Abnormal engagement "thud" when accelerating from coasting

[plechac123]

Hello All !

Fresh owner of gt2 2020 88000 km here with LSD rear diff.
Super happy about the car and first time owner of the 8-speed AT that isnt DSG.

I have noticed that when car is coasting and I start acelerating, i can feel a small "thud"/jerk i assume that is when drivetrain reengages. It is very subtle, i only feel it in my "guts". I have noticed it only because when accelerating, the shifts are buttery smooth.

Also when putting into reverse, there is small jerk (going forward is smooth)

Is it normal, or should I take some look on the drive train ? (Defferential, AT etc ? ) I have nothing to compare it with, therefore i am looking for experience of others

 

[stoopid]

What drive mode are you in when this happens? Maybe try comfort/eco if you're not already, see if the problem persists.

I doubt this is good news, to me it sounds like a transmission issue with the clutch not engaging/slipping slightly. Sticky brake pads can also 'release' from the disc or move in place, creating a clunk sound/sensation. But coasting that wouldn't be the case unless you're braking lightly when coasting then feel it when starting to accelerate.

One of the things that would scare me buying a Stinger used is whether the prior owner beat the snot out of it, or had mods they removed for resale to 'hide' the fact they were. They may have even been experiencing this issue and 'knew the time to sell was now'. Anyway, best of luck with the problem. Hopefully you bought it from someplace that provides a used car warranty.

 

[Thomby]

What drive mode are you in when this happens? Maybe try comfort/eco if you're not already, see if the problem persists.

If it's truly Coasting (engine disengaged and at idle, "Coasting" message displayed), it'll only happen in Eco or Eco-Smart ("Smart" displayed in green). The other drive modes won't disengage the engine.

@plechac123 is this what you're talking about? I can feel it both when the engine disengages, and when it reengages, and it's more noticeable if you jump back on the pedal vs. easing onto throttle. I'm not sure if it'll always pick the highest reasonable gear for your speed (to minimize the rpm increase needed) or if hammering the throttle will make it go straight to a lower gear, but the latter would certainly make you feel it more.

Unless it's really aggressive I don't think it's a concern, there's just going to be some drag as the engine is pulled up to wheel speed. Think about putting a manual back into top gear from a cruise, without blipping the throttle.

 

[stoopid]

If it's truly Coasting (engine disengaged and at idle, "Coasting" message displayed), it'll only happen in Eco or Eco-Smart ("Smart" displayed in green). The other drive modes won't disengage the engine...

... I can feel it both when the engine disengages, and when it reengages, and it's more noticeable if you jump back on the pedal vs. easing onto throttle. I'm not sure if it'll always pick the highest reasonable gear for your speed (to minimize the rpm increase needed) or if hammering the throttle will make it go straight to a lower gear, but the latter would certainly make you feel it more.

Unless it's really aggressive I don't think it's a concern, there's just going to be some drag as the engine is pulled up to wheel speed. Think about putting a manual back into top gear from a cruise, without blipping the throttle.

I drive in comfort and can't remember having experienced this. That at least lines up with what you're saying.

 

[Snowwhite]

you can turn it off in the settings, or at least the older cars can turn it off. it's called smart coasting or something like that.

 

[plechac123]

What drive mode are you in when this happens? Maybe try comfort/eco if you're not already, see if the problem persists.

I doubt this is good news, to me it sounds like a transmission issue with the clutch not engaging/slipping slightly. Sticky brake pads can also 'release' from the disc or move in place, creating a clunk sound/sensation. But coasting that wouldn't be the case unless you're braking lightly when coasting then feel it when starting to accelerate.

One of the things that would scare me buying a Stinger used is whether the prior owner beat the snot out of it, or had mods they removed for resale to 'hide' the fact they were. They may have even been experiencing this issue and 'knew the time to sell was now'. Anyway, best of luck with the problem. Hopefully you bought it from someplace that provides a used car warranty.

