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Bucking under light throttle in gears 1-3

Z

zero10

Ready to race!
Location
Vernon, BC, Canada
Is anybody else experiencing this?

I have a 2017 Golf, 5 speed, 1.8T, and I find as I try to hold a very light throttle position in a low gear (I only observe this in gears 1-3, though it might be too weak to feel in 4/5th gears) I get a strong bucking. I find this happens at what feels like 5-10% throttle, not completely off the accelerator but very light pressure. It feels like something snapping open/shut in the intake - maybe a diverter valve or something? It is at about a 2-3 second frequency.

This annoys the crap out of me because it happens at exactly the throttle position needed to hold constant speed in first or second gear on flat ground. I've tried wedging my foot against the center of the car to ensure that I am not allowing my foot to move at all when this is happening.
 
Z

zero10

Ready to race!
Location
Vernon, BC, Canada
This happens anywhere from 1800-3000RPM, haven't really tried any higher than that since I usually shift in that range. This generally happens when I'm stuck in slow moving traffic and just trying to hold a low steady speed.

I've got an obdeleven but I am not sure what to log to try to pin down what is happening - or if I even can log with it?...
 
R

robmarktoo

Ready to race!
Location
Oregon
I occasionally will get into a weird bucking syncopation at low throttle and speed, but only in 1st gear. Once was on a cold engine and once was on a very warm engine. Felt like I was just at this weird "sweet spot" which confused the ECU the one time it did it where it was really bad for a few seconds ,which was after being stuck in bumper to bumper traffic for like an hour.
 
MonkeyMD

MonkeyMD

Autocross Champion
Mine started doing this recently. Just assumed my engine and trans mounts are worn out after 65k miles. And partial throttle might be causing the engine to rock back and forth from power on to off. Nothing to support this of course. Just speculation
 
oddspyke

oddspyke

Autocross Champion
Location
Delaware
Car(s)
2016 GTI, 2018 ZL1
Any chance it's related to the intake runner flaps? If they're cycling due to being in a weird transitional spot it could make the engine act strange. There should be sufficient dead band to keep that from happening, but it would explain what the OP is describing.
 
Z

zero10

Ready to race!
Location
Vernon, BC, Canada
oddspyke said:
Any chance it's related to the intake runner flaps? If they're cycling due to being in a weird transitional spot it could make the engine act strange. There should be sufficient dead band to keep that from happening, but it would explain what the OP is describing.
Click to expand...

I am now going to google what these flaps are but by the name they sound related. Any chance you can explain more? Is this something I can log or check?

*EDIT*
Found somebody with a similar complaint here:
https://www.golfmk7.com/forums/showpost.php?p=163741

Not much to go on from there though, but that thread has pointed out something else I am getting which is a couple significant surges as I ease off the accelerator before changing gears. I figured that was just a poor stock tune / boost map, but maybe there's something up with my flaps.
 
Last edited:
R

robmarktoo

Ready to race!
Location
Oregon
MonkeyMD said:
Mine started doing this recently. Just assumed my engine and trans mounts are worn out after 65k miles. And partial throttle might be causing the engine to rock back and forth from power on to off. Nothing to support this of course. Just speculation
Click to expand...

This is what it "felt like" to me.
 
oddspyke

oddspyke

Autocross Champion
Location
Delaware
Car(s)
2016 GTI, 2018 ZL1
zero10 said:
I am now going to google what these flaps are but by the name they sound related. Any chance you can explain more? Is this something I can log or check?

*EDIT*
Found somebody with a similar complaint here:
https://www.golfmk7.com/forums/showpost.php?p=163741

Not much to go on from there though, but that thread has pointed out something else I am getting which is a couple significant surges as I ease off the accelerator before changing gears. I figured that was just a poor stock tune / boost map, but maybe there's something up with my flaps.
Click to expand...

I believe you can log demanded position and actual position, but I haven't ever done it; hopefully someone who has can chime in.

Basically there are two intake paths - one prioritizes velocity (long path) for low end torque and light throttle response, the other prioritizes maximum air flow (short path) for higher rpm power. The flaps select which is in use; cycling at low load could disrupt air flow enough to make it feel surging.

My only other guess would be a DV issue. You should be able to log DV demanded position and boost pressure to see if something is going on there. Are you on the stock DV?
 
Z

zero10

Ready to race!
Location
Vernon, BC, Canada
oddspyke said:
I believe you can log demanded position and actual position, but I haven't ever done it; hopefully someone who has can chime in.

Basically there are two intake paths - one prioritizes velocity (long path) for low end torque and light throttle response, the other prioritizes maximum air flow (short path) for higher rpm power. The flaps select which is in use; cycling at low load could disrupt air flow enough to make it feel surging.

My only other guess would be a DV issue. You should be able to log DV demanded position and boost pressure to see if something is going on there. Are you on the stock DV?
Click to expand...

Thanks for the explanation and ideas!

I am 100% stock.

I'll see what I can do with obdeleven to log those things - this morning I had it happen about 3 different times, RPM was about 2100, 2300 and 2400 each time I had it happen. It's definitely happening with the throttle not at 0% but near it, maybe 5-15% (based on pedal travel). First I need to learn to use obdeleven better - back when I had my mk4 I used VAG-COM extensively but sold it with the car and so now I've got obdeleven instead.
 
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