99 TA with a LS376, what causes the idle swings???

[dermotw]

Ok, I am hoping that I missed something really obvious..

My car has quite a lot of mods, in fact more or less only the ECU and the sump are left in the engine area!

Since bolting in this engine, I made reasonable progress with setting it up (more or less from scratch) and it's quite driveable, the fueling is where I'd expect (I'm not trying any real WOT stuff at this point, and the top end timing is deliberately reduced a bit).

However, I can't get rid of the idle instability. See the log. That wasn't from fully cold but you see that it swings all over the place, then when closed loop starts, it (eventually) more or less settles (but I still think it's not right, it still cycles slightly, I reduced that by tweaking the table [12483] Closed loop prop idle gain vs. 02 err...). Playing with the startup airflow doesn't help.

I've looked at and tweaked all sorts of stuff, none of it seems to make any difference to the open loop swinging? It has a 16lb flywheel and slightly bigger injectors (36's). The MAF is recurved, the injection data loaded, the VE is done in idle area at least and when driving, the fueling does what I'd expect, given my limited knowledge, i.e. 14.5-15.5:1 at cruising/general driving, around 12:1 if I boot the throttle hard & higher rpm....

Any suggestions as to what I missed?? There has to be something obvious!

Thanks in advance...

-D

[La RPM]

it appears you would need more base idle airflow to keep a cam like that idling without surging.

[dermotw]

Nope, not that, I played with it already, via the RTT...

I got rid of the initial startup surges when I realised (today!) that in fact there are 2 Startup Airflow tables, and the one that it appears I needed to change is "Friction Airflow Initial". The other one, "Startup Airflow initial" appears to do something different, despite the name (and of course, it was that one I originally changed... oops). But what does the other one do?? In the standard tune, it's all zero'd...

Capture.JPG

However, the original issue is still there, ok, it's not bad, but...?

-D

[mowton]

The Friction air is a short "burst" of air at the very beginning of the startup cycle to compensate for metal to metal/cold oil loads...essentially get the rotating parts turning. You can control the delay (point at which the initial value begins to decay..cam revolutions) and the decay rate, Startup air varies from platform to platform but in Gen III's I find about 7 seconds is the norm. Again you can delay the decay decay start point and the decay rate. There is a trade off between RAF and these two tables as they are somewhat interrelated. What I mean is, you can compensate for insufficient Friction/Startup by increased RAF, but you will find when you log your idle desired air you will be pulling a fair amount of air with the LT/STIT's. If so take the air out and increase your friction/startup tables if starting becomes an issue.

In all cases the values in these 3 tables will equate to some Throttle blade/IAC opening during the startup cycle. Log that as well.

Just some thoughts on the subject.

Ed M

[dermotw]

Ok, thanks for the help/info... I still suspect these tables are actually labelled "backwards" in my old '99 ECU tables 'tho.. The Friction Air one seems to be the one that handles the startup, the other one provides the burst!

I tried swapping values around between them, and the only way it seems to behave as I'd expect, is when the startup air values are in the friction table! Then it's fine (and in fact, that is how the original '99 standard tune was..)... can it be that GM also was just.. wrong..? Weird. But anyway, it behaves ok now, so hopefully I can kick the rest of it into shape!

Thanks again, D.