924Board.org

Bockscar · Joined: 16 Nov 2010 Posts: 392 Location: Sandy, Oregon

I'm currently in the middle of putting the fuel mixture components back onto the engine and I've come across the decel valve. After examining the valve and the engine, I have no idea where to put it. I went over both manuals I have and the Hanes manual says 1981 turbos don't even have a decel valve and the Porsche shop manual just shows a diagram of the vacuum connections, but doesn't actually talk about the decel valve.

This engine has enough plumbing to last a lifetime; if the engine doesn't need the decel valve, I'd just assume get rid of it. However, looking at the diagram in the shop manual, it looks like a critical component.

According to the diagram, the little hose connector connects the decel valve to the Control pressure regulator; that sounds important.. Also, it looks like the decel valve has connections to the freq valve and the auxiliary air valve. If I were to delete the decel valve completely, how do I connect the network of vacuum hoses with it out there?
_________________
Brian
'79 924 - Restoration In Progress
'81 924 Turbo w/EFI - Scrapped

stevekat · Joined: 19 Jan 2008 Posts: 719 Location: Los Angeles, CA

In my experience with a defective decel valve on my 931, my revs do not fall between shifts, and may increase a bit. Very annoying and probably not good.
_________________
"Never get out of the boat. Absolutely goddamn right. Unless you are goin' all the way."

Gone to new home: '81 924T, US version, CGT Intercooler, UTCIS-PT, Euro DITC, Greddy Trust MBC, Forged Fuchs Flat Dish 6's, Factory Recaro's.

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

You're reading the diagram wrong. I'll take a pic for you tomorrow.
_________________
Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

Bockscar · Joined: 16 Nov 2010 Posts: 392 Location: Sandy, Oregon

Let me make sure I have this right.

I believe the far end of the black metal hose connects to the intake manifold, near the cold start valve, and both the back and brass metal hoses end up connecting to the axillary air regulator.

Also, according the the diagram, the part of the far end of the black hose that bends off to the right apparently somehow connects to the frequency valve. I'm not sure that is correct..
_________________
Brian
'79 924 - Restoration In Progress
'81 924 Turbo w/EFI - Scrapped

Smoothie · Posted: Tue Mar 29, 2011 11:41 pm Post subject:

The decel valve prevents a spike in A:F mix richness whenever the throttle is released. It does this by providing an air bypass around the the throttle plate that closes gradually. A spike in vacuum when the throttle shuts, opens the decel valve, then it closes slowly. Result is a more gradual drop in rpms. Without it, rpms should actually drop faster, so you have another problem that's keeping rpms up between shifts - maybe sticky throttle..
Decel valve also prevents backfires between shifts (which would be caused by the overly rich mix (unburnt fuel) entering the exhaust.

None of those lines would be going to the frequency valve. The FV is like an EFI fuel injector with only fuel in-out and an electrical connector (no vacuum lines).

_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Bockscar · Joined: 16 Nov 2010 Posts: 392 Location: Sandy, Oregon

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

Venturi tube for the brake booster.
_________________
erstwhile owner of just about every 924 variant ever made

Bockscar · Joined: 16 Nov 2010 Posts: 392 Location: Sandy, Oregon