CAR IS ON THE ROAD! Idle hanging a bit, CIS troubleshooting tips

[ADDvanced]

Hey Boys! CAR IS BACK ON THE ROAD! Stoked.

1977 911, CIS still there, engine out refresh, replaced all hoses, gaskets, etc.

The car ran perfectly before I redid everything, and I put 75 miles on it, the only issues I noticed were 3 things:.

1. Idle hang. When I come to a stop the idle is at 1600ish, and slowly goes down to the real idle speed of 950ish. All of my throttle bushings are new, so it isn't that. I've been reading that maybe it's the cold start valve dripping fuel and not turning off right away? Any other tips on what to look for?

2. Noticed breakup around 5k RPM 2 times in over an hour of driving. I do have a pertronix system installed, but some people have had issues? Not even sure where to begin on that one, as it seemed not repeatable.

3. After starting/stopping/off/on and driving 75 miles, I got home and parked in my driveway. 30 minutes later I wanted to pull into the garage, car would crank but wouldn't start. Took like 3 tries and then it came to life. Didn't do that previously.

Not sure if any of this is related, but posting so you have as much info as I do.

[Schulisco]

Vacuum leaks, wrong basic setup of the CIS.

Don't get me wrong - as lang as you haven't proved (with a smoke generator with pump) that there is no vacuum leak I don't believe your statement ("replaced all hoses, gaskets, etc." and "therefore everything is tight").

1. Check for vacuum leaks with a professional smoke generator and close all other openings of the CIS and pressurize it with the smoke generator. Pressurizing is important because some leaks are too tiny to see smoke coming out of with no pressure.
2. To determine if there too much fuel or air that kept idle high use a starter spray.
3. A proper running CIS engine must start on the first turn, no matter if it's cold or hot. Period.
4. No experience with the Pertronix ignition, but if it's not clear that the mixture is perfect under all conditions (idle and cruising with AFR 14.7; accelerating and WOT at 12.5) I tend to say that the ignition is not the problem.
5.Point #3 might hang together with a wrong control pressure from the WUR (check the pressures in all conditions!) or either the fuel accumulator or the check valve at the fuel pump is bad.

Fun fact alongside: We had in the german forum a case where the oil filler cap seal was the culprit for a temporary vacuum leak, not predictable, but it showed up in general when the engine was warm after a longer drive....

Check out for more info https://jimsbasementworkshop.com/CIS/pages/test_troubleshoot.html and https://jimsbasementworkshop.com/CIS/Troubleshooting%20Guide/Notebook%20Spiral.html

Thomas

[berndtj]

Also on point 3 as this just bit me, spread the pins on the fuel pump relay a bit to make sure you are making good contact. I spent like a week tracking down the same sort of symptom and that was it.

[gsxrken]

Congrats on the successful overhaul. Can’t help with the N/A driveability issues but wishing you a quick resolution. I will say like the other poster that over the years here I’ve noticed that vacuum leaks in those years are often the culprit even when the owner swears it can’t be that. And then it is.
The driving season is upon us. Good luck!

[ADDvanced]

Just find it hard to believe, since all the typical sources of vacuum leaks are either brand new hoses or my sealed/glued/screwed airbox.

[ahh911]

3. A proper running CIS engine must start on the first turn, no matter if it's cold or hot. Period.


Thomas,
How do you know this? It does not make sense. The cold start valve is timed so that somewhere between when the cold start injector turns on (key turned) to the mid point of the allotted time it will remain active is the correct number of turns.

Meaning, if the temperature is such that 8 seconds of cold start valve operation is available, then most likely 4 seconds is the correct start time, not the first turn. I think starting on 1 turn is a defect of the aged system. I've moved away from that easy solution (overly rich first crank startup, to several cranks. Takes more detailed work to achieve this.)

Phil

[Funracer]

Subbed

[Funracer]

Originally Posted by ADDvanced

Just find it hard to believe, since all the typical sources of vacuum leaks are either brand new hoses or my sealed/glued/screwed airbox.

Along the same lines did you have WUR or FD rebuilt? New fuel filter or accumulator or pump? I did my WUR was good right out of the box (thanks Tony) but my “calibrated” FD was set to the wrong system pressure. I assumed it was correct and never checked it before reinstall. Took me two months to find after the motor was back in the car.

My point is unfortunately because something is new or redone it can still cause an issue.

[PeteKz]

Back to the basics: When you turn on the key, do you hear the CDI box whine? Does the fuel pump run? I don't remember if the 77's have the airflow switch that controls the relay to the fuel pump. If so, raise the air metering plate and see if the pump runs, and listen for the injectors squeaking.

[Showdown]

At the risk of beating the dead horse… cis pressure test is the only way to find out what part of the system isn’t working. It could be something stupid simple but you’ll never know without data.

