Getting ready for my first emissions control inspection I was in the engine (1984 928S 4.7L) compartment last night checking lines and noticed a missing vacuum line.

On the passenger of the engine attached to the throttle body is a large (3") vacuum diaphragm but there was no vacuum line attached to the nipple in the middle of the diaphragm. According to the WSM Emission Control section it looked like the line going from the Deceleration Valve to the "Vacuum Box" which is attached/part of the "Pressure Converter" (all one unit?). Following other vacuum lines it looks like that part is on the passenger side engine compartment wheel well but I couldn't find any corresponding nipples that would receive the line from the Decel. Valve. Am I looking at the wrong part? And once I find it what size vacuum line goes there?

The car runs fine and I've never noticed any deceleration problems. Only real problem seems to be poor gas mileage. Always around 10-12MPG, never any better.

Thanks for any help,
Don

 

Not to worry. That line is disconnected on many cars. The decel valve lets extra air bypass the throttle when you lift the pedal sudenly. They get sticky and start to hold the idle up.

If there isn't an un-capped hose laying nearby I'd ignore it.

 

Should I cap the exposed end?

 

So there is a hose for it? Yes, by all means cap it.

I expected (for no good reason) that it might be capped at the throttle body.

 

No hose; but capping it at the throttle body is what I meant.