77 911s Cold Idle Speed

Avanti · 05-21-2016, 04:11 PM

Is the cold idle speed adjustable? I can't seem to find anything in the Bentley manual except for adjusting the warm idle. My cold idle speed is too high IMO, around 2400 rpm.

Avanti · 05-22-2016, 09:48 AM

Mmm. Is there no cold idle adjustment and its controlled by the WUR pressure? Which would mean if my cold idle is too high I would need to adjust the WUR?

Or... I seem to remember reading about an air valve that gradually shuts off after warm up. It sounds like it works like an idle air control valve (IAC) on a modern car. I may be way off but does this air valve adjust the cold idle? If this does exist, where is it?

Not near my CIS books now. Thanks,

gazzerr · 01:37 PM

I have an '82 but it's the AAR (Auxiliary Air Regulator) that's sits between the number 5 & 6 intake runners on the right hand side looking into the engine compartment. 12V is supplied to the bimetallic strip inside the AAR and it closes the rotary valve over a number of minutes. There is a minimum spec (ohms) to the heating element that wraps around the bimetallic strip. This ensures the air valve is slowly closed over the correct timespan. I don't recall what that should be but you should be able to look it up easily enough.

Assuming you have no vacuum leaks, your AAR is good, WUR cold control pressure is in spec and your idle mixture setting is correct the throttle body bypass that the AAR provides will control the idle speed. I don't believe the adjustment screw in the throttle body gives you any cold adjustment. The default height of the sensor plate may also play a factor. I'm also assuming your timing is correct and your plugs and wires are functioning properly.

I'll let the real experts chime in here.

Avanti · 05-22-2016, 01:42 PM

Quote:

Originally Posted by gazzerr

I have an '82 but it's the AAR (Auxiliary Air Regulator) that's sits between the number 5 & 6 intake runners on the right hand side looking into the engine compartment. 12V is supplied to the bimetallic strip inside the AAR and it closes the rotary valve over a number of minutes. There is a minimum spec (ohms) to the heating element that wraps around the bimetallic strip. This ensures the air valve is slowly closed over the correct timespan. I don't recall what that should be but you should be able to look it up easily enough.

Assuming you have no vacuum leaks, your AAR is good and your WUR cold control pressure is in spec the throttle body bypass that the AAR provides will control the idle speed. I don't believe the adjustment screw in the throttle body gives you any cold adjustment. The default height of the sensor plate may also play a factor.

I'll let the real experts chime in here.

Thanks Gazzerr, I'll check the sensor plate height because I haven't done that yet since getting this running after an 18 year hiatus. BTW, all is good during warm-up and the idle settles to a normal 900 rpm. If I remember correctly, my initial cold fuel pressure is about 5 lbs off, I have to check my notes from a recent test.

stlrj · 07:36 AM

The AAR is adjustable for whatever cold start rpm you desire. It's just a matter of tapping the round pin down in either direction to adjust the amount of air that is allowed to pass through. With a little trial and error you'll have it set just where you want it.

Leave the sensor plate or fuel pressure alone. It has nothing to do with your annoying cold idle speed.

Cheers,

Joe