Ok, Just How Does That Deceleration Valve Work?

Sunroof · 12-12-2005, 05:20 PM

I think I finally have my CIS (1973.5T) set up okay with the cold temps upon us, but I do notice sometimes that the idle will drop to the bottom and come back up to its 900-1000 rpm slot. Its only on occasion, but it does raise the question of just how the decel valve works. Its that pretty little bell shaped unit behind the throttle body.

Can it be adjusted? How can it be tested? Is it responsible for my needle dropping? Where does it come into play with the other components?

Thanks
Bob

arcsine · 12-12-2005, 05:24 PM

It is responsible for slowing the RPM drop when coming down to idle after shutting off the throttle. It is not "adjustable" but there there are threads that speak to "crushing" is slightly to get it to work properly.

Jubbie · 12-12-2005, 07:41 PM

911sc deacceleration valve

Scroll down to bottom for how to adjust.

ianc · 12-12-2005, 09:07 PM

Have not read the thread, but yes, the earlier ones are adjustable. On the top you'll see a shaft with flats and a locknut. This is the adjustment mechanism. HTH,

ianc

Noel · 12-13-2005, 06:21 AM

My underatanding is that it is an emissions part that helps to burn the remaining fuel after a sudden off the thottle event. Otherwise the car would go very rich for a short time. This is often when you hear cars backfire or the popping of the exhaust.

KNS · 12-13-2005, 07:30 AM

There is a small rubber line coming off the Decel Valve. Does anyone know where the other end goes to?

Gunter · Gunter's Garage

Kurt: There are different Deceleration Valves for different years.
Up to '79, the valve is adjustable as described above.
On your '80, the valve is not adjustable.
To undestand how the lines connect, do a search on the Pelican diagrams

arcsine · 12-13-2005, 09:32 AM

I stand corrected on my "adjustability" answer.

KNS · 12-13-2005, 10:17 AM

Gunter, Thanks

sammyg2 · 12-13-2005, 10:55 AM

The decell valve is a vacuum operated valve that opens under high vacuum and allows air to flow around the closed throttle valve.

At idle, the engine will pull a certain amount of vacuum, prolly around 15 or 16 "HG.
Under decelleration, that number jumps, sometimes up to 20"HG or more.
Thje valve is designed to be closed at idle but open when the vacuum get higher than normal idle vacuum.

When you let off the throttle the vacuum jumps, and a hose sends that vacuum to the valve. The higher than normal vacuum overcomes the spring that holds the valve closed. The valve opens and stays open until the vacuum drops to near idle.
If you increase the spring pressure, you make the valve stay closed longer. If you decrease the spring prressure the valve will open at a lower vacuum signal and stay open longer.

Basically it is a waste gate that operates by vacuum instead of boost.