Best way to address clunk when reapply throttle at low speeds

[jtrosky]

I have a 2012 Impala with the 3.6L LFX engine and a 6T70 6-speed transmission. Ever since I bought the car @ 19k miles, it's had a pretty annoying "clunk" when reapplying the throttle after coasting or braking at low speeds. For example, if I'm cruising along and then need to slow down for a red light which turns green before I come to a stop, when I reapply the throttle at low speeds (15-25mph), I get a very annoying "clunk". Only happens at low speeds - don't have the same issue when slowing to 40mph and reapplying the throttle, for example. Seems to be common on these vehicles, so I don't think it's a hardware issue that is specific to my vehicle.

I'm trying to come up with a way to address this. Originally, I was trying to address from the transmission side of things, but since there isn't any shifting going on when I reapply the throttle, I think I need to address it from the engine side of things. Not sure how to address is though.

Maybe by using a VERY small positive value at the zero pedal position in the driver demand tables at those speeds (so there is always a very minor positive torque value at those speeds)? Or slightly increasing the rolling idle speed at those speeds in the Rolling Idle table? I'm a noob, so these are just some best guesses at how I can address this issue. Any help would be GREATLY appreciated!! Have already tried disabling TCC lock-up at those speeds, which doesn't address the problem.

Thank you.

[jtrosky]

FYI, so far it seems like I was able to eliminate (or at least GREATLY reduce) this issue be doing a few different things:

1. Slightly lowered the Driver Demand table for throttle positions 1-12%.
2. Slightly increased the Rolling Idle RPMs in 3rd gear at low speeds.

So far, with these two changes (after a few tweaks), I haven't had the problem happen again...

Not sure if this is really a tune "problem" or if I am just covering up some sort of hardware issue, but I'm not really sure I care at this point. The car did this ever since I got it with 19k miles on it - I'm just happy to have a smoother driving car! :-)

[cp-the-nerd]

I've been told this sensation is drivetrain lash, the mechanical parts "catching up" after deceleration. Seems like you minimized the shock with those changes, good troubleshooting.

[jtrosky]

Yeah, I have heard the same thing (that it could be driveline lash). Never experienced this with any other car, but maybe it's just a result of the higher engine power and drive-by-wire in the newer cars? Not really sure.

It's amazing how long it takes to address minor issues like this - TONS of research and then TONS of time setting up the VCM Scanner to log the required into and then TONS more time to scan, review and try making changes and see what happens. RInse and repeat... Really makes you respect the work of the more advanced tuners out there!!

I really like the results of adjusting the driver demand table - really lets you customize your throttle response to the way you like it. It's not like it was on older vehicles where there was a TAC module involved (apparently, you could fry your ECM if you used values outside of the "range" of the TAC module). That is no longer an issue with these newer TAC-less setups though.

Having a lot of fun learning about this stuff and dialing in the car to my liking though...

[cp-the-nerd]

I posed this question in another thread just a minute ago, but since you're here: where is the driver demand table? I can't find it.

[jtrosky]

Responded in the other thread:

http://www.hptuners.com/forum/showth...l=1#post382213