Should I use the vacuum pump with the 2 larger hoses disconnected?

Yes. Should work to test it. If this fails the diaphragm is broken and the valve needs to be replaced. Repair afaik not possible, at least never read before. Additional test: Trying to blow air through the bigger outputs with/without applying vacuum on the thin control output. With vacuum must be open, without not.

Joe Engineer has some written and made videos about it:
https://rb.gy/dr2hwv

Thomas

So just tested it. Without vacuum the center port on the backside is closed when blowing into it. When applying Vacuum at around 17 I can blow through both backside ports. So I can consider the Decel valve is good correct?

Yes. The diaphragm seems to be ok still, just hanging sometimes. Therefore I recommended earlier to make it more flexible again with some shots of silicone spray...

You can also try squishing the pancake of this precision device, flatten it by 1mm.

That is supposed to help with "unstable idle".

Correct, remembered it while reading your posting...

Porsche Technical Bulletin 81-07 for all 911

https://i.imgur.com/eXA0kWo.png

But issue seems to be solved anyhow, even without knocking/pressing the decelleration valve by 1mm ...

Haven't taken it out for a test drive yet after spraying some silicone spray but I'll try to push it in a bit as per the bulletin. Keep you posted
Thanks

I would press it only when problem persists and won't be sorted with silicone spray ... They're very pricey...200-260$ depends on the supplier...

Thomas

Should I use a vice on the bracket side to press it?

Don't know. Never did before. Problem is - you can easily apply way too much force on it... Would use a caliper to check the way...

More good stuff from here ...

https://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies.html#post7528901

in fact, it says so explicitly.

http://forums.pelicanparts.com/uploa...1724369883.jpg