Clunk from rear when ISG restarts

[KPB-StingerGT]

Recently on first drive of the day in the cold I get a significant thump from rear the first time the ISG restarts. Only seems to happen on the first time. Never experience this any other times. 2021 GT1 AWD.

 

[rtv900]

interesting

 

[Snicklefritz]

Turn ISG OFF and you'll never hear it again if the "thump" is connected to ISG. If you hear the "thump" again then it is something else. I typically don't worry about cold/heat extreme related noises because they go away all the other times that the temperature is not extreme.

 

[KPB-StingerGT]

Turn ISG OFF and you'll never hear it again if the "thump" is connected to ISG. If you hear the "thump" again then it is something else. I typically don't worry about cold/heat extreme related noises because they go away all the other times that the temperature is not extreme.

Thump does not occur if ISG is not active. Yes I could turn it off on cold mornings to solve issue. However it is a pretty solid clunk that comes from rear drive train. So something is not right. Hoping someone has experienced it and had it fixed.

 

[Snicklefritz]

Thump does not occur if ISG is not active. Yes I could turn it off on cold mornings to solve issue. However it is a pretty solid clunk that comes from rear drive train. So something is not right. Hoping someone has experienced it and had it fixed.

This is a first report in my experience. I don't know what would engage the rear when ISG is left ON. I've always assumed that the OFF button merely disconnects the computer module that controls the starter motor interface with the ISG app. Why that would cause any sort of issue elsewhere is an interesting question. Which is why I signed onto this thread.

 

[rtv900]

have you tried just shutting off the car and restarting it normally in those same conditions to see if it's the same?

 

[KPB-StingerGT]

have you tried just shutting off the car and restarting it normally in those same conditions to see if it's the same?

It does not happen if I shut car off, place in park, then re-start and shift into gear. I could try shutting off while in drive, place in neutral (car will not start in drive), restart, move to drive. Not sure what this would accomplish though. I think this occurs because the ISG stops the engine in drive and restarts it in drive.

 

[Snicklefritz]

I think this occurs because the ISG stops the engine in drive and restarts it in drive.

That's good reasoning. I hadn't thought of that. What we want to find out is if ISG decouples the trans with the engine off, and recouples when you get off the brake and touch the accelerator. I bet there is nothing fancy about KIA's ISG, they are all probably bog standard across the car manufacturing industry. But Ima here to learn things.

 

[Kodiack]

My thought is if was a drivetrain issue you would hear the same cluck going from park to drive, that puts the same load as the ISG does on the rear drivetrain.
If that doesnt happen I would look at the exhaust system, it could be hitting something on restart if a hanger slipped off.
Restarting the engine under load could cause abnormal movement in the exhaust you wouldnt see on normal startup.
Just a thought

 

[KPB-StingerGT]

That's good reasoning. I hadn't thought of that. What we want to find out is if ISG decouples the trans with the engine off, and recouples when you get off the brake and touch the accelerator. I bet there is nothing fancy about KIA's ISG, they are all probably bog standard across the car manufacturing industry. But Ima here to learn things.

My thought is if was a drivetrain issue you would hear the same cluck going from park to drive, that puts the same load as the ISG does on the rear drivetrain.
If that doesnt happen I would look at the exhaust system, it could be hitting something on restart if a hanger slipped off.
Restarting the engine under load could cause abnormal movement in the exhaust you wouldnt see on normal startup.
Just a thought

I tried shutting off while in drive, putting in neutral, starting and shifting into drive: No clunk.

 

[KPB-StingerGT]

I tried shutting off while in drive, putting in neutral, starting and shifting into drive: No clunk.

There is not only a clunk noise but you can feel the car jerk. And again only occurs when cold and ISG starts engine.

 

[Kodiack]

I wonder if its has something to do with the back brakes. I think they activate during ISG and then release when it restarts.

 

[Snicklefritz]

I wonder if its has something to do with the back brakes. I think they activate during ISG and then release when it restarts.

No. That is auto hold. ISG stops when you release the brake, or even won't engage if you gently press the brake. Auto hold works the same way, you have to firmly press the brake pedal to engage either feature. You can use both at the same time. Then ISG will disengage when you press the gas pedal.

 

[Alpinesting]

What tires are you running?

