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?

Maybe this should be in the tuning forum?

Bueller...

Intake gaskets ?

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...

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.

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.

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

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?

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.

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!

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.

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.

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.

Here are the throttle maps that I settled on for anyone that is interested.

Just remembered this thread. In case anyone is interested, the bucking was fixed when I swapped my winters on... so this should be moved from tuning.

 

Turns out, the tires I was running were just over 1/32nd off, front to back. Apparently that's enough to cause a light bind in the drivetrain, which was releasing at very low speed with light throttle inputs. Running new tires, the binding / on-off feeling is completely gone. Thanks to SBT for mentioning this as a possibility in another thread.

^ very interesting...