up, down, up, down. is their anything that would cause idle surging besides a miscalibrated TPS or a vacuum leak. i know the TPS is set right and i MAY still have a vacuum leak. i know everyones gonna say "its a vacuum leak" but thats not my question. i just wanna know if anything else would cause this? thanks.

 

bump.

 

http://www.teamfc3s.org/info/articles/idle.html

 

yes ive read that before. i need to know if anything else causes SURGING though. you know, rpm goes up 500 RPM and then goes down 500RPM.. up and down up and down. like a vacuum leak. what else can cause it though. i dont want to know the idle circuit theory.

 

cmon... no one knows?

 

if your injectors are clogged and the car is running lean at idle.. it will hunt like you are talking about..

I see it a lot with haltech's, especially on ported motors. My car like to idle from 12.6- 12.9 anything higher than that and it hunt hunt hunts

Only real way to tell if its lean for sure is to hook it up to a wideband.

 

I've had this problem twice, and in both cases it was caused by the idle speed being set too high. The ECU uses closed loop operation of the BAC valve to control idle speed (i.e. if idle speed is too high reduce bypass airflow, if idle speed is too low increase bypass airflow), but if the idle speed is too far from the ECU's 750rpm set-point it repeatedly over-corrects as the idle speed swings back and forth past the set-point, hence the surging.

The first time was straight after the TB mod, and was fixed by using the primary throttle stop screw to lower the idle speed. The second time turned out to be a leaking BAC valve that was bypassing too much air, which raised idle speed too high.

 

thanks for the info. no BAC valve on my car though. is there anything without the bac valve that would cause it?