low idle after deceleration

[anc]

my 76 911S has a very low idle after decelerating (idle approx 1oo rpms), if i lightly touch the accelerator pedal, the rpms go back to normal, approx 900-950 rpm, the car rides great except for this problem, any suggestions?

[warment]

Sounds like an adjustment to the "microswitch" may be in order? Somebody correct me if I am wrong...

Bill

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William Armentrout
1973 911T
2.7 carerra rs specs
www.geocities.com/william_armentrout

[john walker's workshop]

set the co to 3.5%. check the dwell (38°), set the timing to 35° total advance with an adjustable advance light. set the idle speed to 1000rpm. if it still has the problem, adjust the deceleration valve that sits to the left of the throttle housing until the idle drops slower. check that there is vacuum from the hose to it, as lots of people have plugged them with BBs.

[This message has been edited by john walker's workshop (edited 11-06-2001).]

[anc]

thanx for the input, but i have neither a microswitch nor a deceleration valve!! what next?

[Superman]

I'd check the throttle mechanism, making sure the throttle comes to rest easily and is not getting hung up by being dirty or something. I'd also check the advance/retard mechanisms, both vacuum and advance for the same reason: To make sure they are smooth and free, not sticky.

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'83 SC

[anc]

i cleaned the throttle and sensor plate, they were gummy, but the problem still exists, it doesn't seem to happen until the vehicle is warmed up. in my garage i was able to replicate the low idle and then played in the engine compartment. there is an auxiliary air valve behind and below the fuel distributor, that if i tap it a few times, the idle comes back to normal. logically, could this be defective? thanx again for your input.

[Superman]

I still have plenty to learn about the Auxilliary Air Valve and the Auxilliary Air Regulator. One of those things is open during cold starts, for the purpose of increasing idle. Like an idle bypass. I am still unsure what the other device does, but I think it may work in concert with the Decel Valve, which (I believe) allows air to momentarily bypass the throttle plate to reduce high vacuum pressures.

I'd say that if you can tap a device and change the troublesome symptom, you are very close to understanding the problem. There are specs for testing just about every device. Find out how the device in question (AAV?) is tested, and test it. Unfortunately, there is no shortcut for fixing CIS systems. Each component must be working properly, or the system is sick. And will stay sick until each component is operating according to spec. Having said that, I'll admit that my Decel Valve vacuum line is plugged with a BB. when it is not, the engine returns to idle VERY VERY slowly. I also have a little exhaust popping during deceleration, and occasional low idle (about 600-700 rpm instead of 900). So, I need to follow my own advice.

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'83 SC

[Bill Douglas]

Guys, John Walkers Workshop, Superman. What's this deceleration valve! Does anyone have an image or can tell me what it looks like? I think my '79SC needs it adjusted as the revs don't drop as fast as they should and end up at about 1700 for 3 or 5 second sometimes. Bill.

[BRink]

Bill:

My '79 SC had the exact same symptoms as your car.

Now, what I call the deceleration valve is located just behind the air flow sensor boot on the left side of the intake/airbox. It is about 4 inches tall and has one vacuum line running out the bottom to the back of the sensor plate and the other line comes off the top and connects to the main vacuum line on the left side of the engine bay via a "t" connector.

It is my understanding the deceleration valve was used for air conditioned cars as a way to keep the idle up while decelerating to an idle.

To test if the valve is functioning properly remove the vacuum line on the top of the deceleration valve and plug the vacuum line with a golf tee. Rev the engine and see if it idles down correctly.

Brian

[Doug Zielke]

Originally posted by Bill Douglas:
Guys, John Walkers Workshop, Superman. What's this deceleration valve! Does anyone have an image or can tell me what it looks like? I think my '79SC needs it adjusted as the revs don't drop as fast as they should and end up at about 1700 for 3 or 5 second sometimes. Bill.

Bill,
$hit-can the decel valve (or just plug the line to it). You don't need it at all. It's a smog thing intended to prevent high manifold vacuum after you lift off the gas. That has some effect on emissions. The effect of a lagging idle is most annoying, so go find a golf tee and suffocate that bugger!



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'81 SC Coupe (aka: "Blue Bomber")
Member: SCWDP
Canada West Region PCA
The Blue Bomber's Website
"If it ain't broke...we'll help you fix it 'til it is!"

[Bill Douglas]

Thanks Doug and Brian. It probably is the thing I pulled the hoses off ang bunged up already (that improved things quite a bit). Any clues for making the idle drop a bit quicker. The car is great in the higher rev range but pottering about town it would be nice to have the revs drop quicker in the low rev range.

[This message has been edited by Bill Douglas (edited 11-06-2001).]

[anc]

thanx for the info, i'll disconnect the decel valve, which by the way, in my bosch fuel injection manual, is called an idle air by pass valve????? hope it works, thanx again

[john walker's workshop]

i thought you didn't have one. you want the idle to stay up rather than drop real low. that's what the decel valve does, if properly adjusted. so if you plug it you gain nothing. did you check the tuneup specs i recommended? a proper tuning base is a prerequisite to any troubleshooting.

[anc]

i didn't know that the idle by pass valve is also called the deceleration valve, my mistake, i checked the other variables, as you suggested, and they're right on. is the deceleration valve adjustable? could the auxiliary air valve be defective and cause the low idle after deceleration problem?

[wckrause]

It's also sometimes called the vacuum limiter.

Item #7 in this pic.

http://www.pelicanparts.com/911/911_Parts/1978-83/1-7-2.JPG

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Bill Krause
'79 911SC Euro
MY PELICAN GALLERY

[Superman]

My Decel Valve looks like Number 39 in the pic. I'll consider adjusting it and placing it back into commission, because I respect John's advice. But at the moment I feel like Dout. My idle drops nicely, not too far. Perfect.

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'83 SC

[anc]

superman, # 39 in the picture, is the auxiliary air valve in my 76 911S, and that is the valve that when i tap it, the idle comes back to 950 rpm's. is this valve adjustable, and if so, how, thanx.
ps if you're saying that #39 is the deceleration valve, then what is #7 called?
BTW i disconnected the deceleration (by pass) valve #7 and the problem still exists!!

[This message has been edited by anc (edited 11-07-2001).]

[Superman]

Okay for now I'm not going to call #39 a Decel Valve, but here are the facts: the #39 device in my car has a big hose in and a big hose out, plus a vacuum hose. When I purshced the car, a BB was in the vacuum hose. When I remove the BB and reconnect the vacuum hose, the car's engine returns to idle VERY VERY slowly. On the highway, this would not be a problem but idle drops so slowly that city driving would be embarrassing. I believe it is the Decel Valve. I don't know what that cylindrical #7 is. I don't believe my car has one.

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'83 SC

[wckrause]

Im my Porsche Parts and Technical Reference Catalog they call #7 a vacuum limiter with the following part #s

1974-1979 911 110 220 00
1980-1983 930 110 190 00

While #39 is called the Additional Air Valve with the following #'s

1976-1979 911 119 273 00
1980-1983 911 110 273 01

My Haynes book calls #7 a "vacuum control" in one diagram (for 78)and a "deceleration valve" in another diagram (for 79); however in a diagram for 83's it calls what looks like #39 a decel valve.

I think that for '80 to '83's, the AAV and the Deceleration valve got combined into one item. This would make us all right, depending on what year we've got.


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Bill Krause
'79 911SC Euro
MY PELICAN GALLERY