it. The tools are expensive, but this *can* be done in your garage if you've got the motivation. You can't diagnose this accurately just wiggling the prop. shaft like that. Check my 6MT rebuild thread if you want a walk-through of the measurement steps involved. My R180 rebuild FAQ is a sticky, as well.
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Old 07-19-2018, 04:56 PM   #3
Snowphun
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Quote:
Originally Posted by mrsaturn7085 View Post
That looks like normal gear lash to me.
I agree that the amount of movement doesn't seem outrageous, but it is loud. If this isn't the cause of the drivetrain slop, where else might it be? I can't see it being in axles and I'm running out of parts after that.
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Old 07-20-2018, 01:49 PM   #4
mrsaturn7085
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2006 Impreza WRX STI
WR Blue

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Slop is hard to gauge over the internet. You need to decide how deep you'd like to dig into this. The 6MT is not difficult to open up and work on, but many of the preloads and clearance checks DO require special tools.

If any single one of your mods caused a change, I would have to guess the carbon prop. shaft as none of the others really could. Not a diagnosis... just helping with the process of elimination.
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