Subaru Legacy Forums

Subaru Legacy Forums (http://legacygt.com/forums/index.php)
-   Tuning (http://legacygt.com/forums/forumdisplay.php?f=84)
-   -   Very light throttle low speed bucking (http://legacygt.com/forums/showthread.php?t=212944)

LatentWagen 08-22-2013 08:51 PM

Very light throttle low speed bucking
 
When creeping in traffic, 1st gear engaged, the car has a tendency to buck noticeably I give it very low throttle. It seems like the trip point is a bit less than 10%, and it always occurs at that exact throttle position.

The car feels as if it's going back and forth between coasting and engaged full decel, as if I were pulsing the throttle back and forth very quickly. I can usually get it to do this on command, but only after the car is completely warmed up, and again, only with the lightest possible amount of throttle in that one spot <10%. The car is a bone stock '08 spec B with 142,000 miles of highway tripping. It runs like a top otherwise.

I have a few suspicions: electronic throttle/tps sensor, throttle body, and/or a bad ground.

Anybody with similar experience?

LatentWagen 08-23-2013 09:21 AM

Maybe this should be in the tuning forum?

LatentWagen 08-28-2013 10:44 AM

Bueller...

GTPpilot 08-28-2013 11:07 AM

Intake gaskets ?

RallyMachine 08-28-2013 12:24 PM

I have this too and I'm stage 3/tuned. it happens in 1st and 2nd for me normally only on a very flat stretch of road.

Has this always been a problem or a recent development?

I think it might be the ecu alternating back and forth between cells in one of the map's parameters because all conditions are being held right on the transition point (be it load, rpm, etc.). The difference between the two cells must be so different that it causes the bucking.

Edit: ^^This is just a guess, I believe the ecu interpolates the values in between the cells but maybe there is a condition where the transition isn't smooth...

LatentWagen 08-28-2013 02:04 PM

Quote:

Originally Posted by RallyMachine (Post 4562113)
I have this too and I'm stage 3/tuned. it happens in 1st and 2nd for me normally only on a very flat stretch of road.

Has this always been a problem or a recent development?

I think it might be the ecu alternating back and forth between cells in one of the map's parameters because all conditions are being held right on the transition point (be it load, rpm, etc.). The difference between the two cells must be so different that it causes the bucking.

Edit: ^^This is just a guess, I believe the ecu interpolates the values in between the cells but maybe there is a condition where the transition isn't smooth...


Sounds like we're talking about the same thing. It's a completely ON-OFF feeling.

The IM gaskets would be an possibility if the issue were cold/startup-based. Unfortunately, I think this is ECU or DBW throttle-related, rather than a mechanical issue.

DeltaNu1142 08-28-2013 02:54 PM

I think what your experiencing is... absolutely normal.

I learned to drive manual in a 2.2L Toyota pickup. Letting out the clutch with insufficient throttle from a start would cause the driveline to buck badly as the ECU fed the engine fuel in an attempt to keep it running at below-idle RPMs.

Unless you're saying that 1st gear is engaged at a higher engine speed than idle (like, during gear braking), and applying throttle causes the car to buck. In that case, I have no idea what your problem is. But if you're bucking while applying throttle at between 5-600 RPMs, I wouldn't lose any sleep over it.

700watts 08-28-2013 02:55 PM

most likely running too rich, should be in tuning thread. I had the same issue....corrected with tip in adjustment and slight fuel adjustment.

RallyMachine 08-28-2013 03:39 PM

Quote:

Originally Posted by DeltaNu1142 (Post 4562435)
I think what your experiencing is... absolutely normal.

I learned to drive manual in a 2.2L Toyota pickup. Letting out the clutch with insufficient throttle from a start would cause the driveline to buck badly as the ECU fed the engine fuel in an attempt to keep it running at below-idle RPMs.

Unless you're saying that 1st gear is engaged at a higher engine speed than idle (like, during gear braking), and applying throttle causes the car to buck. In that case, I have no idea what your problem is. But if you're bucking while applying throttle at between 5-600 RPMs, I wouldn't lose any sleep over it.

For me it generally occurs between 2-3k rpm.

Also. Doesn't tip-in only apply when the throttle plate opening is changing?

LatentWagen 08-28-2013 08:41 PM

Quote:

Originally Posted by DeltaNu1142 (Post 4562435)
I think what your experiencing is... absolutely normal.

I learned to drive manual in a 2.2L Toyota pickup. Letting out the clutch with insufficient throttle from a start would cause the driveline to buck badly as the ECU fed the engine fuel in an attempt to keep it running at below-idle RPMs.

Unless you're saying that 1st gear is engaged at a higher engine speed than idle (like, during gear braking), and applying throttle causes the car to buck. In that case, I have no idea what your problem is. But if you're bucking while applying throttle at between 5-600 RPMs, I wouldn't lose any sleep over it.

What I'm getting is not the 'oops I almost stalled' bucking you're referencing. This is occurring above that RPM and speed... for instance, 1st gear, slightly downhill, attempt to maintain ~5MPH speed, so consistent and smooth <10% throttle, ON/OFF bucking.

LatentWagen 08-28-2013 08:42 PM

Quote:

Originally Posted by 700watts (Post 4562437)
most likely running too rich, should be in tuning thread. I had the same issue....corrected with tip in adjustment and slight fuel adjustment.

Will be checking into this, thanks!

DeltaNu1142 08-29-2013 08:02 AM

Quote:

Originally Posted by LatentWagen (Post 4562856)
What I'm getting is not the 'oops I almost stalled' bucking you're referencing. This is occurring above that RPM and speed... for instance, 1st gear, slightly downhill, attempt to maintain ~5MPH speed, so consistent and smooth <10% throttle, ON/OFF bucking.

More info is helpful. Yes, this is definitely not what I described.

Willson556 09-01-2013 12:09 PM

I had a similar thing. My diagnosis was drivetrain slop coupled with an overly sensitive throttle. I changed my DBW throttle maps to make it less sensitive at the low end as well as remove the RPM dependence so it functioned more like a cable throttle car. I'll post up the tables if you're interested.

MDW25gt 09-02-2013 01:12 PM

I had similar with Stage II Cobb. Unhooked the battery over night and started it in the morning and let it warm up fully before touching the throttle. Took care of it.

Willson556 09-03-2013 04:59 AM

Throttle Maps
 
1 Attachment(s)
Here are the throttle maps that I settled on for anyone that is interested.


All times are GMT -6. The time now is 06:47 PM.


LegacyGT.com