Decel Valve Issue

[drola]

i have been having a problem with high idle after warm up on my '76 2.7. after much research here i'm pretty sure that the decel valve is the culprit.
the valve has 3 hoses. one on top, middle and bottom. when i plug the top hose (vacuum) nothing happens. when i plug the middle hose the motor stalls at idle but runs fine if i give it some throttle. when i plug the bottom hose nothing happens.
am i right in thinking that the middle hose is letting air into the system? when i plug it up the mixture is too rich for it to idle hence the stall.
do these things go bad for no reason? can i just plug up the hose fittings at the back of the throttle body and the vacuum hose on top and be done with it? are they repairable? if it is stuck couldn't i spray some lube in it and see if it loosens up?

[Gunter]

The Decel valve is adjustable. The top is a hollow tube-spindle with a lock nut.

The top hose-connection should have a T meaning: a branch of the hose goes into the top of the Decel Valve and another branch goes to the bottom of the WUR.

Do you see a nipple on the bottom of the WUR?

Middle- and bottom hoses on the Decel Valve are coming from the back of the TB. They have to be in the correct position. In your case, I would have to see a picture of the rear-connections to be sure the hoses are connected right .

The Decel valve is there so you can adjust "hang-time" when decelerating to ease the sudden change in mixture when the butterfly closes. Adjustment is done by trial and error to suit.

[mpwr99]

Gunter here is a picture of what he is talking about.... I am having the same problem. Car starts up at about 900 cold then when warm is at about 2k.

[Gunter]

Yes, I have that part-diagram but it's not complete (I have the complete diagram) plus it doesn't show me the actual connections on his, or your, engine.

The Decel Valve has to be connected right or it won't function.
Meaning: correct connections to top and bottom of the butterfly by the nipples in the back of the TB plus the top hose has usually a "T" going into the DV but continues on to the WUR.
There can also be a Thermo Time Valve (TTV) in this line.

Without pix of your actual connections, and verifying that your WUR has a nipple on the bottom plus a possible TTV, I cannot help.

Besides, your problem may be different. AAR or AAV or..........?

[Joe Bob]

The decel valve is a smog control. It's designed to reduce unburned hydrocarbons from escaping the tail pipe when decelerating quickly from high rpms and when downshifting in spirited driving.

I've had good luck pulling it off and giving it the float test. If it doesn't float, problem solved.

[drola]

yep, that's the one.

[Superman]

My decel valve looks more like a flying saucer, located next to the AAR over the #5 and 6 cylinder heads. It is not adjustable, except that you can crush them a little, and hope for the best.

I do not like decel valves. If I wanted the idle to hang up, I could just keep my foot on the accelerator pedal. So.....I plug the hoses to disable the valve. Problem solved.

[boyt911sc]

Originally Posted by Superman

My decel valve looks more like a flying saucer, located next to the AAR over the #5 and 6 cylinder heads. It is not adjustable, except that you can crush them a little, and hope for the best.

I do not like decel valves. If I wanted the idle to hang up, I could just keep my foot on the accelerator pedal. So.....I plug the hoses to disable the valve. Problem solved.

Superman,

CIS cars '76-'79 have the adjustable decel valve shown in the previous post. While the later SC's ('80-'83) have the type of decel valve that looks like an AAR with a nipple. These are two (2) different types of decel valves but with the same application. HTH.

Tony

[drola]

thanks for the input. but i have this habit of wanting to know how everything works so i figured i'd ask.
please tell me if i got this right: the vacuum hose on top opens the valve under heavy vacuum. the valve sucks in atmosphere air from the fat middle hose and lets it into the system (under the butterfly) via the bottom hose. is this right?
so when i plug the the middle hose it cuts off the excess air and richens the mixture.
the vacuum hose t's and goes into the top of the decel valve. the other end goes into the top of the wur. the bottom nipple on the wur is hooked up to the top nipple on the front of the throttle body (by front i mean when you are looking at it which would actually be the back).

[Gunter]

Not quite.
Ignore advise from owners with Lambda systems having a non-adjustable DV.

Post pictures of the back of the TB showing the DV and the nipples/connections on the TB.

