1978 911 SC high idle problem

[loemerich]

New member here.

I recently purchased a 1978 911 SC with 126k miles.

When my car is cold it idles at 2000rpm (which seems high even for being cold). When warmed up, idle is at 1100rpm. My understanding is that warm idle should be around 900rpm.

What is the best approach to tackle this problem?
TIA

[Funracer]

Welcome!

Did you get any maintenance records with the car? Do you get the sense it has been kept up and driven regularly, or has it been sitting for a while? Is the stock CIS in place? Do you have any experience working on cars and aircooled Porsches in particular? Will you be doing the work or looking for a shop to work on it? Does the engine run smoothly and car drive well (no stuttering or hesitations) when cold?

A lot of things can cause high idle.

Number one by far are air leaks. Tons of info here about how to find and fix. Use the search function.

Yours is not too far off where it should be. Both cold and warm idle are about 200 high. Do you know how to turn the idle down using the idle adjustment (air bypass) screw on the upper left side of the CIS? This is kind of cheating but you cant really hurt anything by turning the screw clockwise to reduce the idle and see if you get lucky. Keep track of how much you turn it so you can put it back where it was if necessary.

Timing. Fuel pressures. Fuel delivery etc etc.

Lots of help here. Ask questions. Get a Bentley repair manual. Read a lot.

And shoot us a pic of your new ride!

[Winter]

Here’s a place to start your CIS journey

https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/31-CIS_Fuel_Injection/31-CIS_Fuel_Injection.htm

There is a TON of info on the forum here, you may want to do a search and read a bunch of threads (which may inspire you or dismay you).

I picked up a 76 targa with CIS last fall. I’ll be the first to admit that I know little about the system, and that the upcoming driving season will be an educational period as I get my car sorted.

But plenty of experts reside here, so learn as much as you can so you can ask the right questions

And we all love pictures of cars, so yeah, post some of your new ride!

[loemerich]

Originally Posted by Funracer

Welcome!

Did you get any maintenance records with the car? Do you get the sense it has been kept up and driven regularly, or has it been sitting for a while? Is the stock CIS in place? Do you have any experience working on cars and aircooled Porsches in particular? Will you be doing the work or looking for a shop to work on it? Does the engine run smoothly and car drive well (no stuttering or hesitations) when cold?

A lot of things can cause high idle.

Number one by far are air leaks. Tons of info here about how to find and fix. Use the search function.

Yours is not too far off where it should be. Both cold and warm idle are about 200 high. Do you know how to turn the idle down using the idle adjustment (air bypass) screw on the upper left side of the CIS? This is kind of cheating but you cant really hurt anything by turning the screw clockwise to reduce the idle and see if you get lucky. Keep track of how much you turn it so you can put it back where it was if necessary.

Timing. Fuel pressures. Fuel delivery etc etc.

Lots of help here. Ask questions. Get a Bentley repair manual. Read a lot.

And shoot us a pic of your new ride!

Thanks for the quick response. I have plenty of records going back to 2008 and it seems to have been serviced by independent specialty shops since then. The last owner of 10 years put over 30k miles on it including longer trips of 200 miles or more so luckily it hasn't been sitting for long. Overall it runs pretty well without hesitation. Stock CIS is in place. Car is all stock except radio and missing air injection pump, A/C and rear spoiler. I've worked on BMW E30/E46 and VW Vanagon's before and will do most work on the SC myself. The Bentley manual should arrive in the next few days..

[loemerich]

Originally Posted by Winter

Here’s a place to start your CIS journey

https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/31-CIS_Fuel_Injection/31-CIS_Fuel_Injection.htm

There is a TON of info on the forum here, you may want to do a search and read a bunch of threads (which may inspire you or dismay you).

I picked up a 76 targa with CIS last fall. I’ll be the first to admit that I know little about the system, and that the upcoming driving season will be an educational period as I get my car sorted.

But plenty of experts reside here, so learn as much as you can so you can ask the right questions

And we all love pictures of cars, so yeah, post some of your new ride!

That's a great thread to check. I've only had my car for 6 weeks and just like you I'll be using the upcoming driving season (winters in MI with salty road don't work that well for a 911) to get things sorted - just thought the idle issue could be done while in the garage...

[PeteKz]

loemerich, welome to the forum.

You will need to buy or borrow a few tools you probably don't already have. You will need a smoke tester to find leaks. I recommend this one, and its on sale right now:
https://www.amazon.com/AutoLine-Pro-Automotive-Detector-Diagnostic/dp/B07JVT1NG2/ref=sr_1_5?crid=4ZUC78OR245M&dib=eyJ2IjoiMSJ9.EB_SN3fGNfpCIZToWHySf2HpcbHJD26Md 2J5OGsXeGOBNyk6Dtlu_YOp9IVYGMMFWsAC1fGvsNJYgpEwj8t Ru8DiQTSG6aWS0RzLYUfdeMXoHACZl-kR3kPVyQHFiDR51O9EjOP7ZpNJq-CePiWp-L1XYk-qllsLJ4B8YYIqR6AH34qOE5wlAY9j4AovDK3xhQ4jQFrl2QvRP FTUTYe0kc_5u8Km9AFnftJ4E-7xX04UtEyNlj14HgLMnl8orE7-PXHCqSmZBhepmQ1MvmY3O7uqTDA5fGerSl0NXoKkoxA.7COFdM tw6A2MpLmCy63t7UviO6LrTPrUTXusa-y-IDs&dib_tag=se&keywords=smoke%2Btester%2Bautomotive&qid=1744000792&s=automotive&sprefix=smoke%2Btewter%2Cautomotive%2C219&sr=1-5&th=1

