MY15 Idle Dip and Misfires
 

I know there have been plenty of threads on fixing the idle dip with a reflash but that was mostly geared towards the MY13's but I have an automatic MY15 and have had this problem since the day I picked it up from the dealer.

It will normally idle around 750 after coming to a stop and then after 5-10 seconds of not moving the car will shudder a bit and drop to 650-700 until I start moving again. During this time there is no knock or weird engine load changes. It normally stays in the 650-700 range but every so often I will get it where it dips to 450-500 and then revs back up to 850 only to drop to 450 then back up to 850 etc and it will repeat this until I start moving. It has nothing to do with the AC/Heat/Defroster/Fan as I literally don't ever use any of them.

The stock tune had this happen and I have used OFT Stage 1 93 on a stock car and still had the dip. Then Stage 2 E85 on a stock car and still had the dip. Then Stage 2 UEL E85 when I replaced the headers and still had the dip.

I have not taken it back to the dealer yet as I thought the '15s were supposed to have this updated so it wouldn't be something a quick flash would solve.

I read through this thread but there was no resolution unfortunately.
http://www.ft86club.com/forums/showthread.php?p=2162799

After that long winded setup....I am also getting occasional misfires. Not enough for the engine to throw a CEL but usually 2 ocurrences in Cylinder's 2 and 3 on cold starts and then occasionally cylinder 1 while driving.

I'm thinking my best bet will be to have the dealer check the coil packs as this seems to be a fairly common failure on the car and can produce these symptoms. I'll make sure to follow up with what they tell me but any info/suggestions in the meantime would be appreciated.

Thanks!

EDIT: Dealer reflash of the TCU per a service bulletin completely solved the idle dip and rpm fluctuations after putting the car in park after a ~20 min drive at highway speeds. Still having misfires on cold starts but not enough for CEL.

Sounds a bit unusual for a 2015 car, it does sound like their is an actual hardware problem probably best to see if dealer will fix it, although if its not throwing and CEL codes and not rearly rough all the time its going to be difficult to convince dealer

The only thing you could try is this tweak

Possible Idle dip and hesitation fix

http://www.ft86club.com/forums/showp...6&postcount=21

set all 3 PI/DI ratio tables to have full DI up to 0.3 g/rev load (see figure)
(changes have red box highlight) make sure you do all three tables cold/warm/hot

http://www.ft86club.com/forums/attac...1&d=1422886610

I also noticed in the OFT idle speed target tables for my 2015 MT, the idle targets are set at 700-650 RPM. I don't know if the automatic 2015 tables are like that, but it's worth looking at maybe adjusting up.

I have to look all my changes up but I changed a few things and I think it's gone now...
1. went to 14.7 FA at idle
2. put cam timing to 0 on both
3. adjusted for my short ram intake
4. raised the idle speed
5. adjusted timing

I'll have to check, but those combined to help and now I think it's fixed...

Since posting this I have had 2 cold starts. Of course with no misfires on startup and then not even a single misfire on ~30 min drives. I swear the car knows I complained on the forums lol.

I did take a look at the target idle tables A-F last night and they are all set to 700 once the coolant is up to temp. I'll mess around with that later.

Thanks for the suggestions!

Most of the time if you have a misfire (with a new car) and it doesn't set a code then it's probably bad fuel. GM cars that are flex fuel cars can tell if there is water in the fuel. But yours doesn't. I would pull the fuel filter and pour the gas in the filter in a cup and see if there's some in it. maybe put some that heat stuff in the gas tank.

I experimented a lot with trying to get the engine to run lean for fuel economy, and reducing the idle speed.

For whatever reason the ECU isn't very smart about handling the throttle when you lift off at low rpm. If you tap the throttle quickly while it's idling it'll go up a bit, then almost stumble with the rpm going below 650, then it'll go up a little, then settle back down.

The lowest rpm the engine will idle at without stumbling when you hit the gas pedal is 600. 580 will stumble. At a 600rpm idle, it'll maybe stumble under heavy alternator load. As you raise the rpm from 600 to 700 the chance of it stumbling drops to 0, assuming you didn't make some other radical change.

Cam timing is always logged as 0 at idle. Doesn't matter what you change it to.

Off idle the response is really really bad though, it's possible that you're getting annoyed by something that only mildly bothers me. Raising idle to 700 should probably take care of it though.

Just noticed my 86 does this too. Just purchased in October. MY16 at. I'm gonna pay more attention to when it occurs. I've also noticed that sometimes when I shift in manual mode, the car hesitates and jersey back. I'm thinking it's the ecu trying to learn driving conditions. It only starts jerking when I'm off the freeway or after driving pretty harsh.

Try this:

Start car in neutral and let it drop idle to 600.

Press both window switches down. Notice the idle will dip and the engine feel unsettled.

It's a normal thing that is most noticeable if you have it on an a/c setting that cycles on/off. Or for when the alternator clutch kicks on. Often the engine will try to compensate and give it a few extra revs. When it drops back down to 600 it can feel a little unsettled, but is normal for the operation of the engine.

That 'engine tries to compensate' also can be noticed when one goes with half pressed clutch / starting to go without gas input, for example when parking backwards. When rpms get too low near stall i often see some rpm waves up even if my leg is not on gas and clutch is kept pressed at same level ..

Mine is a 2012 and does it all the time

Updated the warranty thread but forgot to update this thread.

I had the dealer reflash the transmission per the service bulletin and I have no more idle dip or any fluctuations when putting the car in park after a longer drive.

I do still have misfires on cold starts, my OFT is currently broken so I can't log which cylinder/how many events. It is not enough still to trigger a CEL but you can definitely feel the car shudder/buck when it starts.

^^ Are the misfires when it first starts up, during the high rev/really loud phase? Mine does that, but immediately stops once the idle settles closer to 1000rpm.

Yep that is when the misfires happen. The cylinders that misfire on startup where typically 2 and 3 with 1 or 2 events per cylinder during the whole startup time. Very very occasionally I have had a single misfire on Cylinder 1 during regular driving when the car was fully up to temp but no shuddering when this happened. From the times that I have logged, i have yet to see a cylinder 4 misfire.

From what I can find occasional misfires during startup, especially fully cold starts, are ok if it is not triggering the CEL. I'm just trying to keep an eye on it so I can see if it starts to get worse.