Idle problems
 

sometimes sitting in traffic, i notice the idle is too high, ie the engine stays at about 1800 RPM and doesnt drop down
to 1000 or so.

it doesnt happen all the time but it does happen. anyone had similar issue and solved it?

thanks so much, people.

anybody? anybody? anybody?

1) Remove the big rubber boot on top of the CIS. Check if the airflow sensor plate and the throttle flap are clean and move easily.

2) Remove the decel valve behind the throttle body. Connect a vacuum line on top, suck and pinch it off to keep the vacuum. The idle air bypass must be open now. Release the vacuum: the bypass must close in, say, 1-2 seconds.

3) Motor off, ignition off. Release the residual fuel pressure (for example, you can loosen one connection of the fuel filter). Now lift the airflow sensor plate by hand. The control plunger must move easily inside the fuel distributor. Let the plate come down again and feel if the plunger follows.

4) Check the microswitch at the throttle housing.

5) If you have a catalysator, unplug the O2 sensor to get a fixed duty cycle for the control pressure frequency valve. If the high idle disappears, the closed loop control may be faulty (I don't think that's your problem, I just wanted to add an item #5)

Hope that helps,

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Robert / 80 Targa

I'm sure what the Euro CIS setup is, but when my US spec 83 did this, it was the de-acceleration valve staying open (a 2" disk on the right side of the engine with two large rubber tubes on the engine side and one small vacuum tube on the other). I just disconnected and plugged the vacuum tube.

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Graham Archer
83 911SC Cab

Robs -- thanks, I've never seen a test procedure for the deceleration valve. I occasionally get that symptom, but only when it's VERY cold outside and the engine hasn't fully warmed up.



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Bill Krause
'79 911SC Euro
MY PELICAN GALLERY

The decel valve is another useless piece of smog crap. It can cause the symptoms you mention, as it gets old and cranky.
As stated before, take off the vac line that goes to the unit. Plug the line with a plastic golf tee secured by a tie-rap. High tech, eh?

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'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website

Doug,

I thought it also served to limit the vacuum, during deceleration, from over leaning the mixture and causing backfiring.

Do you have a problem with backfiring when you lift the throttle?

Not at all, Bill.
My mechanic told me it had to do with the engine bringing the idle down more gradually when you lift off the gas. This apparently impacts the level of emmisions. My Alfa Romeo 2.0 Spider had a similar valve. All part of the Smog-Nazi agenda, I suppose.

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'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website

there was also a bulletin about squeezing the decel valve a certain amount to adjust it, being that it's not adjustable externally. easier to just pull off the hose and plug it. i've heard complaints of the odd car dying between shifts when this was done, but that usually that can be remedied thru tuning adjustments.

I get that symptom some times too, but it comes and goes. Sounds like I've got a flakey decel valve. I'll try disconnecting it, and see what happens.



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Bill Krause
'79 911SC Euro
MY PELICAN GALLERY

My '78 decel valve is adjustable. I backed off the lock nut, turned the threaded post clockwise (which has flats, about 6/7mm) and I no longer have this problem: car comes down smartly to standard idle.

As my problem was identical to yours, maybe this is the fix if you car is running the same unit?

Good luck,
Jw

[This message has been edited by Jdub (edited 08-16-2001).]

A good irritating stress reliver when that occasionally happens to me....let the clutch out slowly with the brake on.....try not to stall it....and it will lower the rpms. Depressing the clutch back and the rpms stay at 950 +/- 50 like it's supposed to.

Then I forget about it till it happens again next month...

JW, is your 78 a US or Euro car? My 78 has this same problem, normally after the car has gotten good and hot, after driving on the highway for a while. After you adjusted the valve, did you have any problems as mentioned earlier?

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plain fan

thank you all so much for your help. these CIS systems sure have a lot of components to them and diagnosing them can be tricky.

well, you build experience and confidence as you move along, i guess.

Plain:

It's a US, non-California, 49-stater. And yes, it ONLY happens after running on the hiway for a bit of time. Car is warm, and the idle just won't turn down; sits about 1800rpm.

My approach solved the problem, period. I am told that you must have *some* decel to prevent the backfire as described above. And note that the locknut is on very, very tight. I thought I would bust the unit before it freed up finally.

This should be done in concert with removing the rubber boot...makes life much simpler, access-wise, and also lets you clean the throttle plate (i.e. butterfly) and the sensor plate from the topside. I think you will find yours is probably dirty, especially if you ever overfilled the oil!

Good luck,
Jw

Thanks Jdub, I'll look at it this weekend and see if I can fix my problem. I've also found that I can let the clutch out a bit to slow the idle back to normal but once the clutch is back in all the way, it jumps back up. Thanks again.

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plain fan

Hey guys, I am going through some problems with my 81SC CIS system. I was at an autocross last week and when the car got hot (second run) the car started to idle poorly while waiting in line for the next run. On the next run, as I let off the accelerator for the first turn, the car backfired badly. It stalled out and I restarted it and continued. It then did the same thing again. After much investigating, I found that one of the vacuum lines off of the deceleration valve is missing and it is sucking air through the tee joint where the line is missing. There is the main vacuum line from the decel valve that goes to a tee joint, where one side goes to a spot just next to the idle adjustment screw. The other end is the missing one, and I have no idea where it is needs to go. Can anyone tell me how long the line should be, and where goes from the tee joint?

Thanks in advance,

Paul
81 911SC/RS

Paul,
check if you have the ROW setup (seems so, b/c you have the tee joint) as shown in
http://www.pelicanparts.com/911/911_Parts/1978-83/1-7-2.JPG
The missing connection should go to the vacuum switch (part #17A, sometimes called thermo valve) and from there to the WUR. If the line to the WUR doesn't carry vacuum, the hot engine idles too rich and may surge. I guess the US setup for a 1981SC should look like
http://www.pelicanparts.com/911/911_Parts/1978-83/1-7-3.JPG
but I don't see a tee joint there.
BTW, if you have a vacuum switch, then it is located near the WUR and has the part number 911 606 106 0X.

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Robert / 80 Targa


[This message has been edited by robs (edited 08-17-2001).]

Robert, Thanks for looking at the parts diagrams here on Pelicans site. I looked at them both before I posted the question, and neither looks like the setup I have. I know it has to be the US setup, (original engine and US VIN#). The setup looks more like the US 81 SC diagram, but with that tee joint. I am going over to the local Euro shop and will look at a couple SCs to see what they have on their decel setup. I put a cap over the end of the tee that is missing a hose, and it did make a big difference in terms of how the car would idle after reving the engine a bit. I have to drive it today after work to see if it changes real world driving conditions. I will also recheck my CO % at the shop tonight and see if it is still within spec.

P.S. thanks againg for looking at the diagrames, at least I know I am not looking at them incorrectly.

Paul
81 911SC/RS

Check that you've got two vacuum lines to the ignition distributor, and one to the WUR. There are not many places that it could go to.



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Bill Krause
'79 911SC Euro
MY PELICAN GALLERY