How To: IAC Rev Mod (Fixing the hanging RPMs)
10-13-2011, 02:58 PM
#81
so i did this mod and it friggin awesome! totaly works and idle was not effected only thing is you WILL have more backfire i had some to begin with so it was a tad more i might end up putting a bigger hole in the copper piece (put 1/4 inch hole) depending how much it bugs me or doesn't overtime.
10-05-2013, 02:50 PM
#83
TECH SAVANT
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10-05-2013, 04:55 PM
#84
1st Gear Member
Join Date: Mar 2013
Location: California
Posts: 91
My car idle hangs a bit, but I'm hesitant to do this because my car idles kind of low when im at a stop and in gear, at about 550. I imagine that doing this would cause my car to shut off since there would be less air going into the engine?
10-06-2013, 08:16 AM
#86
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10-06-2013, 08:54 PM
#87
1st Gear Member
Join Date: Aug 2013
Location: California
Posts: 102
Jeez I love this forum, I can't wait to try this trick, I'll hit the hardware store for the copper cap on the way to work tomorrow.
Cliff, you're the man, now dump that Mercedes and come back home to Mustang Land where you truly belong.
Cliff, you're the man, now dump that Mercedes and come back home to Mustang Land where you truly belong.
10-07-2013, 08:19 AM
#90
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Join Date: Dec 2006
Location: Saint Augustine, FL
Posts: 10,938
It is not the same thing, as the screw in question's function is not to control or set the idle; that is the PCM's job, using the IAC and ignition timing.
The throttle stop screw sets the position of the throttle plate when "closed" so that a certain fixed amount of air "beeds" passed the throttle plate. Properly set this is about 0.44 lb/m of air¹--not enough air to support idle. The PCM then opens the Idle Air Control valve, ideally to about 35 to 40% of fully open, to maintain the idle speed set in the tune.
In fact a better way to set the stop screw is to monitor the duty cycle of the IAC control signal and adjust the screw until the d/c is in the 35 to 40% range. It is NOT an idle adjustment screw and it should not be used as one.
Idle speed is set in the tune and the PCM will do all it can (again, using the IAC and ignition timing) to maintain that idle speed regardless of the stop screw's setting. Because of this if the stop screw is not properly adjusted all sorts of idle problems can occur.
If too far open too much air will bleed passed the plate and the PCM will not be able to reduce idle speed as needed (engine speed will hang between shifts, backfiring on throttle cut, etc.), even if it closes the IAC; the AFR at idle can go quite lean if far too much air is passing through the TB at closed throttle.
Too little bleed air and the PCM will not be able to add enough air, via the IAC, to increase idle speed when additional engine load is added (such as the AC, generator, p/s pump).
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¹ - This value is set in the tune, usually labeled Air Flow Through Throttle Body. The PCM needs to know this value, and the stop screw need to be adjusted so that the actual flow is near this value, so that it can better control AFR at idle where the MAF output is not very precise.
The throttle stop screw sets the position of the throttle plate when "closed" so that a certain fixed amount of air "beeds" passed the throttle plate. Properly set this is about 0.44 lb/m of air¹--not enough air to support idle. The PCM then opens the Idle Air Control valve, ideally to about 35 to 40% of fully open, to maintain the idle speed set in the tune.
In fact a better way to set the stop screw is to monitor the duty cycle of the IAC control signal and adjust the screw until the d/c is in the 35 to 40% range. It is NOT an idle adjustment screw and it should not be used as one.
Idle speed is set in the tune and the PCM will do all it can (again, using the IAC and ignition timing) to maintain that idle speed regardless of the stop screw's setting. Because of this if the stop screw is not properly adjusted all sorts of idle problems can occur.
If too far open too much air will bleed passed the plate and the PCM will not be able to reduce idle speed as needed (engine speed will hang between shifts, backfiring on throttle cut, etc.), even if it closes the IAC; the AFR at idle can go quite lean if far too much air is passing through the TB at closed throttle.
Too little bleed air and the PCM will not be able to add enough air, via the IAC, to increase idle speed when additional engine load is added (such as the AC, generator, p/s pump).
-------------------------------------------------------------
¹ - This value is set in the tune, usually labeled Air Flow Through Throttle Body. The PCM needs to know this value, and the stop screw need to be adjusted so that the actual flow is near this value, so that it can better control AFR at idle where the MAF output is not very precise.