911SC idle problem

[hardt45]

Hi - new to the forum and this is my first post. Looking for help with my 1980 911SC. The car was running fairly poorly after my purchase a year ago - mainly fluttering idle when stopped, and sometimes turning off completely. I recently did a tune-up (new plugs, wires, rotor and cap) and I am very happy with the results.

I do have one problem. When the engine is cold (>200 deg), the idle is perfect and I have no issues. At stops, it is constant around 900-1k. However, once the car has been driven for a while and warms up over 200 deg, the idle once stopped begins climbing steadily and stops at about 2k. Very annoying, and causing me to feather the clutch to keep the idle down while in stop and go traffic.

Any ideas? I have looked at threads about the AAR, AAV, and decel valves. I am thinking that maybe I need to disconnect the decel valve, but not sure. Bear with me, I'm new to this and have novice capabilities when it comes to this stuff. Just trying to fix this idle issue without having to take to the shop. Thanks for any help you guys can provide!!

Chad

[Nitrometano]

the decel valve can be a problem. But a cheap test that you can do is buy carburetor cleaner and clean the throttle body inside and his plate and the CIS plate. This are the ones that are under the big rubber boot. In that parts, the gunk that acumulates there cause some rpm problems. Another test can be to check the air fuel mixtures. Incorrect mixture can cause some problems too. The decel valve cause problems too, but i dont remenber how to test it. Also you can check the throttle plate for free movement and if the spring have the proper tension to close the plate.

maybe this help you

[ruf-porsche]

More likely the AAV if this is happening after the engine is warmed.

[pryord]

Subscribed, because my SC does exactly the same thing. What's the "AAC"?

[mca]

Plug the DECEL VALVE vacuum line with a golf tee. This is the cheapest and easiest test.

Take it for a drive ... I can almost promise that this will fix the problem.

If it does fix it, pull the decel valve out and squeeze in a vice EVER SO SLIGHTLY (make it more pancake shaped - don't squeeze on edges of perimeter). Replace and all will be well again.

I just did this to mine a few months back.

PM me for further questions if needed.

[hardt45]

mca,
I'm going to find a golf tee and try this out. A few questions - is the vacuum line the one on top of the valve in the attached picture? Where is the decel valve in the engine bay? Any pics? And last one - if plugging the vacuum line fixes the problem, why not just plug the line more permanently or remove the valve altogether rather than squeezing it and re-installing?

[hardt45]

WOW - it's fixed. I took the vacuum line off of the decel valve and plugged it with a vacuum plug. Took her out for a drive, and it's amazing.

mca - you're the man.

pryord - you gotta do this.

Now my question is, should I be alright to just leave it like this? The vacuum plug is on there tight, and I can quick-tie the line in the engine bay so it doesn't interfere with anything. I would really rather not have to remove the valve and mess with it if I can avoid it. I love the way the car's driving - just want to make sure there are no long term ill effects.

[Bill Douglas]

Good skills mca

I pluged my decel valve hose 10 years ago and just left it - enjoy the car.

[Joe Bob]

Originally Posted by hardt45

WOW - it's fixed. I took the vacuum line off of the decel valve and plugged it with a vacuum plug. Took her out for a drive, and it's amazing.

mca - you're the man.

pryord - you gotta do this.

Now my question is, should I be alright to just leave it like this? The vacuum plug is on there tight, and I can quick-tie the line in the engine bay so it doesn't interfere with anything. I would really rather not have to remove the valve and mess with it if I can avoid it. I love the way the car's driving - just want to make sure there are no long term ill effects.

It's a smog thing....unless you go State side wid da car bradah....hang loose and enjoy....

[Zacko]

Originally Posted by hardt45

mca,
I'm going to find a golf tee and try this out. A few questions - is the vacuum line the one on top of the valve in the attached picture? Where is the decel valve in the engine bay? Any pics? And last one - if plugging the vacuum line fixes the problem, why not just plug the line more permanently or remove the valve altogether rather than squeezing it and re-installing?

hardt45,
Here is pic of decel valve(vacuum limiter???)
Which vacuum line did you plug?? Did you plug it in both direction??
My vacuum line goes from the top of the decel valve - to thermo valve - to warm-up valve.
What does this decel valve do??


/Hans
911 SC 3.0 -79

[Gunter]

Originally Posted by hardt45

mca,
I'm going to find a golf tee and try this out. A few questions - is the vacuum line the one on top of the valve in the attached picture? Where is the decel valve in the engine bay? Any pics? And last one - if plugging the vacuum line fixes the problem, why not just plug the line more permanently or remove the valve altogether rather than squeezing it and re-installing?

