Its that time again for a weekly low idle question! I have searched the forums, but found some conflicting info, and now Im confused! I have a 1987 S4 Tii which I just rebuilt, replaced all the vacuum lines, so on so forth. The emissions system is deleted. The first time I started it, It idled at ~1100 RPM, then went down, I think this means the thermowax in the throttle body is working, it idled around 750 RPM, perfect! Now the fun part, a couple of days later, I took it on its first actual drive. It was about 8 miles. When I stop, the Idle goes down to almost below 500, and it sounds like it is running rough/one rotor. Im a little scared! I was reading my FSM, and it only shows the idle adjustment to be on the BAC valve, I have an USDM engine, so it is there. Should I adjust this screw on the BAC, or do you think its another issue?

Do I even need the BAC valve? I dont have any accessories, only the engine and the alternator? Ill keep it on and not take it off for now, I dont wanna mess with the coolant system.

 

I will also mention that the fuel system is basically new. New filter, New Walbro 255 with sock, New wiring to the fuel pump. Parallel fuel system running at 43psi with aftermarket FPR from Aeromotive. When I first start the idle AFR is around 10-12, good imo.

 

I would do the initial set coupler and set the idle air fuel mixture as prescribed in the field service manual. Section 4A page 77 . . .Check TPS as well. (foxed.ca)

I would next check the Brake Booster by using a hand diaphragm pump and checking if it holds vacuum. Do does your idle crap out when you pump your brakes?

The BAC valve increases air into the intake to keep idle right when the engine encounters a load like A/C or other electrical items.

 

I’ll give that a go, I guess I don’t know if it drops with brakes applied, the brakes don’t work lol. They do now but the diff mount went out