Allen bolt at the back of the TB

What does the large allen bolt at the back of the throttle body do aside from lowering the idle when closed?

when opened or closed does it affect gas mileage? engine performance?
If I remember correctly it sort of sets the scope that the electronic Idle Air Control Valve IACV can work within, so closing it will force a lower idle, opening it will allow a higher idle to what the IACV will allow. No real effect on gas mileage unless it's not set properly I'd say.
Thanks, I thought all along setting it close had caused my plugs rich black... might have to look somewhere else what is causing the problem...


Club Member
i would be checking the coolant temp sensor centi, i ran mine with 2 sensors wired in, to shorten the coldstart enrichment time