I am still under factory warranty ... i just wanna get my grasp wheather it is something "faulty" or normal behavior, as i have never driven torque-convertor AT before

 

[Thomby]

I am still under factory warranty ... i just wanna get my grasp wheather it is something "faulty" or normal behavior, as i have never driven torque-convertor AT before

Maybe others can chime in with their impressions, but some level of lurch/deceleration is inevitable, unless the car were to blip the throttle and rev match before reengaging the torque converter (theoretically possible but I've only heard of on manuals, like the 370Z).

If you can get a video of it, we can see if it looks excessive, but again if it's as subtle as you say I don't think there's anything wrong. In my Stinger, the little bit of drag is less noticeable than I remember from my last automatic (10+ years ago) when I'd shift into neutral at speed, coast at idle, then reengage drive (dragging the engine speed up and the car speed down).

 

[5tinger]

Problem with smooth and refined vehicles is that minor imperfections can be amplified. I believe a subtle clunk or thud is normal when increasing acceleration from coast position. However it should be subtle enough that only driver would notice it.

 

[Ah GetFd]

Maybe others can chime in with their impressions, but some level of lurch/deceleration is inevitable, unless the car were to blip the throttle and rev match before reengaging the torque converter (theoretically possible but I've only heard of on manuals, like the 370Z).

If you can get a video of it, we can see if it looks excessive, but again if it's as subtle as you say I don't think there's anything wrong. In my Stinger, the little bit of drag is less noticeable than I remember from my last automatic (10+ years ago) when I'd shift into neutral at speed, coast at idle, then reengage drive (dragging the engine speed up and the car speed down).

"I remember from my last automatic (10+ years ago) when I'd shift into neutral at speed, coast at idle, then reengage drive (dragging the engine speed up and the car speed down)."

Know for decades as "Angel Gear"

Definition:
Angel Gear is neutral, a most dangerous practice going downhill to save fuel or to pick up speed from the weight of the vehicle & the incline. Save fuel maybe - cause accidents probably. With no engine braking this practice is exceptionally dangerous.

Coasting on the other hand is managed by Kia to immediately re engage on acceleration or braking.

 

[Thomby]

Angel Gear is neutral, a most dangerous practice going downhill to save fuel or to pick up speed from the weight of the vehicle & the incline. Save fuel maybe - cause accidents probably. With no engine braking this practice is exceptionally dangerous.

I'm not sure what gaggle of lawyers and HR ladies wrote the "most dangerous", "exceptionally dangerous" language, but it's little different from kicking a manual into neutral to coast to a redlight and save clutch wear. Maybe it comes from the days when overheating old brakes on mountain descents was a thing?

I do remember some finger-wagging in driver's ed ("you might need to floor it and forget you're not in gear!"), but I'm pretty sure everyone I knew did it to try to save gas, and I can't imagine it being in the top 50 causes of accidents...

 

[Rejay14]

I read this thread yesterday and since I was taking a 250km trip today, I figured that I would test it. I rarely use the Smart mode, mostly because I don’t know what it’s for. I usually drive in Eco or Custom (everything Sport except the seat bolsters).

I actually did find that I felt what the OP was talking about. Buttery re-engagement in Eco, minute thud re-engaging in Smart. Feathered the accelerator, so that’s not the problem

 

[killinghall]

I can confirm my 22 GT lets out a slight thud when the accelerator is pressed when in coasting mode. It used to bother me initially but no longer does.

The thud has become much less noticeable over time as well.

Nothing to worry about, I reckon. Just the transmission and engine re-coupling...

 

[Thomby]

I rarely use the Smart mode, mostly because I don’t know what it’s for. I usually drive in Eco or Custom (everything Sport except the seat bolsters).

Smart lets the car manage it based on your driving (mostly throttle input). If you baby it, Smart will stay green (Smart Eco).

A bit more throttle (in my experience, even enough for a single downshift going uphill) and it'll turn white (Smart Comfort); more aggressive throttle and it'll go red (Smart Sport).

It definitely changes throttle sensitivity and shift points, but it doesn't seem to do the seat bolsters for sport. Not sure about suspension, steering, and exhaust.