Q's on tune - split BLM's @ idle and idle-speed

**atlantadan** · 03-24-2017

Car is a 1995 LT1, automatic, 383 with cam specs 224/230 .560/.544 112 LSA. The car's IAC's are currently around 40, blades against the throttle stop in the stock position, volts @ .67.. Initially, IAC's were around 70, so I started drilling the IAC passageway in the TB and that got me to where I am today. The problem I am having is that the car has crazy split BLM's at idle - 120'ish on bank 1 and 140'ish on bank 2, but as soon as you come off idle, everything goes to 128 (or so) as it should. Car drives great - just these splits are driving me batty. At one point, before I drilled out the TB, I was able to get them much closer at idle by opening the stop, slotting the TPS and generally fooling around with it, but I couldn't get it to idle as well as it does now. I stopped drilling because most people drilled 11/32 for their 383's and that was enough. Mine appears to want more, if I am to get the IAC's down to 25 or so (which is where I read they need to be?) but I stopped at 13/32. So drill more, or start screwing around with the throttle stop and TPS as before?

Also, the car idles at 900 warm, in gear or out, and despite having a 3k stall - it still pulls against the brakes at a light. Should I drop the idle down to 825-850 or keep it where it is?

Finally, how hard is it to adjust the idle in the tune myself? It currently calls for a 900 RPM hot idle, but I really don't want to hassle the tuner with this. He's worked his ass off tuning this car and he's got everything else just about spot-on. I have the laptop and have uploaded plenty of tunes in the past - I've just not adjusted anything myself before.

Any assistance or advice you could offer would be most appreciated.

**steveo** · 03-24-2017

Car drives great - just these splits are driving me batty. At one point, before I drilled out the TB, I was able to get them much closer at idle by opening the stop, slotting the TPS and generally fooling around with it, but I couldn't get it to idle as well as it does now. I stopped drilling because most people drilled 11/32 for their 383's and that was enough. Mine appears to want more, if I am to get the IAC's down to 25 or so (which is where I read they need to be?) but I stopped at 13/32. So drill more, or start screwing around with the throttle stop and TPS as before?

that split isn't so bad. your IAC counts are just fine, i usually aim a bit higher, but the idea is to ensure most of your air comes from the idle passages and not the throttle plates. however, if opening the plates slightly helps your blm split, go ahead and do it, usually it's the opposite. you don't have to adjust the TPS voltage for a bit of throttle plate adjustment unless you go way out of voltage range, the LT1 tps is self-adjusting.

this might be a true imbalance, and the split might be correcting it for you. use eehack to reset your BLMs and see if it runs better or worse. if it runs worse with your trims locked back at 128, then you might have to tune idle fueling per-cylinder to correct this properly.

Finally, how hard is it to adjust the idle in the tune myself? It currently calls for a 900 RPM hot idle, but I really don't want to hassle the tuner with this. He's worked his ass off tuning this car and he's got everything else just about spot-on. I have the laptop and have uploaded plenty of tunes in the past - I've just not adjusted anything myself before.

super easy. are you using my XDF? there's an in gear and in neutral table for idle speed targets, it's hard to miss.