2013 BRZ "slipping?" "bucking?" on highway ...
 

Good morning everyone. I'm having a weird issue with my BRZ and after working it on it for a few weeks and searching and searching forums and the internet, researching , reading service manual, I am at my wit's end ...

2013 Subaru BRZ 6MT DD - mechanically stock except for K&N drop in filter and IP coil-packs (plugs done as well) I bought the car with 52,000 miles have changed all fluids and had to replace clutch/flywheel from fused throw-out bearing, new battery earlier this year.

When driving down the highway at a constant speed I am getting an intermittent random bucking or shudder. It's not violent per se, like a bucking bull or anything like that. It's really hard to describe in text. It almost feels like maybe the tires skipping on very small different sized black-ice patches or something, but its of course on dry-pavement. I'm not getting any CELs, no obvious noises, no obvious deviations in engine RPM. I've cleaned MAF, air filter, changed fuel ...

I have no idea where to go from here. We recently moved from WY to AZ and I know I cannot currently get Ethanol free fuel here, which I always ran before, but that is going to be a difficult solution. Without any CELs I don't even know if it's a fueling issue, or maybe something in the drivetrain?? I am at a loss, but need this thing to commute to work and would like to get it fixed.

Does anyone have any suggestions? Or run into this issue before? I've went through a lot of forum posts and couldn't pinpoint this specific item...

Check tire pressures?

ETA: Have you tried turning all traction control off?

Wheel balance?

The tires and wheels are in good shape. This doesn't feel like anything tire related.

The aftermarket coil packs might clue us in on something. If you suspect its engine related, I'd get a bluetooth OBD II reader and torque app and start logging. Try logging timing advance, and short and long term (LT) fuel trims.

Or maybe you're experiencing wheel hop mixed with drivetrain slack over mixed pavement.

I'm having a hard time suspecting the engine with there being no CELs. I would really think the ECU would have picked up on something if I can feel it the way I do. Is there a specific item to search for the OBD Reader and App? I'd definitely be willing to try it.

The roads are pretty crappy here ... Any way to test your second suggestion? I would love it to just be this, but I feel it's gotten progressively worse and more frequent, so I'm not sure.

@GJR726WY I used an OBDlink bluetooth OBDII reader and "torque app" (usual recommended android phone app) to log various things from the car. You just point the app to all the different listed functions and tell it to log:

IAM ignition advance multiplier
long term and short term fuel trims
flkc and fbkc knock correction
AVCS logging

it usually takes quite a lot for the check engine light to come on.

i'm also suspicious of the coil packs/spark plugs-- aftermarket or otherwise. what plugs were used?

Don't think Torque has those PIDs built-in, he may need OBD Fusion + addon if he wants to monitor those values.

Potential misfiring? If you hold the revs in neutral, do you get the aforementioned bucking?

No, not at all.

Everything that I read about coil pack malfunctions was producing a misfire CEL.
I used the OEM spark plugs when I changed them.


I've ordered an OBD reader, I will start logging as soon as it comes in.

I'm still at a loss because half the time the thing runs like a dream. I can't seem to pinpoint anything specific causing the issue ...

Have you tried with all traction control off?

On the way home from work yesterday it finally reared it's head worse than it has to date and popped a CEL. Misfire Cylinder 1 .... I can apparently just tell something is wrong long before the computer can.

Does anyone know of anything besides the coil packs/plugs causing these misfires? The plugs are OEM so I doubt it's them ... I'm hoping IP will warrant the coils, but somehow I doubt that as well ...

The misfiring was probably within the ECU's tolerance as a CEL is only lit when a misfire continuously happens and not one-off, as I recall.

Since it seems to be getting 'worse' - electrical or connection issue?

I believe it only pops a code when it detects so many misfires within a certain period.

I mean there are a lot of things that CAN cause a misfire, coil pack is a common one. I suggest moving the coil off #1 and swapping it with a different cylinder to verify the issue.