Slight surging on decel

[5_Liter_Eater]

I have posted before about bucking when in transition from little to zero throttle. This is on a '99 vette (drive by wire)

http://www.hptuners.com/forum/showthread.php?t=4523

Lately I am noticing that if I am in decel, a lot of times the motor drags and then stops dragging, like I'm pressing on the gas very lightly, but I'm not.

Also, my LTFT's are great, -1 to -3 pretty much everywhere but my decel cells are all double digit positive. I don't know if the surging is a result of the extreme positive LTrims or why they are so far off.

[EC_Tune]

Your MAF cal could be off "down low" but since you're not logging MAF Hz we don't know where you are at exactly.

Try this config: VETune
It has a bunch of status bits you should be looking at, MAF in HZ, commanded AFR, AFR Error (If you're using a wideband, if not it reads about 12.8 AFR all the time)

[jdaniels]

My car does something very similar, it wasn't bad until yesterday when I got my SF3000 stall put in... now it's completely unbearable.

[5_Liter_Eater]

Attached is a new log with the above config. I don't see anything jumping out at me.

[EC_Tune]

BTW: Why is your intake air temp reading -40 all the time?

The LTFT vs MAF Hz is showing + 20 on decel. Not normal. Usually +3 to +5 is normal. You need to work on your MAF if the VE table is good to go.

EC

[WS6FirebirdTA00]

yes, decel being that lean will cause the issue granted you have DFCO disabled

[5_Liter_Eater]

Not sure why the IAT is stuck. Attached is log from <30 minutes prior to that one and the IAT is reading fine. Maybe I had a loose connection on the ALDL because I have no DTC's for it.

I think it does come down to my MAF beign off on the lower airflow part of the scale. I will crunch some #'s and see where it needs tweaking.

[5_Liter_Eater]

Well it took a few days to get the decel fuel trims negative. They are a little more negative than I'd like (~-7 to -9) but that's OK. IT took two steps to get there. First refuced the MAF table up to 3500 Hz by 10%, LTFTs still positive and bucking reduced (maybe my imagination) but still there. Reduced the MAF table by another 10% up to 3500 Hz and FTs are negative. Now I did go against the scientific method and changed some other things. I enabled DFCO. This is apparently not enabled on fly by wire vehicles, or at least it isn't in my stock tune, so I'm not sure what the effect is on a vehicle without an IAC but in any case, the surging/bucking is gone!

I am havign some problems with stalling but I need to do some research on RAF/LTIT's etc. to see how to fix that.

Thanks to everyone! I wouldn't have thought that positive decel fuel trims were what was causing the problem. Actually I'm still not sure that is what fixed it or if it was the DFCO but in any case it's gone now and all my FTs are negative.

[EC_Tune]

I don't try to get the decel trims negative. Just not more than +5 LTFT from 15 kpa tapering to +2 or +1 by 35-40 kpa. Helps fuel economy a bunch.

BTW: I like to disable LTIT so there is no "learning". Then you only have to log STIT. When you are all done with idle trims put the stock values back in to the LTIT In Gear and P/N learning. Leave the filter values alone.

EC

[5_Liter_Eater]

I think my stalling has to do with the fact that I have enabled DFCO on the vette which has it disabled stock. It only really stalls when I push in the clutch while in decel where the RPMs are <=~2000. So I'm in DFCO and when I push in the clutch there is no fuel to keep it running. There is the whole section on DFCO Clutch Transition and it is all disabled. guess I need to play with that. Anyone know what this is exactly?

[silverTA2002]

If it's all disabled in your stock tune..... it's disabled for a reason. Don't mess.


FWIW, the bucking I've had recently was solved by hand smoothing the timing cells in those areas I was having bucking.

For me, it was under .26 g/cyl and at 1400-1800 rpm. I got rid of any sharp spark transitions, and the bucking went away mostly.

[silverTA2002]

I'm looking at your timing table now.... What's up with the decrease of 8* at 2400 and 2800 rpm?

Also, your timing seems a bit high in your decelleration cells (.08 to .2 g/cyl) at 40*. Can anyone else confirm this?

You've even got negative timing values on the table (at 400 rpm) but still....

All in all, it looks like the timing table was done with absolute values (add everywhere... subtract everywhere, etc) rather than tuning to what the engine likes.

Maybe spend some more time with the timing table.

[silverTA2002]

Just a few other things I notice about your tune....

Your VE table isn't smooth, it's a bit choppy.

And what's with your PE table? You go from 1.14 to 1.32??? (the number should be close to the same for your commanded A/F if everything else is right, like VE and MAF calibration).

[5_Liter_Eater]

Thanks for the analysis Silver.

