like the title says, when having the fan on(with or without AC) my idle sits nicely at 800 but if a kill the fan i drops really low(~500ish) never let i go lower but guessing it will stall.

what can cause that? isnt it usually the other way around?

 

Have you tried adhusting your idle? Sounds if your idle screw is set low.. so when you turn on the fan.. the extra load kicks the BAC on... and when you turn off the fan, BAC stops operting and idle goes low again.

Also check for vac leaks, tps, etc.

 

just changed the tps cuz the old one was shot but havent done any calibration to it, did checj the ohm values and well didnt go as it should but the car runs fine(used to studder under load and had pulsating idle) did adust the screw on the bac to get the idle high enough altho that only changed it like +-100rpm

 

My S5 TII acted the same way. Idle had been fine for literally years since the engine was insttalled (@ 25,000 miles), but this spring, noticed the idle down around 600/700, but smooth. With the air on the idle was perfect, so having a JSpec Bac and no air adjustment screw, I raised the idle via the throttle stop screw. Everything fine.