Ok, just finished up installing my buddies 408 longblock in his 35th SSLE. I did a few things from the stock tune, like changing cylinder volume from .71xx for a 5.7L to .82 for a 6.8L (408ci???). Cold idle at 1000rpms, warm-hot is 950rpms all 4 modes. Then reduced the Main VE from 1200rpm down by .8/.7/.6/.55 (1200 on down). I increased base spark, idle spark, Hi and Lo octane spark, adjusted for the 42# inj, reduced inj min. time as well, Tq red., cleared necessary codes and whatever else. Start up was fine, it catches idle and seems good, but then it starts falling on its face, requiring a pumping of the throttle, like a carb would if it were choking out/flooding. I drilled the NW 90mm TB a new hole, small one, and it didnt help, so then I opened up both holes one more step larger, still no change. It will idle ~1000rpm, then for no apparent reason drop down to a stuttering 650'ish. When it drops, again it takes a fair amount of pedal modulation to clear it up. Fuel pressure no change 60psi. O2's fluctuate about as good as can be expected with 1 7/8-2" stepped uncoated SS Kooks Headers, mine seem a little lazier with the 1 7/8" headers for comparison.

Off idle, driving the car is a GD Beast no doubt. When you first go WOT it spits out a small puff of rich black smoke as it propels itself forward at an amazing feeling thrust like feeling. I will be trying to teach Larry how to tune since he now has his car running and HPTuners of his own. He'll probably pop in here and ask lots of questions, so go easy on him and I will try to help him out first of course. Thanks.

Charlie

 

I had to go fairly large on the hole sizes in the TB. I tried adjusting the tb set screw and got it to where the car would hold an idle. I knew then it was not getting enough air. I also had to drill the stock NW hole larger and a second hold at 9/32s. I got my IAC counts down to about 60ish and then went onto RAF and finally VE. It idles like a champ now and I have no complaints. I can't get the RAF down perfect because it won't hold an idle that way. My car is tuned in open loop SD by the way.


Dougie

 

Here are the things that i would try first.

To start, you're going to need a wideband (LC-1) so you can later work on dialing in your VE table at idle.
Next, you're going to have to put the car into OLSD (Open Loop Speed Density) and log your RAFIG from a cold start and see what the car is doing as far as cutting/adding air. If it appears that the car is cutting a lot of air, you've probably gone too far with the holes in the blade.
Next, you're going to have to dial in your VE table, a wacked out VE table will cause lots of driveability/idle issues. This will also help with your fuel trims if they're maxing out, which could be a source of your problem as well.
Once you've done that, and you're still having problems, you're going to want to try and work on your spark timing tables. If it seems like its having idle hunting problems once the engine is warmed up, you're giong to have to experiment and try lowering or raising the the spark at the idle areas. From my experience, try lowering it first, if that doesn't work or makes it worse, then try raising it. On another note for this, you want your hi/lo and base spark tables to all be the same, because large swings in these tables will cause problems. So copy with labels hi to the rest of the tables. You may also consider smoothing these tables out.
Next, you're most likely going to want to zero out all of your spark adjustments for ECT and IAT's because they tend to mess with things more then you'd want.
Once you have your idle nailed, and its solid, no hunting, try putting lots of load on the engine to see if you'll have bogging issues. (turn wheel, turning on air, defroster and lights all at the same time) If it boggs and dies, if it does, you'll have to keep working on it, if not, then you're ready to start driving and logging the rest of you VE table.
If you've done all of this, and still having problems, I found that zeroing out all of your Idle learning tables (which disables idle learning airflow corrections) will solve problems. If you find that it does, then you're going to have to tweak these as well. I had to eventually lower mine a lot to fix problems.
Now idle transitions from drive ect is a whole other story, which involves pretty much messing with the throttle cracker and follower tables. But I gotta go now, i'll check back if you've got more questions!

Best Regards,
Adrian

 

Dougie thanks man. I am not sure if the owner wants to go OLSD mode for good or not, still debating. But that would make life a bit easier in the long run.

Adrian, wow, thats a pretty comprehensive list, but I got it thanks. I would have normally gone through most of that, however, I never tuned a car with this issue. I have been into about 12 or so cars, but this one, totally threw me for a loop. Start up sometimes no issue and idles fine. Then, for an unknown reason just drops into this 650'ish or so idle and either comes out whenever it decides, or you can attempt to throttle the hell out of it. Kind of made me think I need to drill larger, but dont want to go there yet without being a little more certain.
Thanks again guys.

Charlie

 

Well not sure if it will matter or not, but the 408 is a 6.7L & a cylinder volume of 0.83749L or thats what I got anyways in my car. Some one please correct me if Im wrong. Everything you've modified is what I have cahnged. I just keep on adjusting all of them till I got the perfect idle. Have you log it to see whats going on at idle?

 

Hey Charlie,
I didn't know how experienced you were so I just thought I would give as much help as possible =D. I actually had a similar problem when coming to idle, it would just suddenly drop to like 300 rpms or something crazy like that. After about 150 tune changes, I have found that disabling the idle airflow control will solve this problem. After disabling it, you can work slowly and starting very small with the idle airflow corrections. Give that a try, which would be to set all of the Idle/learning tables to zero, and see what it does.

Best Regards,
Adrian

 

Vann, I was in a pinch and called a buddy from here who thought it was a 6.8L, I doubt that being so little off will matter, but for S&G I will update that. Thanks for that tidbit, and I will definately post if it changes anything significant.

Adrian, I really appreciate the help, please dont take it the wrong way. I havent had to disable the idle airflow corrections before, so guess this will be new territory for me.

One thing I been wondering about. If the car is idling, and I use the controls to increase the IAC Steps, and then it idles smoother, that would mean I need to drill the blade out more correct? Or is there something else to do better?

Charlie

 

No problem man. I didnt figure it would change any of your problems but just figured Id point it out. I to am having to mess with drilling the TB to get the idle set. Its becoming a real PITA. Ive adjusted & adjusted some more but with not much change. Im going to start a small szie bigger & go from there. What size hole is in the TB now?

 

I didnt measure, but IIRC its about 3/16". GL

Charlie