CIS Gurus. 1980 911SC High Idle w/ Idle Screw In

[Ltanderso]

Hi,

Sorry for the novel here. The gist is that I have to adjust the idle screw all the way in to get my car to idle at 950 rpm, which implies a vacuum leak, but I can’t find one. The history of repairs and refreshing is spelled out below.

I’m new to 911 ownership, but I’m fairly good with cars and have spent dozens of hours learning about the vehicle that I purchased. I’m hoping that I’ve run down all the obvious answers that are routinely thrown around because those are easy to identify through searching the forums.

I bought a 1980 911SC with 70k miles. It’s mostly stock, but it wasn’t driven much over the last 10 years. I’ve been going through all the bits that degraded by sitting up (suspension bushings, rubber seals and o-rings, etc.). I’ve been going through the CIS system. Somehow I had a cracked airbox, but it was cracked above one of the mounting points, not a seam crack that is typical through an intake backfire. The rubber mounts were trash, so I think they allows the airbox to hit the metal bracket. Because of this, I took the whole CIS system out and rebuilt/replaced a whole lot of components. I replaced the following:

1) Airbox
2) Intake runner seals
3) Intake boots at airbox
4) Injector boots and both o-rings
5) Soaked/cleaned injectors
6) soaked/cleaned fuel distributor
7) All new seals around throttle body and airflow meter
8) New seals on triangle of death items on top of engine

In addition to these, I confirmed
1) control/system/cold fuel pressures are all in spec
2) the AAR closes when 12v is applied
3) there were no visible vacuum leaks anywhere with a smoke test
4) the idle drops when I remove the oil filler cap
5) timing at idle is good (plugs, wires, cap, rotor are all new)
6) frequency valve is function by unplugging the relay under the passenger seat
7) AAV is disconnected and the vacuum hose is plugged

I have one limitations on my ability to test items. I don’t have a CO meter, but I set the fuel mixture using a wideband O2 sensor, and it’s set around 14.1 at idle. I also know that one headstud on cylinder #3 is broken, but I do not hear any leaks. I will likely have to pull the engine to replace, but that is a summer 2025 project. I have a place to replace it in place, so we’ll see.

The only other item that has appeared since the CIS refreshing is that one time on startup up, it ran like garbage. Really rough idle and it was backfiring pretty bad. I drove it gingerly for 20 minutes, and it finally cleared up. I’ve started and run the car 6-8 times since then, and this hasn’t reoccurred. In my mind, something mechanical stuck in the wrong position, but I haven’t been able to recreate this to prevent a further issue.

So I’m looking for advice on where I could have a vacuum leaks that wouldn’t appear during a smoke test. Or, if you think that there could be something else wrong, I’d love to hear some ideas.

Thanks.

[walt]

Probably good on vacuum if it passed the smoke test and you had an rpm drop with removal of the oil filler cap. Might pay just to pop off the throttle linkage to verify the throttle plate is fully seated.

You did a lot of work on the CIS, did you check the clearance around the sensor plate.

[Glenfield]

Might double check your ignition advance. I may be totally off (and someone will correct me if so), but believe too much advance at idle will keep RPMs too high. I had a similar experience earlier this year after car was at mechanic … turns out they volunteered some adjustments to CIS and set advance too high.

Edit: sorry, re-read your post and sounds like you’ve already confirmed idle.

[pmax]

Originally Posted by Ltanderso

I bought a 1980 911SC with 70k miles. It’s mostly stock, but it wasn’t driven much over the last 10 years.
...
The only other item that has appeared since the CIS refreshing is that one time on startup up, it ran like garbage. Really rough idle and it was backfiring pretty bad. I drove it gingerly for 20 minutes, and it finally cleared up.
...

Did you clean out the fuel system from the fuel tank, filter to the CIS etc

That one time rough startup could be due to crud.

[Ltanderso]

Thanks. I’ve been thinking about checking the throttle plate base position. The set screw appears to be in an original position, but it doesn’t hurt to check. I have checked the timing, and the advance at idle is properly set.

I did clean out the fuel system from the filter to the injectors, so crud could be the explanation there.

[Ltanderso]

I also checked and adjusted the air meter sensor plate during the whole process.

[boyt911sc]

What are the last 3 digits on your WUR? What is the heater resistance (Ohms) at room temp. or cold motor? What are the values of your CCP and WCP? I suspect you are getting unmetered air into your system. Double check it.

Tony

[dkirk]

Could be a sticking auxiliary air device. Pretty straight forward removal and clean with an aerosol spray carb cleaner.

[boyt911sc]

How did you perform the smoke test? And how did you isolate the airbox from atmosphere during the test?