 

[rtv900]

There is not only a clunk noise but you can feel the car jerk. And again only occurs when cold and ISG starts engine.

If it's really only when cold and only starting in Drive with the system doing that I'd consider maybe your trans is just in need of a fluid change.
I'm guessing the system just restarts in drive no different than sitting at a light, so in that case your torque converter restarts with it in gear. Maybe with old cold fluid it's just too viscous for a smooth start but goes away once it's thinned out a bit?

How many miles you got and have you ever changed it?

 

[KPB-StingerGT]

If it's really only when cold and only starting in Drive with the system doing that I'd consider maybe your trans is just in need of a fluid change.
I'm guessing the system just restarts in drive no different than sitting at a light, so in that case your torque converter restarts with it in gear. Maybe with old cold fluid it's just too viscous for a smooth start but goes away once it's thinned out a bit?

How many miles you got and have you ever changed it?

The transmission was replaced under warranty 2,000 miles ago due to leak and slip between 1 and 2. That started after engine Re&Re to replace rear main seal. Car has 26K on it. Yes had the trunk too rattle (fixed myself following 2 TSB) and build up on rotors from crap pads....so pretty much all the common issues. I do not abuse it. Never used launch control and always let it warm up properly......

 

[KPB-StingerGT]

What tires are you running?

Round ones.......OEM

 

[rtv900]

Round ones.......OEM

nice choice going with the round

 

[Thomby]

I could try shutting off while in drive, place in neutral (car will not start in drive), restart, move to drive.

Doesn't the car automatically shift to Park if you shut the engine off in D/N/R? I think I tried it once last year, but didn't want to make a habit of it (unless I'm remembering incorrectly and it just set the Parking Brake).

What we want to find out is if ISG decouples the trans with the engine off, and recouples when you get off the brake and touch the accelerator.

I don't think there's a way to "decouple" aside from shifting into Neutral. Normally when you're at a crawl/stop, your torque converter impeller is spinning with the engine and sending a little bit of torque through the fluid to the turbine, which is how you creep forward by partially braking.

ISG only activating with a certain brake threshold is probably about ensuring you have enough brake pressure not to creep/lurch upon restart (with the added benefit that the engine restarts again before you fully release the brake, so it's ready when you hit the throttle).

I could see some combination of brake wear or pedal adjustment changing this relationship such that the engine/TC apply torque in a way that takes up the slip/slack and gives you a clunk. @KPB-StingerGT have you tried easing off the brake at various rates, e.g. just enough to restart the engine without fully releasing it? Maybe worth a pad check, brake bleed, and fluid top-off, if only to eliminate that.

nice choice going with the round

Maybe @D.J. can show off his square ones

 

[D.J.]

Recently on first drive of the day in the cold I get a significant thump from rear the first time the ISG restarts. Only seems to happen on the first time. Never experience this any other times. 2021 GT1 AWD.

My car did something like what you are describing, about 4 or 5 years ago.

Only 1nce per drive cycle (cold mornings), it would not happen more than once per cold drive cycle.. (once on the way to work, and then once again on the way home from work).

My symptoms were, on the first restart from the first ISG event, there would be what felt like an impact, as if I was gently rear-ended by a car from behind.

Very shortly after experiencing this a few times, I disabled ISG for all future drives, and I never debugged the situation, but surely it's related to the engine starting while in gear, and the torque converter spinning up

Turn ISG OFF and you'll never hear it again if the "thump" is connected to ISG. If you hear the "thump" again then it is something else. I typically don't worry about cold/heat extreme related noises because they go away all the other times that the temperature is not extreme.

Same here!
Very low concern.

Also:
Automate pressing both "Auto Hold" - "Auto Stop and go" each drive cycle. 2018 Kia Stinger

https://imgur.com/a/RltYt8v

Doesn't the car automatically shift to Park if you shut the engine off in D/N/R? I think I tried it once last year, but didn't want to make a habit of it

This is actually what I do 100% of the time.

When I park, I just hit the engine stop button (while in drive or reverse) and leave.
Since I also always use auto hold, the EPB engages automatically, while the transmission automatically goes into park. (Absolute minimal effort on my part)

Maybe @D.J. can show off his square ones

View attachment 89146

Lol... Yup, that's what our wheel Ike up here, I can not lie!.