And a picture of the WUR.
Do you have a Thermo Time Valve? It looks like a small version of the AAV with 2 nipples for vacuum lines usually between DV and WUR.

Did you check the AAR hot and cold for correct function?

Once I see the pix, I can suggest connections and adjustments.

[T77911S]

the decel valve does NOT change the mixture. as with the AAR and the AAV, they all bypass air around the throttle plate, but it is metered air, IE after the AFM, so the mixture does not change from these devices.

you can connect a hand vac pump to the decel vac and pull a vac to check it opening vacuum. you can also use to check the other ports to make sure they dont leak, but that would just change the idle.

if you motor has a lot of miles, you could be getting some blowby. oil blowing out the breather and back into the intake. when i pulled my throttle body off, my decel valve was full of oil. the oil was making the idle hang up, IE, it was holding around 2k for a long time before it would drop.

it could be that the decel vavle is constantly adding to the idle, due to leaks, and over time, it has been compensated for with idle adjustments.

[drola]

gunter, i do not have access to take a pic of it at this time. i should be able to take one in the next few days.

i recently completely disassembled the cis to go through it. the thottle body was clean with no oil inside. the motor ran great when i put it back in other than a nasty oil leak. afterwards the motor sat for a month waiting for me to drop it to change the flywheel seal. then when i put it back together it started with this idle issue. i've checked for vacuum leaks and haven't found anything. besides i just replaced every gasket and o-ring in the whole system.
searching the forums it seems like everyone who has the exact same symptoms as me found it to be the d.c.
i'm thinking of removing it and see how it drives. if the idle issue is gone then i know what the problem was. i do mostly city/highway driving and would rather have the valve in place than not have it.

[Gunter]

O.K. wait for pix.

I like the adjustable DV and purposely use it on my SS 3.2.

Over the years, I found that people get confused with the vacuum line connections front and rear of the TB and the WUR and possibly the TTV.

Vacuum connections are very different for different years.

That's why pictures help to avoid more confusion.

Is your distributor vacuum retard? (Not advance?)
Was the distributor ever serviced?
Is there a Thermo Time Valve?
Do you have Cruise Control?

[Superman]

I believe the Decel Valve does not impact either mixture or ignition timing. In other words, all it does is to cause your engine to return to idle more slowly when you remove your foot from the Go Pedal. I am quite comfortable allowing my engine to return to idle quickly. I do not prefer a slow return to idle. So.....my Decel Valve is something of a useless engine decoration. Taking it out of the intake air loop also reduces the number of intake air (vacuum) connections that might fail.

[boyt911sc]

Originally Posted by Superman

I believe the Decel Valve does not impact either mixture or ignition timing. In other words, all it does is to cause your engine to return to idle more slowly when you remove your foot from the Go Pedal. I am quite comfortable allowing my engine to return to idle quickly. I do not prefer a slow return to idle. So.....my Decel Valve is something of a useless engine decoration. Taking it out of the intake air loop also reduces the number of intake air (vacuum) connections that might fail.

Mr. Superman,

I'm totally flabbergasted after reading your response about the decel valve. While I agree with you that the DV does not impact either the mixture or ignition timing, your thinking how a decel valve works or function is flawed!!!!! If you think i'ts main function is to slow down the RPM drop to idle is incorrect. And to proclaim that the DV is an useless engine decoration only adds merit to the myth!!!!!

With due respect to a senior member with 16,000 posts, I beg to disagree in your analysis. Or are you interested to find out what DV does to the exhaust gas? Please do some technical reading maybe it could help clear things up. Just my two-cents.

Tony

[Superman]

Tony, lean back. Deep breaths. First, if the DV performs an important or necessary function, then please help us (and me) out with an explanation. I am aware that the DV's function is largely (or entirely) related to emissions. When a throttle plate snaps shut at high RPM's, perhaps there is a brief time during which a vacuum (and no incoming air) is present while injectors are still spraying, and perhaps during this time there is insufficient air to ignite that fuel. On the other hand, the air flow sensor plate position is set by incoming air volume/force.....so it won't take much time after throttle plate closure for the plate to fall to "idle" position. In fact, I wonder if perhaps the airflow sensor plate doesn't return to idle position BEFORE the engine slows down to idle RPM. I don't mind being educated or corrected, regardless of the number of exclamation points, but you haven't done that. You've criticized without following up with an explanation or description of your position. To answer some of your questions:

I have done some technical reading, thank you. If you have information or suggestions for me, post them here.

