How to bypass decel valve?

[Speedy Squirrel]

What made you deduce that the decel valve is bad? The reason I ask is that the decel valve does not bypass any air around the throttle at idle. If it does, then it is broken. It also has no emissions function. It's only job is to limit the maximum vacuum during decels, hence its name. To make this function more clear, they renamed it to vacuum limiting valve for the C2 turbos. As has been mentioned, it keeps the compressor from surging during shifts. Without it, you will hear what is described as a horse whinny sound, which is the noise your compressor makes when it is surging, during a hard decel.

You should test it by pulling a vacuum of 20" Hg on the vacuum hose connection to see if it will hold it. If so, the diaphram is good. If not, it is toast. If the diaphram is good, but the bypass is leaking, you can sometimes fix that with a swab and some carb cleaner while applying vacuum to the hose connection.

[Thierry25]

Originally Posted by 911st

I know most want to remove the throttle bypass valve as they think of it as a smog device.

I have noted this before but it also helps minimize turbo stall between shifts in a way similar to the compressor bypass valve dose.

This helps keep boost up between shifts and makes things easer on the turbo shaft and bearings between shifts.

It also adds a little safety as if the CBV fails you will not be as likely to snap a turbo shaft.

The AAV is just a cold start feature.

If either are removed, yes, block them off and use clamps to prevent blow off.

+1 it helps to keep boost up between shift

It also help to burn the gas which is delivered by the CIS between shift and decceleration .

[equality72521]

Originally Posted by Speedy Squirrel

You should test it by pulling a vacuum of 20" Hg on the vacuum hose connection to see if it will hold it. If so, the diaphram is good. If not, it is toast.

That is why I pulled it, it will not hold a vacuum at all.

[beepbeep]

Bypass valve is mainly there to reduce emissions and popping on overrun. It has little effect on "boost between shifts". That's a job for recirculating valve (the one with piston and two rubber gaskets).

You can remove the deccel valve and plug all hoses going to it. It's one of those things that starts leaking when it gets old. You might get flames on deccel when on track (there is no deccel fuel shutoff built in K-Jet , KE-Jet has it though).

[mark houghton]

If your valve is still working and you simply want to run without it's affect, how about just pulling and plugging the vacuum line going to it so it won't' open under full vacuum/decell conditions?

[911st]

Yes, I believe the throttle bypass valve better allows the extra fuel that is typically of CIS to be more burned in the combustion chamber than in the turbo or exhaust system. Sometimes sending fuel down to the turbo turns it into a jet motor which can be good for spool but bad for the life of the turbo.

Mark, I am not sure that disconnecting the vac line will tottaly disable the TBV. If there is enough of a boost spike between shifts (effectiveness of the CBV) it still might be pushed past to some degree. Just a guess.

[equality72521]

Is there an alternative to the decel valve that operates the same way that isn't $500? I'm not opposed to having the system functioning the way it should but I can't see spending that much for it, especially since I will buying new fuel pumps now. I'm going to replace both since the front is bad and the rear is also 30 years old.

[JFairman]

Sure.. it's called your foot on the gas pedal. Just don't lift it off all the way during high speed upshifts. You might even learn to heel and toe in the process.

That will do the exact same thing your saucer valve was doing for you.
..and now you are in control if you wanna be and you have the option of lifting your foot off the gas all the way during slower speed upshifts so the engine rpms drop faster in a similar way as a lighter flywheel would do.

but thats all preference. you can install a dozen golden saucer valves on your car if you or someone else thinks it will make something better...

[equality72521]

So is it designed to allow circulation (open) when vacuum is applied and block circulation while boosting (closed)?

[JFairman]

It doesn't recirculate anything... it just bypasses air around the throttle body from the intercooler or recirculation manifold into the intake manifold through the 2 larger hoses attached to it.
That is no different than holding the throttle open a tiny bit.

If you don't understand that, look up the word "bypass" in a dictionary.

It takes about 21+ inches of vacuum to open the golden vacuum limiter saucer valve and the only time a very low compression 930 can possibly ever have intake manifold that high is while decelerating with the throttle completely closed.
You can check how much vacuum it takes to open the thing with a mightyvac tool.

[equality72521]

Originally Posted by JFairman

If you don't understand that, look up the word "bypass" in a dictionary.

