Hi Folks,

I'm tracking down a suspected vacuum leak on my '86 S2. As per pic, there's a y shaped connector on the left of the intake. The bottom one goes to the brake booster and this holds vacuum after a lot of pumping. The top one doesn't when I attach the mitivac, it looks like it goes down to the throttle body so is this normal without the engine running?
Couldn't see it in PET to understand its purpose, oil vapour maybe?

Appreciate any guidance, thanks
Gary

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Hi Gazfish, that is a venturi tube. It is designed to increase vacuum at the Y-connector by the action of the flow through the straight connectors. IE, the flow through the internal venturi creates a low pressure area at the Y-connector. There is an O-ring inside that seals it. To my knowledge, the O-ring is not a Porsche serviceable part. However, internet/Rennlist lore has this on the O-ring: 3/8" ID x 3/32" cross section, Dash no. 110.

Here are photos of the venturi tube from my Red Witch:




Venturi tube and hose to brake booster check valve.






Venturi tube.






Venturi tube disassembled. Note the thin green O-ring on the insert at left. The green makes me wonder if it is not an O-ring from an A/C O-ring set...


Here is a page from PET for ROW 1986 928, under 'wheels and brakes':



Item #1, 035 133 753 A ,venturi tube



Good Luck!

 

Thanks for the detailed response, appreciate it.

 

You are very welcome!

 

Gary,

To answer your original query- the air passing through this system is metered air and is a relatively small amount. This stream by-passes the throttle plate and the ISV the idea being that no matter what the throttle position the air flow through the venturi nozzle will pull a low vacuum by nature of the accelerated flow creating a localised pressure drop wherein the branch being specifically placed sees tis localised vacuum and this ensures that the brake booster always has vacuum on it to give brake assist on demand. When you try to pull a vacuum on the line going to the upstream side of the throttle plate you will not pull any vacuum as it is open to atmosphere.

 

Makes sense, thanks Fred.