IAC flaring on EFI Hardware Throttle Body

[someguy360]

Hi All,

Bit of a weird one but I thought I'd reach out to the brains trust for ideas.

I'm currently tuning a 304 with an Edelbrock dual plane manifold and an EFI Hardware 750CFM throttle body, it runs the stock GM IAC with the smaller pintle (Pre LS) (taken directly from the OEM throttle body and is a known working unit).


The problem I'm experiencing is randomly at idle the IAC is shooting up to around 100-150 steps then slowly closes back down to it's resting idle state of around 10-15 steps. Also when changing from Park to Drive the IAC will flare up to around 100+ steps then drop back down over 5-6 seconds to it's resting idle position, the same thing then happens again when I go from drive back to park it will open the IAC right up and idle at 1500-1600RPM for 10 or so seconds then drop down.

To then add to that when cruising at 50/60kph in 4th the IAC starts to creep open when coasting to around 60-70 steps and basically keeps accelerating the car when my foots off the accelerator.

It cold starts perfectly with my desired warm up steps etc, and when watching the warm up process on the live dashboard I can see the steps behaving, but once the cars warmed up it's all over the place. If I disconnect the IAC and run solely off the throttle stop screw the car is perfect, the problem is I have an astra power steering pump and a BF thermo fan and I like to have the IAC to compensate for when the thermo kicks in and adds load, otherwise to be honest in the Adelaide climate I don't really need idle control that much other than for cold start in winter.

I've swapped over to my spare IAC to test to make sure it's not a fault sensor and the behavior is the same.

I've tried adjusting the maximum base parameters for drive/park etc and nothing seems to make much of a difference, the IAC just has a mind of it's own.


Any ideas would be much appreciated.

Thanks

[Gareth]

What ECU? 808?

Is the VSS connected and showing a sane number in Tuner pro?

[immortality]

I believe the normal idle setting is around the 20-30 steps? (or am I thinking V6 stuff?)

Throttle showing 0% at idle when coasting?

Try lowering the values in the IAC- Throttle Crack Offset Vs RPM table a bit.

[someguy360]

What ECU? 808?

Is the VSS connected and showing a sane number in Tuner pro?

It's a 424 PCM.

VSS is connected and accurate.

[someguy360]

I believe the normal idle setting is around the 20-30 steps? (or am I thinking V6 stuff?)

Throttle showing 0% at idle when coasting?

Try lowering the values in the IAC- Throttle Crack Offset Vs RPM table a bit.

Yeah I think anywhere from around 10 steps to 30 steps at operating temp is considered normal.

And it does idle at an acceptable level, but when I change from park to D or D to Park or if I'm coasting at cruise it keeps opening to a ridiculous amount of steps for a brief period.

Will have a look at the throttle crack offset, however it seems weird as changing from drive to park etc doesn't have any throttle crack/input when it decides it's going to flare out.

For example last night, I had it at operating temperature, reversed it into the garage and put it in park, when I put it in park the IAC went wide open to 196 steps (which revved it up to about 1800RPM) then spent the next 20 seconds slowly creeping back down to it's desired set point.

[immortality]

It's cammed right? (I can't remember what you have sorry). What kpa does it idle at?

I wonder if the IAC Kpa Vs RPM threshold needs adjustment.

[someguy360]

It's cammed right? (I can't remember what you have sorry). What kpa does it idle at?

I wonder if the IAC Kpa Vs RPM threshold needs adjustment.

Thats a good point, with 109LSA it doesn't have much vacuum at idle, it idles between 75-80kpa in park

[immortality]

This is from the 11p Overview PDF,

Closed Throttle Volumetric Efficiency (VE)
On engines with low and varying vacuum at idle it can be problematic using the
MAP sensor signal for fueling. A closed throttle VE vs RPM table has been added.
To enable the use of this table set “[xxxx] set = Closed Throttle VE Option”. The
E value from the table “Volumetric Efficiency (VE) – Closed Throttle VE vs RPM”
s used instead of the value in the main VE table whenever the throttle is closed.

You many need to play with this

[someguy360]

This is from the 11p Overview PDF,

Closed Throttle Volumetric Efficiency (VE)
On engines with low and varying vacuum at idle it can be problematic using the
MAP sensor signal for fueling. A closed throttle VE vs RPM table has been added.
To enable the use of this table set “[xxxx] set = Closed Throttle VE Option”. The
E value from the table “Volumetric Efficiency (VE) – Closed Throttle VE vs RPM”
s used instead of the value in the main VE table whenever the throttle is closed.

You many need to play with this

Cool thanks I'll have a look around tonight, I haven't used the Closed Throttle VE functionality but it's worth exploring.

I've been running the car for the last 11 months since I got it back on the road and since then I'd just been running it straight off the idle set screw and warming it up for a few minutes when I drove it as the IAC issues were annoying me.

But I've been more motivated lately to actually get proper idle control working, especially before the 355 goes in as that will be a lot more rowdy and need more help idling I reckon.

[someguy360]

Doing a bit more reading too, some ford guys with big TB's have needed to put restrictor plates in the IAC passage as they are too big and let too much air through and need a smaller passage to restrict the air flow to a more controlable level.

Looking at a pic of the throttle body it is quite a large open passage to let air in.