911 SC Coupe - Low speed makes car go "jumpy"

[PorscheCollecto]

Hi all,

Searching for few good ideas/hits to fast find the most plausible root cause of my 911 SC Coupé from 1979 weird behavior at low speed.

At very low speed (low acceleration - below 3000 rpms) in 1. gear, the car is running "jumpy", I honestly don't know the right word in English. But I think you get the idea when I try to explain it. In 1. gear it feels like you're running in a higher gear, which is too high to the low speed, so the car runs "jumpy".

At high speed or high revs the engine runs great and pulls strong. And there's no problems.

What would recommend to go through to find the root cause of the problem?

Ps: I got a few Porsche 911 SC's in my time, so I know this is not normal behavior.

Big thanks for any help.

[spunky.goat]

Does this happen when the engine is warm or cold or both?

[Bill Douglas]

Mine occasionally does this surging thing. Not often enough for me to try to fix it.

I just put the clutch in and it stops surging, then let the clutch out again and give it some gas.

[Schulisco]

In case of these "inspecific" problems there's always a list of tasks have to performed to get the whole picture of the CIS system.

1. Check the sensor plate height and the mixture (with either a certified gas tester or even a lambda gauge). Chase for vacuum leaks with a smoke generator. Mostly when vacuum leaks and won't identified, people enrichen the mixture by manipulating the sensor plate height and "adjusting" the mixture by turning the CO screw. The sensor plate height affects the mixture over all rpm band. If this is misadjusted the mixture won't be correct either for low or high rpms, bad or good cold start etc. Too low level of the sensor plate makes the lever make more distance which equals in richer mixture, too high level leans it.

https://forums.pelicanparts.com/porsche-911-technical-forum/1153872-no-fuel-no-start-3.html#post12174131
https://forums.pelicanparts.com/porsche-911-technical-forum/1153872-no-fuel-no-start-4.html#post12174201

2. Also check the ignition timing. Ignition maintenance is also recommended if this was long time ago (plugs, wires, dizzy rotor & cap).

3. Check the fuel pressures (control and system pressure)

4. Also a big concern may be the fuel accumulator. This is an often oveerseen part of the CIS. The manuals say it is okay if it keeps the pressure as described. But this prevents imho a mandatory check of it. I experienced on my car that it kept the pressure but there were still problems after almost everything on the CIS was maintained. So I took it out and tested it outside the car with an air compressor. I closed the output with a manometer and pressurized it. Then, after confirming that it holds the pressure, I opened the output wihtout depressurizing it first so that pressure relieved suddenly. Then the spring should immediately follow the pressure relieve. Mostly elder FA don't, they follow with hesitation! This is caused by wear or corrosion! This affects the mixture as well.After replacing it with a new one the mixture has to be readjusted. Also check the check valve at the fuel pump. Only together with the check valve the FA can do it's job.

Thomas

[MBAtarga]

O2 sensor maybe? I've heard it can cause surges.

[mike sampsel]

Originally Posted by Schulisco

In case of these "inspecific" problems there's always a list of tasks have to performed to get the whole picture of the CIS system.

1. Check the sensor plate height and the mixture (with either a certified gas tester or even a lambda gauge). Chase for vacuum leaks with a smoke generator. Mostly when vacuum leaks and won't identified, people enrichen the mixture by manipulating the sensor plate height and "adjusting" the mixture by turning the CO screw. The sensor plate height affects the mixture over all rpm band. If this is misadjusted the mixture won't be correct either for low or high rpms, bad or good cold start etc. Too low level of the sensor plate makes the lever make more distance which equals in richer mixture, too high level leans it.

https://forums.pelicanparts.com/porsche-911-technical-forum/1153872-no-fuel-no-start-3.html#post12174131
https://forums.pelicanparts.com/porsche-911-technical-forum/1153872-no-fuel-no-start-4.html#post12174201

2. Also check the ignition timing. Ignition maintenance is also recommended if this was long time ago (plugs, wires, dizzy rotor & cap).

3. Check the fuel pressures (control and system pressure)

4. Also a big concern may be the fuel accumulator. This is an often oveerseen part of the CIS. The manuals say it is okay if it keeps the pressure as described. But this prevents imho a mandatory check of it. I experienced on my car that it kept the pressure but there were still problems after almost everything on the CIS was maintained. So I took it out and tested it outside the car with an air compressor. I closed the output with a manometer and pressurized it. Then, after confirming that it holds the pressure, I opened the output wihtout depressurizing it first so that pressure relieved suddenly. Then the spring should immediately follow the pressure relieve. Mostly elder FA don't, they follow with hesitation! This is caused by wear or corrosion! This affects the mixture as well.After replacing it with a new one the mixture has to be readjusted. Also check the check valve at the fuel pump. Only together with the check valve the FA can do it's job.

Thomas

And after this check (if no success) check the fuel injector spray patterns and volume. Easy to do in you have the flexible fuel lines. You did not specify what year. A good test anyway to be sure all is well with the FD volume and the injector spray pattern.

[AndrewCologne]

Originally Posted by MBAtarga

O2 sensor maybe? I've heard it can cause surges.

See above, ... SC from 1979 – so no OX sensor present

[pmax]

Originally Posted by PorscheCollecto

At very low speed (low acceleration - below 3000 rpms) in 1. gear, the car is running "jumpy", .

Low speed or low acceleration ?

Here's the 911SC rpm vs speed tranny chart.