How to bypass decel valve?
 

Do I connect the two hoses on the back of the decel valve (#9) to each other or do I cap them both off? I removed the valve because it's faulty. I also removed the AAR (#4). Now the car won't start. Is it because I capped both hoses?

http://forums.pelicanparts.com/uploa...1252864467.jpg

Looking at the pic above, my car does not have #s 4, 5, 7, 9, 13, 14, 15, and 16

When I recently removed my decel valve, I removed all the hoses and capped-off all associated ports (including the tiny one on the throttle body) . . .

That's what I just did, along with the AAR and it won't fire. I crank it and it catches for a second and then dies.

cap all the connections that went to the decel valve.
since you removed and capped the cold start aux airslide valve you'll have to hold the gas pedal down about a 1/4" or so to let enough air in to start the car and keep it running.

you could install a porsche 944/BMW 320i plastic heater valve in place of the aux air slide valve. it fits the hoses perfectly and with a long cable would make a nice hand throttle for CIS.

all those silly throttle body air bypass devices do the exact same thing as holding the gas pedal down just a little bit beyond idle speed at certain times like decelerating (the saucer shaped deceleration vacuum limiter valve and saucer shaped AAR valve used on the 79 and earlier 930) so nitrogen dioxide emissions or NO2 are reduced and or at cold start ( the cast aluminum auxilliary air slide valve or whatever you want to call it today) so the motor will keep running without you feathering the throttle when it's cold.
they do nothing more than that... totally optional.

here's my 1987 saucer valve.
charming isn't it?

it lives in a box and will never go near my car again.http://forums.pelicanparts.com/uploa...1252867775.jpg

Well, I figured out why it won't start, I keep blowing the fuel pump fuse but I have no idea why. It ran before and I didn't touch anything electrical. I'm stumped.

I've blown that fuse in the past. I wonder if I have a bad fuel pump, both are original. $400+ for the pair, does anyone have an aftermarket source?

Do a search on ebay and amazon for current items and here's a link I saved.
The 044 pump is a good one, second down this list. I think you need to use an adaptor for the line connection.
I've seen it on sale for less so try a search.
http://www.boschfuelpumps.com/

Thanks. A little fuse blowing testing shows pump number one is the culprit. When I remove the relay (new) and run only on pump 2 it doesn't blow the fuse. I've been having power issues lately and now I'm wondering if this is related.

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.

I removed it because it is no good and I'm not spending $500+ to replace it.

...and you can just not let your foot off the accelerator all the way between high speed shifts and do the exact same thing without being married to it if you believe that theory.

along with that, by removing this emission device you get quicker deceleration of the engine during low speed around town gear shifts so the engine rpms match perfectly during casual upshifts making pleasent smooth driving.

and by golly you'll also have a much more stable idle speed with one less artifact from the early days of fuel injection thats just waiting for you to close the hood so it can play vacuum leak and mess with you.

All points to consider.

On a normally aspirated car I will be the first guy to pull the throttle bypass valve.

On a 930 I want it to work but would have a hard time paying $500 for a new one.

The best.

I removed all that stuff from my engine a decade ago. You learn to match revs between shifts very quickly and it becomes second nature. I like having that control to keep spool up when running through the gears at less than full throttle.

Well, it's definitely the front fuel pump causing it not to run. I disconnected the wires and checked the hot wire to ground and had nearly ∞ resistance. I then checked resistance across the fuel pump terminals. The reading was .5Ω Ohm's Law tells me that it's pulling 24 amps. I guess I'll start looking for a pump. The bad thing is the rear probably isn't too far behind (no pun intended).

I never had any issue on my 930 or C2 Turbo with the throtle bypass valve installed.

I learned of it's benifit from a respected 930 tuner may years back but there are more than one way to skin a cat.

I'm only pulling it because it's bad. I'd leave it otherwise, doesn't matter to me.

Well my front pump died in a different fashion. The "o" ring seal inside the housing gave up the ghost. First I could notice a slight gas smell, but could not find a source. Then a few days latter I saw a small drip on my garage floor when I
parked it that evening. Fired it up the next morning to go to the office....shower head city!! Man could not believe the volume of gas that could came out past that dead old "o"ring, glad it happened while at home.

I bought my pump through "Performance **********" new Bosch for $240.00 (sorry host) fit perfect, works perfect.

Mark

I dought it absoulity necessary for performance to pass air around the throttle body.

We just need enough bypass somewhere to minimized the Turbo from slowing down between shifts.

If boost gage drops between shifts enough that we can watch it build up- or you feel the power building back for a one or one-two count, then the throttle bypass might help to a degree. (And again, it s a little insurance if the CBV fails.)

On my car the combo of the TBV and a higher flow aftermarket Compressor Bypass Valve plus a couple of other tricks gave me instant full boost between sport shifts so I did not have to wait for boost to build back up helping me maintain higher average HP. However I mostly attribute that to the higher flow CBV.

In should say I have never tried running without the TBV. Again, I was told to keep the TBV by a respected 930 tuner.

If it works well enough with out it that is all that matters.

On a c2 turbo it would be a benifit to get rid of it if not needed just for the ease of removing and instaiilig the intercooler. I was wondering if it could be removed. I canned the aaa valve a while back as it is easy to just set in car and play with the throttle when cold.

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.

+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 .

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

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).

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?

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.

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.

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...

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

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.