IAC question...

[XfireZ51]

Originally Posted by Ccmano

Roadster, after your recent adventures you may be the resident expert on this subject. Good info, I will go back through and re-evaluate. My idle is currently running 950-1050. As was yours, when first starting the car from cold and the throttle untouched it will settle in to about 750ish. But once driven it will not go below 950.
Thanks
H

Hans,

Of course if IAC is at 0, then u have a vacuum leak but that could be at primary throttle blade. I posted a procedure for adjusting Min Air a few months ago.
Basically u want motor to idle at just below ur target w minimal IAC intervention
ie 10-15 steps. TPS V will need to be adjusted after each change.

[Ccmano]

Dom,

I'd did the reset of the IAC motor today with no improvement. I saw your post on the Min Air Adj., that's up next. Good write up! I'll keep you posted.
H

[Hib Halverson]

Do not change the minimum air adjustment. That's a bandaid fix. You've replaced the IAC motor so you know that's not the issue. The engine ran ok at some time in the past with the existing min. air setting, so that's not an issue.

The only time IAC counts go to zero and stay there is if you have air entering the engine other than through the TB.

This can be "vacuum leaks", worn TB, primary throttle blade not returning to closed position or, maybe, wrong PCV valves.

[Ccmano]

So... I pulled the TB and it appears the issue was a combination of a sticky primary throttle plate and air leakage past the secondary Plates to a small degree. I converted the min air adjustment screw a bolt head type with counter nut. Verified all the related potential vacuum leak hoses and replaced a few. Added new TB gaskets from Jerry's (thank you as always).

Put it all back together and did the min air adjustment setting per Dom & Pauls instructions. Idle is now ~750 right at target. IAC's are running ~6 and throttle position voltage is 0.54volts. Turn the a/c on and IAC's jump to ~35 and maintain idle at ~750. A/C off and it all settles back to normal.

Operation a success!

Headers, exhaust, clutch, flywheel and engine mounts are next....

Thanks to all.
H

[XfireZ51]

Hans,

U may find a bit crisper throttle response.

[Ccmano]

Originally Posted by XfireZ51

Hans,

U may find a bit crisper throttle response.

I did actually notice that on the test drive afterwards. When the Fidanza goes in I expect further enhancement to throttle response.
H

[tpepmeie]

Originally Posted by XfireZ51

Hans,

U may find a bit crisper throttle response.

Dom,
why would that be?

[XfireZ51]

Originally Posted by tpepmeie

Dom,
why would that be?

Less bypass air. Mire air going thru throttle blade, better velocity.

[Paul Workman]

Originally Posted by XfireZ51

Less bypass air. Mire air going thru throttle blade, better velocity.

I agree - the throttle is crisper; no doubt on that. But, ya lost me on the "why" of it, unless you're throwing CAMS into the mix as well.

More air going through the throttle blades is a function of lower IAC counts...which were likely set after installing bigger cams, yes? And, as for velocity...you'll have to "splain it Lucy!"

I think the response crispness comes from (according to the FSM) the ~ .4 seconds for the SPT system to fully comply with the "ON" signal, but only in certain situations.

In normal driving i.e., the throttle is opened more or less gradually and the SPTs are activated w/o the driver being conscience of when the .4 second lag interval began. Therefore, the response time isn't relevant (or even noticed?). But! If any time the throttles are snapped to WOT, in anticipation of an immediate and FULL POWER ON response from the motor - and the quicker the better - .4 seconds lag seems like an eternity, especially when at the drag strip that guy in the adjacent lane has jumped a car length on ya!

Well, whatever the reason, the crispness IS there (the FSM says so!), and if one has an aluminum FW, the "seat-o-the-pants" meter will definitely agree every time the the throttle is whacked!

[XfireZ51]

Paul,

The crispness of the throttle or the sensitivity of the throttle to changes is not dependent on the SPT opening or closing. I'm describing what I feel even at minute throttle changes, cams or no cams. I have done that for both configurations. The IAC position is dependent on throttle opening not the other way around. There are delays and filters for IAC opening and closing that follow the throttle position.
When tuning, the objective is to minimize the ECM intervention. Prior to computerized engine management, u tuned a carb to maximize vacuum. Same concept w Min Air Adjustment except that u DO want to leave some margin for the IAC to intervene in cases such as coast down or sudden throttle closing.
In a tuning strategy, u limit the amount of IAC steps for part throttle but maximize it for WOT to get as much air as possible. So better response botom to mid range, top end more air. Can also use the delays as a means of controlling rpm drop when shifting.
When the secondaries are disabled, I eliminate the delay in the calibration.