Tony

[PeteKz]

Ltanderso: It sounds like you read through much of the CIS information here and/or in a manual. DO you have a shop manual or Bentley manual for the SC? If not, get one. I prefer the Bentley because it's one book, and covers almost everything the official shop manual covers.

You mentioned that it didn't get driven much for 10 years. It's a commonplace here that CIS does not like to sit for long periods, especially since the fuel supply got contaminated with ethanol. I noted that you cleaned out much of the fuel system, but I still recommend that you put a bottle of Chevron Techron Concentrate in it, then go drive it hard and continuously. That may improve its operation.

The WUR should not cause a high idle, but as Tony asked, post the number on the top of your WUR to help in troubleshooting, and the number on your Fuel Distributor. And, describe your smoke test.

Check your power brake booster vacuum hose tat goes from the left side of the air box forward to the tunnel and frunk. Several people here recently have found that circuit leaking, which would provide the false air that could cause your idle to be high.

You do not need a CO meter if you have an AFR/lambda meter. I think those are much more accurate than a CO meter that you stuff up the tailpipe. You can look online for the equivalent lambda or AFR that corresponds to a particular CO reading. IIRC, 14.1 is in the ballpark.

Re the broken head stud: Do not let this go for long, and keep a close watch on the other studs. You do not want the head get loose on the cylinder or you will be doing more repair work. These engines had lower studs made of Dilivar, which is supposed to expand with heat more than steel studs, to keep the tension on the head and cylinders more even. However, as you may know, the Dilivar has severe problems with stress cracking and failure in that application. It is not a question of "if" but "when" they will break. When you drop the engine, remove both banks of heads and cylinders and replace ALL the lower studs with steel studs (that's what is in the top row). DO NOT disassemble the case just because you are "already in there." With only 70Kmi, the lower end is still almost new. Leave it alone unless something is wrong.

But first do the Techron treatment and drive it hard and long, then report back whether it has improved.

[Ltanderso]

Hey guys,

A few updates here. I’ve performed it several times, but I just when through a fuel pressure test to see if anything changed, and all pressures are within spec using the Bentley manual (system, cold and warm). I ran a smoke test by 1) removing the rubber intake boot, 2) sealing the throttle body and the the two lines that run into it, and 3) connecting a smoke machine to the line coming out of the driver’s side of the lower 1/2 of the airbox.

As an additional test, I also clamped the rubber brake vacuum line near the pedal box to make sure there was no vacuum leak in the brake booster. I did recently replace the master cylinder because it was leaking, so I thought that the booster could be damaged due to the small amount of brake fluid/corrosion that existed (which I cleaned during the master cylinder replacement). Clamping this line did nothing to the high idle.


I did get a little more information yesterday. I rigged my wideband to be able to drive down the road with it connected to work on the stutter under acceleration. I adjusted the AFR at idle to be a little rich, at 13.9. I learned that when driving down the road the mixture leans out as the RPM increases. It goes all the way up to almost 16. What would cause the fuel mixture to be proper at idle but lean out as driving?

[silverlock]

We beg you to post your fuel pressure test data and your WUR model.

[Ltanderso]

WUR is model -072.

Cold pressure at 16 C is 2.1 bar
System pressure is 4.7 bar
Control pressure warm is 3.6 bar

[Ltanderso]

Also, just to answer the question, the WUR resistance across the two terminals is 10.2 ohms; although, I’m still unclear how this is relevant. I’d love for someone to explain it to me. The only thing I can reason is that resistance that is outside of spec would just control the time for the WUR to “activate” but wouldn’t have any impact on fuel pressures once activated.

[pmax]

Originally Posted by Ltanderso

...

I did get a little more information yesterday. I rigged my wideband to be able to drive down the road with it connected to work on the stutter under acceleration. I adjusted the AFR at idle to be a little rich, at 13.9. I learned that when driving down the road the mixture leans out as the RPM increases. It goes all the way up to almost 16. What would cause the fuel mixture to be proper at idle but lean out as driving?

80SC and onwards CIS systems, assuming it's a stateside car and hasn't been mucked up, are nicely improved with a O2 sensor lambda controlled closed loop system.

The signal from the O2 sensor goes to the control box under the passenger side (for US) seat which sends a 70 (not sure if this is correct freq) Hz signal to the frequency valve which modulates the FP to the fuel distributor.

The CIS primer is a good read here thanks to Mr Jim Williams (and RarlyL8 ) ...

https://cis911primer.com/home.html

Originally Posted by Ltanderso

WUR is model -072.

Cold pressure at 16 C is 2.1 bar
System pressure is 4.7 bar
Control pressure warm is 3.6 bar

072 FP chart is here

https://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies-4.html#post9671930