Hold on there!
If you have a US '80 SC, you don't have this Deceleration Valve!
The US '80 has Lambda and the Decel. Valve is totally different and non-adjustable unless someone changed the set-up!

Zacko:
Your '79 has the adjustable Decel. Valve shown in the scetch (No Lambda)
For any increase in idle when warm, I would test the AAR first to make sure that it closes when hot. (Do a search about how to test)
The AAR is the one with the small electrical plug and sits between #5-6 intake runner.

[mca]

Originally Posted by hardt45

WOW - it's fixed. I took the vacuum line off of the decel valve and plugged it with a vacuum plug. Took her out for a drive, and it's amazing.

mca - you're the man.

pryord - you gotta do this.

Now my question is, should I be alright to just leave it like this? The vacuum plug is on there tight, and I can quick-tie the line in the engine bay so it doesn't interfere with anything. I would really rather not have to remove the valve and mess with it if I can avoid it. I love the way the car's driving - just want to make sure there are no long term ill effects.

I forgot that you have a different type of decel valve. On mine (82 SC) the decel vavle is pancake shaped and is on the right side of the engine.

I believe yours may be adjustable via a screw. I know that there is info on the board pertaining to decel valves for your particular year.

The decel valve keeps RPMs up between shifts. It also is a smog component.

I prefer to have it hooked up - others don't. I made the decision based on the fact that it stopped exhaust smoke from pouring out of my tail pipe in between high rpm shifts (engine has worn valves/guides). Also, it will help me at the track since I don't yet know how to heel-toe in order to keep rmps up for smoother/faster shifting.

It is fine to leave it plugged.

[mca]

Originally Posted by Gunter

Hold on there!
If you have a US '80 SC, you don't have this Deceleration Valve!
Th '80 has Lambda and the Decel. Valve is totally different and non-adjustable unless someone changed the set-up!

I believe he has the same as mine on the 82 SC. Although the decel is non-adjustable via a screw like the earlier types, they are still adjustable by squeezing them in a vice. Apparently this technique was provided to Porsche specialists in a tech bulletin.

I read and read and read about this topic. In fact, I had another decel vavle in my possession but I wanted to try the vice squeeze method. It works perfectly ... and I saved $100.

[Gunter]

Once squeezed, the later Decel. Valves cannot be changed back if the problem is something else.

For me, with engine hot, the first step would be to pop off the large hose on the side of the AAR and with the help of a small mirror check the inside of the AAR to see if it 's closed.
And go from there.
Getting to the AAR, AAV and Decel. Valve is not that easy especially if you have A/C.

[mca]

Gunter -

I completely agree with you.

In my case the decel line had been plugged since I had the car. One day I decided to reconnect the line and see what kind of difference it made. It was nice until the car warmed up to operating temps. The idle would not come back down to 950/1000 range unless I did so but using the clutch. That let me know that the decel vavle was giving me the problem.

I hear ya on the difficulty on getting to the decel. I had bloody nuckles for weeks following the repair.

[ruf-porsche]

Originally Posted by pryord

Subscribed, because my SC does exactly the same thing. What's the "AAC"?

Don't know what's an "AAC" , but the "AAV" is auxillary air valve.

[Porsche-poor]

I have a 79 and did the golf tee thing for a while but it gave me fits if I drove some where and stopped for a while and the car didi not cool all the way down. Serveral times after leaving the XXX on a Saturday it would die at every light on the way out of town till it got warmed back up.

[Poohsan]

I'm still fuzzy on this whole golf tee thing. On an 81 (pancake shaped) where do I place the Tee? Does anyone have and image?

Thanks!

[Paulporsche]

Pooshan,

The tee is often used to plug the line coming off the top of the decel valve as shown in Gunter's post. If you find the corresponding one on your valve you could try plugging that. Often, people squeeze the unit a little as mca mentioned.

On my early type valve I simply adjusted the port to be more open (pulled out) because I found that plugging caused backfiring and popping. Could be a property of my mongrel engine.

[mca]

Originally Posted by Poohsan

I'm still fuzzy on this whole golf tee thing. On an 81 (pancake shaped) where do I place the Tee? Does anyone have and image?

Thanks!

You should have a vacuum line coming off of one end of the decel valve - will be pointing towards the right side of engine (oil filter side). Pull this line off of the valve and stick a golf tee in the vacuum line.