[ADDvanced]

Guys, I put 100 miles on the car in the past 2 days. It runs pretty good. It starts up on the first crank every time except for last night. Prior to the refresh, it ran awesome so I didn't adjust the WUR or FD, I did not change any of the CIS system components, just cleaned them, redid the airbox and all the hoses/gaskets.. Yes, I get CDI box whine, again, the car runs and drives pretty great.

[Schulisco]

Originally Posted by ahh911

3. A proper running CIS engine must start on the first turn, no matter if it's cold or hot. Period.


Thomas,
How do you know this? It does not make sense. The cold start valve is timed so that somewhere between when the cold start injector turns on (key turned) to the mid point of the allotted time it will remain active is the correct number of turns.

Meaning, if the temperature is such that 8 seconds of cold start valve operation is available, then most likely 4 seconds is the correct start time, not the first turn. I think starting on 1 turn is a defect of the aged system. I've moved away from that easy solution (overly rich first crank startup, to several cranks. Takes more detailed work to achieve this.)

Phil

Personal experience. I replaced the bad TTS on my SC with a new original from Porsche.

The CSV is nothing else than squirting start spray additionally into the airbox to make the engine start easier and quicker.
If the CSV doesn't work, the engine will also start, but not that quick and reliable. The TTS controls the CSV. The TTS (and therefore the CSV) is connected in parallel to the starter motor. While you crank the engine, the CSV squirts a bit extra fuel into the bottom area of the airbox (later SC have the "spider" pipes therefore in which the CSV injects the fuel). The CSV / TTS works only when the starter motor is engaging! When engine starts and you let snap the key back to ignition=on the CSV/TTS also stops as the starter motor does!
The max. time the TTS let the CSV squirt fuel is 8sec to prevent flooding the engine with fuel in the case of longer cranking if necesssary.

The old (electromechanic) version of the TTS only squirts up to 35°C / 95°F engine temp, the newer TTS version is fully electronic and squirts at any temperature half a second to easier and quicker start the engine.

This, in combination with other things like a correct control pressure giving WUR, a new fuel accumulator (the old FA was still tight, but worn and it often hanged anywhere so that free diaphragm movement wasn't sure anymore) and a new Bosch fuel pump with a new check valve brought a huge improvement on starting and operating the engine. Alongside I let overhaul the fuel distributor (I measured differing outputs far more then 10% which is too much to balance them with the screws) and put in new fuel injectors. All the named parts either were bad or not appropriate to the car. Surprinsingly the engine ran with the old / wrong stuff anyway, but it was harder to start, the engine consumed a lot more fuel (also due to a bad basic setup) and the performance was not that what I expected from a light and small 180HP sports car of the 80ies...

Thomas

[proporsche]

i did not read the whole post....
did you try to adjust the deceleration valve? 7mm open wrench and 13mm
take a picture of it if i could see it how do you have it?

Ivan

this one
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[ADDvanced]

Update: so previously, I just marked where my distributor was and installed it in the same location, but i snagged a timing light and timed the car yesterday over lunch, and..... low and behold, breakup above 4k rpm is gone. Pulls good. Still a little idle hang, but less?

Biggest issue now is hot start. Sometimes no problem, other times I need to crank and it sounds like it runs on one cylinder, pumping the gas doesn't help, eventually it will slowly run and I pump the gas a bit and then BAM it wakes up and idles/runs fine. Not sure what that is about, because sometimes it doesn't do that at all.

[Showdown]

Look at the residual fuel pressure. Check it with the gauges and you might find that it's dropping too quickly before the WUR resets.

There's a small O ring that if cracked or knicked will result in a rapid drop in fuel pressure. You also might need to shim the spring. All really easy fixes but you NEED to have gauges to set the pressure correctly.

[ADDvanced]

thanks for the tips! I'll look into gauges now, not sure if I can rent them or what.

[Schulisco]

See how such a pressure gauge can be built by yourself or purchased:

https://jimsbasementworkshop.com/CIS/pages/test_pressures.html

https://toolaid.com/product/33800

https://cis-jetronic.com/index.php?rt=product/product&product_id=309

https://www.vwvortex.com/threads/diy-fuel-pressure-tester.6060634/#post-82272967

https://www.klassikats.com/2021/08/21/how-to-fix-your-porsche-c-i-s-fuel-injection-system/

Thomas

[ADDvanced]

thank you!

[Schulisco]

Welcome.

I heavily recommend to watch the YT Channel of KlassikATS and his series about the 2,7 CIS 911: https://www.youtube.com/playlist?list=PLkWFofpgz2OgF9FdliKnlzuY0pygqRkMj

Thomas

[ahh911]

Originally Posted by Schulisco

the newer TTS version is fully electronic and squirts at any temperature half a second to easier and quicker start the engine.

Thomas

Interesting.
Thanks,
Phil