I'm going to give you the benefit of the doubt that you aren't being a smart ass with this comment.

I simply want to know if the device is closed under boost and open under vacuum.

I realize you are set on NOT having one in but I'm not yet convinced that I don't. I am wondering if a simple recirculating diverter valve will accomplish the same end as the decel valve.

[equality72521]

Originally Posted by JFairman

You can check how much vacuum it takes to open the thing with a mightyvac tool.

Also, I cannot check it because it's bad, it will not hold ANY vacuum.

[JFairman]

Ok, another bypass valve could work if it doesn't open below the opening vacuum the 930 bypass valve opens. If you find one that does it will open at idle because there is intake manifold vacuum at idle and you'll have an idle speed around 1500rpms that you can't lower.

Some Mercedes from the 25+ years ago had deceleration/vacuum limiting bypass valves with an external threaded adjustment with locknut for the spring tension against the internal diaphram that opens the valve seat.
Maybe you can find one of those somewhere.
Most of the ones I've seen had smaller hose fittings though so they won't flow as much air as the large saucer shaped one on a 930.

I'm almost sorry if I sounded sarcastic with my comment... i'm not perfect and don't pretend to be.

[RarlyL8]

Mark,
You are over thinking this. That valve is one of a dozen useless POS piled on the CIS to help with smog control or to make the engine easier to operate sans EFI as the Carrera had.
Just unplug and cap everything then use your foot. You will enjoy the control and ease of shifting when matching revs. And yes holding the throttle does help maintain spool between gears, I do it every day and really enjoy the participation.
If you decide you don't like the foot thing then it is very easy to reverse. I'd find a good used one as there is no reason to spend a lot of money on this device.

[911st]

Makes sense. If it has failed and it is expensive to replace and you do you want to put in a used one, just deleted it. Try it. It seems to work well for others.

As I believe was noted, it dose take a lot of pressure from the turbos boost spike to open these valves. They are not like the de-accel valves we saw on the early EFI cars that make the rpms drop with great delay from what I could tell on my turbos. Yes, they do benefit emissions but they can also help performance (boost recover between shifts) without hurting drivability.

Again, save the money. There are some very knowledgeable owners here running without it and they seem very happy doing so.

Enjoy.

[equality72521]

Ok, ok. ;-) I just wanted to be damned sure I wasn't going to do any damage to the turbo. Thanks for all the advice guys, I'll just leave it all plugged.

[964 T #304]

Mark I love your little statement below your name . That is my motto also ,just never saw it put into words . Good luck withyour valve delete I think you will like it.

[mark houghton]

Originally Posted by 911st

As I believe was noted, it dose take a lot of pressure from the turbos boost spike to open these valves. Enjoy.

I don't understand your comment. If I understand the flying saucer, it opens in response to 21 inches of vacuum from a small vacuum line...and only when the throttle is completely closed (thus, de-accelerating). When properly functioning, boost does not and will not open the valve - not unless the diaphram is ruptured, in which case you would see a perpetual high idle from all the air being introduced.

[JFairman]

If you like the chirp and wooshing noises of adjustable blow off valves on EFI cars you could try hooking one of those up where the deceleration valve was on the intercooler side of the throttle body and let it vent to atmosphere for the noise while your current blow off valve continues to recirculate air back to the turbo inlet.
Of course you would plug the hose inlet on the intake manifold.

Just adjust the spring tension on the blow off valve tight so it only opens a little on deceleration between shifts or experiment with it.

[Speedy Squirrel]

If I have a properly functioning decel valve and you don't, under hard braking I will not be in the ridiculous position of standing on the brake AND giving it a little gas at the same time while I downshift.

When you are upshifting, the throttle slams closed and all that momentum for the airflow converts to pressure. You have a big boost spike, and little airflow. Now, go look at a compressor map at a low airflow and 2.0 pressure ratio. What do you see? Nothing. It is to the left of the surge line. The compressor can't run there so it stalls until it gets to a pressure it can run at, and you've just lost your turbo speed. Not the fast way out of the corner. Don't think your blowoff valve is going to save the situation either. It's way too slow. The decel valve is the "first responder" and keeps you out of trouble from the boost spike 'till the blowoff valve can bypass the compressor.

Also, without the decel valve you can get 35-40 inches Hg vacuum under decels, which sucks oil down the intake valve guides and mucks up your plugs.