IAC drill mod Q's
IAC drill mod Q's Since my cam swap ive had some funky idle characteristics. I Contacted PCMforless and Bryan reccomended i do this drill mod to the IAC and it should smooth somethings out.
The issues im dealing with arnt major, the cam is aggressive and i can deal with it, but if it can be smoothed out i would be alot happier. The car will idle and the tach bounces about 150rpm in either direction. When she is warmed up you go from reverse to drive or drive to reverse and rpms drop to like 400 or so. Then in park or nuetral if your give it throttle, say 2000rpm and try to hold it there it will surge up and down, cant really find a steady rpm.
Ive searched and obviously "drill mod" refers alot more to M6 cars then IAC on the TB. Im just trying to figure out how i do this, and what size drill bit to use?
If anyone can help that would be great. Thanks for reading.
The issues im dealing with arnt major, the cam is aggressive and i can deal with it, but if it can be smoothed out i would be alot happier. The car will idle and the tach bounces about 150rpm in either direction. When she is warmed up you go from reverse to drive or drive to reverse and rpms drop to like 400 or so. Then in park or nuetral if your give it throttle, say 2000rpm and try to hold it there it will surge up and down, cant really find a steady rpm.
Ive searched and obviously "drill mod" refers alot more to M6 cars then IAC on the TB. Im just trying to figure out how i do this, and what size drill bit to use?
If anyone can help that would be great. Thanks for reading.
Look at your stock throttle body and you aftermarket one. You'll see the iac port has a tiny hole in it on a stock tb. Aftermarket doesn't and also is exposed to plenum air. You want to isolate the iac path from the blades and drill a small hole too if the iac counts are high at hot idle.
Here ya go:
http://members.***.net/chipsbyal/page/idle/index.html
Another method:
http://www.noid.org/~lj/PCM%20Tutorial/SplitBLMinfo.htm
I used some magnum steel from the parts store and molded it into the passage between the IAC hole and the butterflies. I then drilled the 1/2" hole to remove the excess magnum steel in the IAC passage. Then drilled the smaller 11/64 hole all the way to the front.
http://members.***.net/chipsbyal/page/idle/index.html
Another method:
http://www.noid.org/~lj/PCM%20Tutorial/SplitBLMinfo.htm
I used some magnum steel from the parts store and molded it into the passage between the IAC hole and the butterflies. I then drilled the 1/2" hole to remove the excess magnum steel in the IAC passage. Then drilled the smaller 11/64 hole all the way to the front.
Last edited by 96LT1355Z28; Jul 17, 2011 at 09:04 PM.
i must say, this is really confusing lol.
So you drill a 1/2" hole where the bleed air hole is and then find a 1/2" peice of copper pipe that you would cut a notch out of it and stick in there?
Whats the difference between doing that and just enlarging the bleed air hole that is already there from its current size to something slightly larger?
So you drill a 1/2" hole where the bleed air hole is and then find a 1/2" peice of copper pipe that you would cut a notch out of it and stick in there?
Whats the difference between doing that and just enlarging the bleed air hole that is already there from its current size to something slightly larger?
No, I did it differently than the two methods shown. Read the description on how the IAC circut works (where the air comes from and goes with the throttle closed) However you do it is up to you. All your doing is preventing the idle air from entering the upper portion of the intake and then making a bleed hole so all the idle air isn't forced past the IAC valve under the TB.
Also, depending on the age and condition of your intake you may need to pull it off and clean/drill the actual IAC passage exits in the runners. I have heard of them ranging in size quite a bit from one to another from the factory - which even if you do the IAC mod can make the idle still act funky if one or more is clogged.
I would match a drill bit to the largest passage hole, then drill them all out to that size. Along with the TB IAC isolation mod you should notice a huge improvement in idle quality.
I would match a drill bit to the largest passage hole, then drill them all out to that size. Along with the TB IAC isolation mod you should notice a huge improvement in idle quality.
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.They are easy to miss if you aren't looking for them, and I would imagine a lot of them are clogged up by gunk on some cars.
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you really want a scan tool to read IAC counts, yours are most likely high. You want them 32 (30-35) at idle with engine at operateing temp
If your TB is stock it has a 1/8" hole. Use drill bit 1/64" bigger and drill hole, measure IAC counts...take BABY steps since 1/64" makes a big diffrence. If still high drill 1/64" larger and measure again. BTW remove TB from car when drilling
You most likely will wind up at 11/64" but don't just drill a hole that size without measureing IAC counts.
Before drilling confirm TPS is .67vdc closed and 4.5 vdc wot using a volt meter (or scan tool) first. If that is off fix first by adjusting stop screw on TB. If you have not touched the stop screw on the TB it should be fine.
Another option is to open blades SLIGHTLY with stop screw and then slot the holes on the TPS so you can rotate that and bring it back to spec. I don't do it that way but some have
Aftermarket TB's don'thave the 1/8" to begin with.
I run a stock TB bored to 52 with the drill mod. had to drill it out more with the 383 and bigger cam
If your TB is stock it has a 1/8" hole. Use drill bit 1/64" bigger and drill hole, measure IAC counts...take BABY steps since 1/64" makes a big diffrence. If still high drill 1/64" larger and measure again. BTW remove TB from car when drilling
You most likely will wind up at 11/64" but don't just drill a hole that size without measureing IAC counts.
Before drilling confirm TPS is .67vdc closed and 4.5 vdc wot using a volt meter (or scan tool) first. If that is off fix first by adjusting stop screw on TB. If you have not touched the stop screw on the TB it should be fine.
Another option is to open blades SLIGHTLY with stop screw and then slot the holes on the TPS so you can rotate that and bring it back to spec. I don't do it that way but some have
Aftermarket TB's don'thave the 1/8" to begin with.
I run a stock TB bored to 52 with the drill mod. had to drill it out more with the 383 and bigger cam
Remember the closed throttle voltage for the TPS can be anywhere from .2v to .9v, not the mythical .67v everyone seems to reference. The PCM will recognize any value between .2v and .9v. Just make sure the WOT voltage is around ~4v higher than the closed throttle voltage.
Remember the closed throttle voltage for the TPS can be anywhere from .2v to .9v, not the mythical .67v everyone seems to reference. The PCM will recognize any value between .2v and .9v. Just make sure the WOT voltage is around ~4v higher than the closed throttle voltage.
I can say for a fact that my throttle response was better with both my TBs after slotting the TPS and resetting idle to .50 after adjusting.
I'm looking at my FSM - it says between .23v and .9v. A smart person would shoot for something in the middle of this range to keep the margin of error as far away from the high/low threshold. I'm saying the mythical .67v is not as crucial as everyone makes it out to be.
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Tps voltage is what the PCM sees for, among other things, for shift points. Your tuner is assuming it is closer to .67vdc than not
If adjusting stop screw (don't recommend) you will effect tPs volts. In some cases taking it out of "ideal" range. This is why people slot the holes to rotate it back
If adjusting stop screw (don't recommend) you will effect tPs volts. In some cases taking it out of "ideal" range. This is why people slot the holes to rotate it back