The second thing you will need is a CIS fuel pressure gauge to check system pressure (SP), control pressure (CP), and residual pressure (RP). I suggest one like this:
https://www.amazon.com/Yonligonju-Injection-Pressure-Tester-Trucks/dp/B0CDVYNYV3/ref=sr_1_7?crid=1KIPEPVBH6G7C&dib=eyJ2IjoiMSJ9.Nrnor8f49paXqObDNEXlnCN3fu1nyN2tH xI_OCNaNGZpRHVO8v2x4eiLf31mmW4lP9ODIBv_JltpdIo0IZG HLG-Mutg4FCuUi4iD9aYMnvuND2tCIHID2CsrK9ooHqqBxw5IQgWQP 4i8lHL1x6O78N05jvJCzSp8QbanF-O4w9n44HVgZJxYuZCDOdA5VHTZSsr2_DIpb3Pe2bFfHNxJxXYu RbMiCjdxN1yrF5G_ueF0gSv9ct1t-qshIHmUsmgjNWc39bHAMu9ydHDjmURCvp9xyxL11ceQJ3cnyxO mXl0.KM6I0Pj00Ue86epF_aq7xh3RcLVHZ5je1wSpvnmwqRg&dib_tag=se&keywords=fuel+injection+pressure+tester+betool&qid=1744001047&s=automotive&sprefix=fuel+injection+pressure+tester+betool%2Cau tomotive%2C168&sr=1-7

As funracer said, the #1 cause of problems with CIS is intake air leaks. Finding and sealing those is the first part of getting it running right. Order those tools and we can guide you to use them.

That said, I suggest that you adjust the Idle Air Screw so that the engine idles at 950RPM fully warm. Then see how that affects the cold start idle. On a properly operating CIS, that will be close to 2000 RPM briefly right after you start it from cold.

Look around the engine to find any cracked, loose or disconnected vacuum lines.

[Mtsurfposse]

AAR auxiliary air regulator. If you’re at 2000 what’s ambient air temp? Is is 30-60 degrees? If it is then it’s pretty close to spec and closes normally after 2 minutes. All normal. Maybe a touch high idle. It should be 900-950 when car has been driven for 15 minutes. That’s how long it takes for a total warm up. If you drop idle 150 then it’s sitting at 1850 for cold start= fine.

At 60-90 degrees cold start it should idle for 1-2 min at 14-1500 ish

Your modern Toyota will warm up at 1800 rpm for 1-2 minutes, you just won’t hear it or feel it

[Funracer]

GPW and lobster? Sweet!

[loemerich]

Originally Posted by Funracer

GPW and lobster? Sweet!

Yes - it looks awesome in person. (IMHO)

[loemerich]

Originally Posted by Mtsurfposse

AAR auxiliary air regulator. If you’re at 2000 what’s ambient air temp? Is is 30-60 degrees? If it is then it’s pretty close to spec and closes normally after 2 minutes. All normal. Maybe a touch high idle. It should be 900-950 when car has been driven for 15 minutes. That’s how long it takes for a total warm up. If you drop idle 150 then it’s sitting at 1850 for cold start= fine.

At 60-90 degrees cold start it should idle for 1-2 min at 14-1500 ish

Your modern Toyota will warm up at 1800 rpm for 1-2 minutes, you just won’t hear it or feel it

I wasn't aware that cold idle is that high...My 2015 X3 cold starts at maybe 1200rpm but is down to 800rpm within 30sec. I guess I need to remind myself that a 1978 fuel injection can't be compared to today's systems.

[blucille]

Agree with PeteKZ, I would start by checking the timing, if it is set properly, I would adjust the idle screw to bring the warm idle down to spec, and then you can see what the cold idle looks like.

When adjusting the idle, it's probably a good idea to note how much you need to turn the screw, in case you find a vacuum leak or something else later on that was causing the high idle. The big rubber boot that connects the air flap to the throttle body often gets cracked, but there's also a few vacuum hoses that should all be checked. But the early CIS cars don't have too much stuff to go wrong.

[Mtsurfposse]

Originally Posted by loemerich

I wasn't aware that cold idle is that high...My 2015 X3 cold starts at maybe 1200rpm but is down to 800rpm within 30sec. I guess I need to remind myself that a 1978 fuel injection can't be compared to today's systems.

You’re right, modern cars arnt that long. I hadn’t thought about the numbers and time for 5 months. My Toyota starts at 1600 for and quickly drops to 1200 for 1-1/2 minutes.

That aside, the CIS and in particular, Aux air is pretty slow to change as it’s waiting for that metal bar to first get warm and then to move instead of electronically actived switches valve ect

[loemerich]

No vacuum leaks to report. Adjusted the idle screw 3/4 of a turn clockwise for a warm idle of 900rpm. Based on some marks, the idle screw had previously been moved. I'll report back as needed.

[mike sampsel]

Idle of 950 - 900 good for stock cams. If it has other cams, someone might set your idle higher from 1000 - 1100 say, which is what some experts here recommend for 20/21 WEB cams.

[loemerich]

Originally Posted by mike sampsel

Idle of 950 - 900 good for stock cams. If it has other cams, someone might set your idle higher from 1000 - 1100 say, which is what some experts here recommend for 20/21 WEB cams.

As far as I know the car is all stock except for the stereo….