Xfi 2.0 question on lt1

[pmrphil]

Originally Posted by L.Fite

So... I'm a carb guy, know zip about FI...
What exactly is the IAC supposed to be doing?
Is that similar to the Idle control that was to up the idle when the A/C was on, back in the carburetor days?
Or something else.

Always curious about how things work.

Idle Air Control - it's a computer controlled (sometimes poorly) air leak into the intake that a race car doesn't normally need.

[L.Fite]

Originally Posted by pmrphil

Idle Air Control - it's a computer controlled (sometimes poorly) air leak into the intake that a race car doesn't normally need.

LOL... ok so that explains what it is...
But why did the engineers decide it was needed originally?

Still curious...

[Jeff Niceswanger]

Originally Posted by L.Fite

LOL... ok so that explains what it is...
But why did the engineers decide it was needed originally?

Still curious...

With an IAC, all idle rpms are consistent. You start your car up when its 0 degrees outside, the car instantly idles at 900 RPM (or whatever).It will stay at 900 all the way through the warmup process. No need to stomp on the carb linkage to keep the choke from reving 2000 RPM in your driveway as it warms up. When you turn on or off the AC, it still stays at 900. When you go from park to drive, 900. If the engineers decide they want 1000 rpm when the air conditioning is on and in gear, but 750 when its out of gear (or vise versa), they just readjust the idle rpms in the program and the car will do whatever they want. If your in a stick car and you have to lock up the breaks at a streetlight, the IAC will catch the situation and not the the car die. If your tugging on the steering wheel and the power steering pump is binding up the engine, the IAC will keep the RPMs exactly where they need to be. With a carb, you needed to set the idle up higher to catch a lot of these situations, no need now, set it as low as you want. I'm sure somehow it saves gas, as it all adds up
There was also something to do with the emissions. I cant remember all the details, but the engines needed to idle faster while getting the 02's up and heated ( they don't work when their cold) so the closed loop could handle the emissions issues. During that short "heating period" time the emissions went out of wack. So they "Idled up" for a few seconds on a startup/restart to keep the emissions down during that short period until they switched over to Closed Loop. Usually around 7 seconds

[L.Fite]

Got it...
Kinda what I suspected...
Is it tied physically to the throttle shaft or just lean/rich the mixture via controlled air leak?...

A friend resto-modded his street car with an LS-X and it hunts at idle...
Doesn't really seem to run like I would have expected a computer controlled car to run.
Seems rich all the time too...

Sorry for the questions but I really don't know anything about modern fuel injection.
Trying to learn.

If it ain't got pills or jets I'm lost!...

[Jeff Niceswanger]

Originally Posted by L.Fite

Got it...
Kinda what I suspected...
Is it tied physically to the throttle shaft or just lean/rich the mixture via controlled air leak?...

A friend resto-modded his street car with an LS-X and it hunts at idle...
Doesn't really seem to run like I would have expected a computer controlled car to run.
Seems rich all the time too...

Sorry for the questions but I really don't know anything about modern fuel injection.
Trying to learn.

If it ain't got pills or jets I'm lost!...

Its hunting because its not adjusted right, but big cammed cars and IAC are a challenge. The IAC has to be timed and adjusted to match up with the basic fuel map. The IAC is a little stepper moto or servo. It has a pindle, basically a big needle like in a needle and seat of a carb. It screws itself backward and forward seating, unseating, screwing just a little bit out, or a lot out. The farther it backs out, the more air it allows to "Go Around" the throttle body plates which in turn increases or decreases rpms. Thays about it, their isn't much to them

[Kdw1403]

Will disconnecting the IAC hurt the performance performance of ET or MPH?

[pmrphil]

Originally Posted by Kdw1403

Will disconnecting the IAC hurt the performance performance of ET or MPH?

Nope.

[L.Fite]

Originally Posted by Jeff Niceswanger

Its hunting because its not adjusted right, but big cammed cars and IAC are a challenge. The IAC has to be timed and adjusted to match up with the basic fuel map. The IAC is a little stepper moto or servo. It has a pindle, basically a big needle like in a needle and seat of a carb. It screws itself backward and forward seating, unseating, screwing just a little bit out, or a lot out. The farther it backs out, the more air it allows to "Go Around" the throttle body plates which in turn increases or decreases rpms. Thays about it, their isn't much to them

Thanks
Makes sense.

Can I PM you some day with more questions?
(rather than stepping on this thread)

[Jeff Niceswanger]

Originally Posted by L.Fite

Thanks
Makes sense.

Can I PM you some day with more questions?
(rather than stepping on this thread)

Sure, anytime

[Dave Gantz]

Originally Posted by L.Fite

Thanks
Makes sense.

Can I PM you some day with more questions?
(rather than stepping on this thread)

Start a new thread. I'm learning. I've had an 87 5.0 Mustang since new. I still know carbs better. lol