Am I interested in what the DV does for exhaust gasses? Nope. Not in the slightest. They are like the autocross cones in my rear view mirror......immaterial. My car lacks a catalytic converter, oxygen sensor or computer. When I take my foot off the pedal, I want a vacuum in the intake plenum, and I want the engine to slow down as quickly as possible.

[boyt911sc]

Originally Posted by Superman

Tony, lean back. Deep breaths. First, if the DV performs an important or necessary function, then please help us (and me) out with an explanation. I am aware that the DV's function is largely (or entirely) related to emissions. When a throttle plate snaps shut at high RPM's, perhaps there is a brief time during which a vacuum (and no incoming air) is present while injectors are still spraying, and perhaps during this time there is insufficient air to ignite that fuel. On the other hand, the air flow sensor plate position is set by incoming air volume/force.....so it won't take much time after throttle plate closure for the plate to fall to "idle" position. In fact, I wonder if perhaps the airflow sensor plate doesn't return to idle position BEFORE the engine slows down to idle RPM. I don't mind being educated or corrected, regardless of the number of exclamation points, but you haven't done that. You've criticized without following up with an explanation or description of your position. To answer some of your questions:

I have done some technical reading, thank you. If you have information or suggestions for me, post them here.

Am I interested in what the DV does for exhaust gasses? Nope. Not in the slightest. They are like the autocross cones in my rear view mirror......immaterial. My car lacks a catalytic converter, oxygen sensor or computer. When I take my foot off the pedal, I want a vacuum in the intake plenum, and I want the engine to slow down as quickly as possible.

Superman,

You just explained how a decel valve works. Removing or disabling the decel valve in a CIS would prevent the additional air to by-pass the throttle body to form a more combustible mixture during high RPM deceleration. This has more effect on the emission than anything else. I don't consider this device to be useless unless you don't care about emission.

The reason why some experience 'hanging' rpm during high RPM deceleration is due to either a defective DV or incorrectly adjusted DV. A leaking decel valve or a vacuum leak would cause slow engine speed drop when you lift you foot from the gas pedal. A defective DV is useless but not a working one.

Tony

[Superman]

Makes sense. I do care about the environment, and my other cars have working, modern fuel and emissions systems. They also have radios and heaters and sound deadening and mufflers and stuff like that. The 911 is my Bad Girl. I wish she had Weber 40IDA's.

[T77911S]

sorry tony, i never really thought about the DV being for emmisions. thnaks for the lesson.
i only removed mine because i put a 3.0 TB on my 2.7 and i could not put the DV back on.

back to the OP.
if your car is idling up after it warms up, i dont see how the DV would do that. engine temp has no effect on how it works. and again, it does not effect mixture.
either you have some vacum hoses wrong or you have a control pressure/mixture problem, along with a possibly bad DV.
it would be nice to know how long after you start it it idles up.

do you have the vacuum controlled WUR? cant remember if they started that in 76 or 77.
as gunter has implied, you could have the vac lines to the TTV, WUR, maybe even the dizzy, and the DV wrong.

if you still feel you have a problem with the DV, the only way to check that it works as it should is to get a hand vac pump and test the middle or lower port to make sure it does not leak, and to pull a vac on the one on top to make sure it opens the path between the middle and the bottom.
or, if you can blow thru the bottom or middle port, it has a leak.

if it is leaking, the idle has been set to compensate for the extra air and is likely why it cuts off when you plug the middle hose. when you plug the bottom hose, you could be pulling unmetered air in thru the bottom port of the DV.
i dont know why it would not pull unmetered air in thru the middle port and out the bottom to the intake, maybe just the design of the DV. tony?

since i dont know exactly how you did your tests, i can only speculate on what was done.

[boyt911sc]

Drola,

When the engine is completely warmed up what idle RPM do you get? If you turn (+/- half turn only) the idle screw on the side of the throttle body, does the idle speed change too? Keep us posted. Thanks.

Tony