LT1_383

Hi everyone-

I just built a LT1 383 with the GM hot cam. I'm just wondering everyone's opinion about where to set the throttle body idle set screw versus IAC position. I have it set now with the IAC reading 70 on scan. Any tips?

Cliff Harris

The exact setting is not important. The ECM will adjust it to get the RPM it wants. The important part is to leave the ECM enough room to modify the idle speed if it needs to.

Some people say the IAC count should be 20 at idle. I like it around 50.

I think the initial IAC setting might have an effect on the RPM when the engine first starts. The ECM sets the IAC to 144 at key on. This creates a "fast idle" effect to make sure the engine stays running when it is first started.

(My personal theory here): The IAC setting works with the throttle opening set by the idle screw. If the throttle is open too much, the RPM could flare up until the ECM can control it. If the throttle opening is too small the engine could stall because it's not getting enough air to run.

ANTI VENOM

It's been awhile since I tuned an LT1 but I think you should also make sure that your TPS isn't reading above 0 at idle. I had to enlarge the holes in my TPS sensor so I could adjust it back to 0. Good luck.

LT1_383

Thanks everyone!

TPS is reading at 0 at idle. I backed the IAC counts down lower. It definitely likes it better. Cold start is much better. I don't think it had enough range for cold start. I may be fighting the tune also. I'm setting up my wide band o2 to start fixing the fuel map. I can't help but drive it. I'm loving the fresh motor!

Cliff Harris

The ECM in the LT1 checks the TPS reading at key on and stores that number in memory and calls it "idle". That's why there is no adjustment.

I'm pretty sure it shouldn't be zero. My recollection is that it should be around 0.6 volts.

ANTI VENOM

What if my foot is part way on the gas when I turn the key on? Does it call that "idle"? There has to be a cutoff somewhere.

And the 0 I was refering to was % of TPS not the voltage output.

Cliff Harris

If your foot is on the pedal at key on then the ECM will be confused about what is "idle". Check your owner's manual. I recall that it tells you NOT to put your foot on the gas pedal at key on.

The TPS percentage is calculated from the TPS voltage at startup. There is a great misunderstanding about how the ECM uses the TPS voltage. For example, my ECM uses anything over 70% as WOT, which comes out to about 3.5 volts (70% of the 5 volt reference voltage). That's why there is no point in trying to "set" the TPS to some arbitrary voltage at WOT. I have seen advice that it has to be over 4.5 volts, but that's just not true.