what exactly does the BISS screw do? does it screw it and out holding the butterfly open enough for users set idle speed? tried finding a diagram of exactly how it functions and what it does but I cant find anything? if someone could break it down that would be great!

 

its just a air bypass for the butterfly.

so in essence, it does the exact same thing as a throttle stop.

 

if you take the tb out you will see that the BISS screw controls the amount of air entering a passageway leading past the throttle plate (toward the intake manifold side). You can fine tune the BISS screw adjustment with Evoscan (along with a properly functioning IAC) to dial in idle, OR troubleshoot idle issues. There are some threads on here that explain this in better detail. Definitely do not tinker on the BISS screw without Evoscan or a way to hold the IAC in place. Otherwise, your idle won't be accurate because the IAC motor is going back/forth compensating for your BISS adjustments.

 

I run an AEM Infinity 506, I have to figure out how to adjust properly without Evoscan

 

I would try unplugging the IAC (power connector) before tinkering with the BISS screw, at least it will stay in one position. I suppose a possible drawback is if the plunger ends up holding at either end of travel (fully extended / retracted). IIRC the purpose of Evoscan was to hold the plunger somewhere near the middle of travel. Someone else feel free to chime in and correct me if wrong. Also, why are you adjusting the BISS screw, are you having idle issues?

 

Also, there are some threads on here where people specified a generic starting point for BISS screw adjustments (based on OEM TB). I believe it was screw it all the way in, and loosen it 1 1/2 turns. From there you will have to trial/error. My car was about 4 turns out before it would even cold start properly.

 

I wanted to try to get to factory so I know its taken care of then get the Infinity correct. I know if the hardware is good then its on the correct track if that makes sense?

 

The only time that I have had any issues with my car running poorly, or the engine wanting to stall out when coming to a stop has been due to the BISS screw. If I recall correctly, the best way to adjust for it is to tighten it tell it stops then start with 1/4 and 1/2 turns (loosening) and adjust accordingly.

When I first bought my EVO 8 off someone it was stalling out whenever you came to a stop sign, I took it to one shop and they wanted to re tune everything, new injectors, fuel pressure regulator, etc. for $3000. I took it to another shop and in one hour he had it running perfectly by simply adjusting the BISS.

 

sorry to thread jack, but this is BISS related. I did a boost test and I'm losing pressure right at the BISS screw. I ordered a new one with O-ring. is this screw something I can remove just by unscrewing it counter clockwise? I was just gonna mark the depth with like a tooth pick and pen or something but was wondering if I can take it out and install the new one just like a normal screw. Thank you in advance.

 

you have the right idea, I did this as well last year. I marked the screw and counted the number of turns before it fully seats (all the way in). Remove the screw, change the ring, screw it all the way in, then loosen the number of turns you noted before. Easier done with the TB removed off the car.

 

After I rebuilt my TB I had to adjust the biss screw and this video on YouTube was great help. .

 

I closed it all the way and opened it up 6 turns as that seems to be my only option. I have my OEM ECU and have Evoscan but I cant find my Tactrix cable but even if I did find it, it would be a pain in the *** to repin the IACV to convert back. I read that if the IACV runs out of travel then the BISS isn't set correctly, does anyone know how many "steps" is the maximum for the OEM throttle body?