Timing: The high octane table is exactly as it came stock with the exception of the timing taken out above .64 and 2000 RPM for the SC. The low octane table is close to stock, the stock low octane map is -12 @ 400 RPMs. My decel timing IS higher than stock on the low octane table, not sure why I did what I did there but homefully I'm not running in the low octane map .

VE: Its a bit choppy yes, but it has been smoothed a couple of times. I haven't tuned VE in a while but I remember when I did it was pretty spot on (IE: FT's were -2 to -4).

PE: I run out of MAF ~5000 RPM and need to use PE to add the extra fuel to feed the SC.

How should I go about finding what the engine likes timing-wise?

[EC_Tune]

Start with your HO spark copied to LO spark then take out a few degrees (1-3 to start) from both in the PE airflow ranges (to keep things safe naturally). Then try reducing fuel until you get KR, reduce spark in that region & then watch your airflow values - Dynamic Cylinder Air is a great one to watch! If reducing fuel & then consequently spark, gives you an increase (.01-.03 is great!) in Dynamic Cylinder air, you're making more HP because you're moving more air & giving the engine what it wants as far as air/fuel & spark. If you move the spark down and the airflow goes up, you just created more HP.
The highest efficiency/highest airflow will give you the best power. Just watch that KR! If it goes *over* 2 deg you gotta attack that area with a vengance.

[silverTA2002]

Running out of Maf, as in the Maf is maxed out?

To be honest, I don't know much about FI tuning, but I'd think it'd be highly unlikely that your supercharged timing would be the same as stock.

I do know that the WOT timing is definitely less than stock for FI, so should he decrease accordingly for part throttle timing??

I can't imagine that PE jump that you have for fueling would be the "best" way to do it, it can't make for a smooth transition. These engines like smooth transitions basically everywhere. Timing, VE, MAF, PE, idle, airflow, fueling, etc. When you've got a big variance somewhere, it causes problems.

[5_Liter_Eater]

Yes, maxing out the MAF. My '99 MAF is at 512 g/s @ 11500 Hz. So at that point the only thing to do is add to the PE. I may be going a bit overboard with the PE. This PE gave me ~12:1 to ~4500 which fell to 11:1 by 5000 and stayed there. On that dyno day my valves were floating though which has since been fixed so I need to re-dyno. I could swith to a later model MAF which would give me a little more room but some long tube headers or a smaller pulley would nix that.

Without a 2 Bar SD tune (which has only become avilable recently) this is the only way to do it and it is how FI LSx's have been tuned.

I've done some things to get to this point that maybe I haven't documented very well. I'm think I'm going to go back and reset and or re-do my VE, MAF, RAF, DFCO, etc.

[5_Liter_Eater]

Start with your HO spark copied to LO spark then take out a few degrees (1-3 to start) from both in the PE airflow ranges (to keep things safe naturally). Then try reducing fuel until you get KR, reduce spark in that region & then watch your airflow values - Dynamic Cylinder Air is a great one to watch! If reducing fuel & then consequently spark, gives you an increase (.01-.03 is great!) in Dynamic Cylinder air, you're making more HP because you're moving more air & giving the engine what it wants as far as air/fuel & spark. If you move the spark down and the airflow goes up, you just created more HP.
The highest efficiency/highest airflow will give you the best power. Just watch that KR! If it goes *over* 2 deg you gotta attack that area with a vengance.

That sounds like a very time consuming process, like something a manufacturer would do on a new engine on a dyno that could simulate all loads and RPMs. IE: hold RPMs at 4000 and a certain load, play with timing and fuel to get the greatest airflow then move to the next RPM/load cell.

For my case I can go off of some assumptions like the more timing the better, but I know that I can only get away with 13 degrees before I start to see > 2 degrees of KR so that is pretty much set. AFR has less of an effect on HP and I know that >12:1 is getting less safe and will not make much more horsepower.

Are we talking about doing the same for other fuel cells like idle, cruise and decel?

[EC_Tune]

I was really just referring to WOT runs.

But you *could* use the scanner to add & subtract spark while holding a load & watching the cyl air.

EC

[5_Liter_Eater]

OK, so enabling DFCO pretty much took care of it but when I wa in decel and pressed in the clutch the car would die so I re-disabled it. This may be why DFCO is disabled on the vette. I tried disabling the throttle cracker and follower but neither of these made a difference.

I have been playing with the decel fueling and I have gotten it ~0 but it seems the longer I drive the LTFTs get larger and larger. Add more fuel in those MAF frequencies and it settles down but is back to large positive values. The timing in the decel cells are stock for both low and high octane.

The "bucking" or slight jerking is still there. Again its almost like I'm tapping the throttle.

I've been doing a lot of MAF logging and tweaking lately and am going to go back and verify my VE particularly in the decel cells next. Maybe I just need to bite the bullet and get a wideband and 2 BAR SD OS. Once the 'autotune' capability is added to the RTT that will make it hard not to.