idle air screw

MX3KEV21 · Post by **MX3KEV21** » January 2nd, 2007, 12:19 pm

does the screw on top of the throttle body control air/fuel mixture? or just the idle speed. coz i adjusted mine and the car seems to have lost a bit of power but only between 3700rpm and 4000rpm just a slight hesitation.

jschrauwen · Post by **jschrauwen** » January 2nd, 2007, 9:45 pm

NEVER EVER adjust the idle air adjust screw unless the car has been placed in a diagnostic mode. What's a diagnostic mode? - check Tunes67 post from today on how it's accomplished. That screw only lets air though a bypass gallery in the TB without moving the butterfly itself. The cause and effect of it all is that there will be an increase or decrease (depending on which way you turned the screw) in vaccum generated into the plenum. That change in vaccum pressure is recognized by the VAF further upstream. The VAF in turn notices this vaccum change and sends the applicable electrical signal to the ECU to adjust fuel levels accordingly. Those new fuel level changes will now be inconsistant with the throttle position sensor/butterfly, hence your poor performance at certain rpm levels. The idle adjust should never be adjusted unless in the diagnostic mode to set a base idle. If the base idle is incorrect, than you have to back up even furhter and do perhaps a TPS adjustment and or a setting of the timing. It would be best to place the idle adjust screw back to it's original position.
If you do have idle issues that precipitated the screw adjust, than it's best to do a search in the FAQ's for Idle, TPS and timing setting.

MX3KEV21 · Post by **MX3KEV21** » January 2nd, 2007, 9:49 pm

well i put the car in diagnostic then done the tps adjustment and then adjusted the screw till the idle was at 750rpm the took the car out of diagnostic setting and the idle sits perfect no hunting or anything. just a slight dip in power at 3900rpm but nowhere else. what could cause this problem?

jschrauwen · Post by **jschrauwen** » January 2nd, 2007, 10:05 pm

Is this 3900rpm dip detected while you are revving the engine at a standstill or while under load on the road? I addressed the possibilities in another thread.