IAC drops to zero. idle surges 3000+

[perrone123]

hello everyone. i have a 1986 s10 that i swapped an lq4 that i got from a van. using the same ecu (p59). my truck started off with a powerglide and over the winter i swapped in a 4l80e with a Manuel valve body ( no electronics). while the truck had a the powerglide it ran well, after i finished the the 4l80e swap. my idle seems to rise up uncontrollably. there are no vac leaks no major mods other than 243 heads with aftermarket headers. the truck will idle okay if i unplug the iac at 250 count. i can place my finger in the iac port and choke the engine to stall out and still feel a vacuum on my finger with the engine off.

my current tune is setup for tuning the VE i deleted the maf. i cleaned the iac. swapped another into it still does the same thing.

i have 2 scanner recordings, 1 is with the iac plugged in and you will see the idle go up and down because of my finger in the iac port, the other one is with the iac un plugged

idle with iac unplugged at count 250.hpl
werid idle lq4.hpl

4l80e open loop VE.hpt

[perrone123]

please help

[Mr. Smith]

In case this helps:


"IAC Relearn Procedure
If you are having idle issues you may need to reset your Idle Air Control (IAC) and TPS, do this before you start drilling holes in your throttle body (TB) blade. I have used this procedure on some huge cammed cars and it worked.
1) Monitor your IAC, Idle Speed, throttle position voltage and your throttle position %.
2) With the key in the ON position, adjust your TB set screw until your throttle position is 2%.
3) Turn your key to the OFF position.
4) Unplug your Throttle Position Sensor (TPS) and IAC.
5) Turn your key to the ON position for a few seconds, and then OFF again.
6) Plug in the TPS and IAC and then turn your key to the ON position and restart logging. Your throttle position will now be reset to 0 degrees and about .45-.55 volts.
7) Start your engine, your IAC should be 40-60 for a cammed engine.
If you are not at your desired IAC position, redo the above procedures in smaller increments."

[perrone123]

okay will try. but my engine has a stock cam

[perrone123]

hellllllpppppopppppp meeeeee

[blindsquirrel]

You have to get it back into a diagnosable condition first, after all your plugging-and-poking things at random. It's probably very confused now. Trying to pinpoint problems while it's in a mixed up state is only going to lead to more confusion no matter how much you whine about not wanting to do the reset procedure.

[perrone123]

You have to get it back into a diagnosable condition first, after all your plugging-and-poking things at random. It's probably very confused now. Trying to pinpoint problems while it's in a mixed up state is only going to lead to more confusion no matter how much you whine about not wanting to do the reset procedure.

okay ill try what mr smith said

[perrone123]

In case this helps:


"IAC Relearn Procedure
If you are having idle issues you may need to reset your Idle Air Control (IAC) and TPS, do this before you start drilling holes in your throttle body (TB) blade. I have used this procedure on some huge cammed cars and it worked.
1) Monitor your IAC, Idle Speed, throttle position voltage and your throttle position %.
2) With the key in the ON position, adjust your TB set screw until your throttle position is 2%.
3) Turn your key to the OFF position.
4) Unplug your Throttle Position Sensor (TPS) and IAC.
5) Turn your key to the ON position for a few seconds, and then OFF again.
6) Plug in the TPS and IAC and then turn your key to the ON position and restart logging. Your throttle position will now be reset to 0 degrees and about .45-.55 volts.
7) Start your engine, your IAC should be 40-60 for a cammed engine.
If you are not at your desired IAC position, redo the above procedures in smaller increments."

i tired it and still nothing. my tps voltage is at .76 at 0

[perrone123]

can somebody please check my tune and scanner recording ??

[perrone123]

thanks everyone

[kevin87turbot]

It certainly could be a faulty IAC. It's got to be getting air from somewhere.
What DTC's do you have? Be sure to disable all of the transmission related DTC's.

[perrone123]

do i set the codes to first error or no error reported to delete it. because for the maf i did mil on first error

[perrone123]

the only dtcs i have are maf and tranny ones

[Mr. Smith]

i tired it and still nothing. my tps voltage is at .76 at 0

Try again/harder. Adjust the throtte (body) stop screw until you get 0.5v at idle and around 4.5v at WOT.
It might not solve your issue but do it anyway. That's ^ idle and IAC basics.

[perrone123]

i got .5v at ilde and 4.5 at wot still the engine wants to rise up. i changed the intake manifold gasket. im at a loss please can someone check my tune and recordings.