2.7s stumbling at low RPM

[kupferbraun77]

Hey everyone,

I’m dealing with an issue on my 1977 Porsche 911 2.7S (CIS) and would love some input (fe is it fuel or ignition related). The car experiences a stumbling/hesitation around 2000RPM. Most notable in 2nd gear, but also in other gears.


Symptom:
The car stumbles around 2000 - 2500 RPM under light/constant load, but runs strong above 3000RPM up to high RPMs.

https://www.youtube.com/shorts/e65OZSPmVq8


Current Setup:
The engine is fully rebuilt. I’ve backdated the exhaust system and deleted the EGR.

Some checks i`ve done:

Fuel Pressures:
- System Pressure: 5.0 bar (within spec)
- Control Pressure: 3.0 bar at operating temperature
- Control Pressure with 500mbar Vacuum: 3.7 bar (seems correct)
- Injector Delivery: all new injectors, delivering same amount of fuel
WUR is rebuild, holds vacuum.


Ignition Timing:
Timing is set to TDC (0° BTDC at idle)

Air/Fuel Mixture:
Currently at 2.5% CO
Tried different AF mixtures. Mixture past 6% (way too high) seems to mask the stumbling. But this is far from ideal.

CIS System is also rebuild, apart from Fuel Distributor.

[fanaudical]

Let's start with ignition.

Which plugs are you using? Plugs gapped correctly?

Checked plug wires for arcing?

Cap and rotor in good shape?

Do you get appropriate ignition advance as engine speed increases?

How much distributor shaft side-play do you have?

Points set correctly? Dwell OK?

[nzrobinz]

Check for vacuum leaks - usually the issue

[Schulisco]

Could you post a picture from the resting position of the sensor plate? (Bottom view). Either there is a spring or probably already the adjustable screw to adjust it.

[choox2]

Just out of curiosity, check your fuel mixture adjustment port (next to fuel distributor). I recently had some assistance rebuilding my fuel system/new fuel distributor and the 3mm hex was left in the adjustment port/screw. I had the EXACT same bucking at +/- 2500, but fine at higher RPMs until we looked around. Tech was embarrassed he left the key in the slot

[Till Krueger]

Definitely do that fuel mixture adjustment port well, those are trifling mistakes that can easily cost a lot.

[kupferbraun77]

Thanks for all the info! I finally found some time to do some checking.

Ignition
- Plugs, Wires -> brand new
- Plug gap -> 0,55mm-ish
- Distributor cap and rotor -> in very good shape
- Distributor gap -> was 0,2mm adjusted to 0,35mm -> Made no difference
- Didn`t check the dwell, because of the simple fact I don`t have a dwell meter

CIS
- No left-behind 3mm hex screwdriver :-)
- Checked airflow meter height -> 0.1mm as specified (see pictures)
- Did a smoke-test -> No leaks at all
- Injectors and O-rings all brand new.
- The plunger of the Fuel Distributor moves freely

Here are some pictures I made along the way:
https://share.icloud.com/photos/089e5gwXh6RyzhdAj68RBVNlQ

My toughs at the moment (for whatever it may be worth):
I noticed that enriching AF mixture, has impact on the problematic RPM-range. For example, it moves down 500. Could this rule out ignition? If CIS is causing the problem, I suspect the AF mixture at some throttle position (low RPM) is not correct, but eases out when RPM climbs (more draft)

Next steps:
- Check ignition timing. Currently 0 degrees at idle, planning to set at specified 35deg@6000RPM
- Deep-dive again in WUR enrichment (although I tested it with pressure meter, control pressure drops when accelerating)
- Deep-dive in Fuel Distributor

[kupferbraun77]

On addition to the steps above, also planning to film the sensor plate while driving. To see if any resonation occurs while entering the problematic RPM zone.

[Schulisco]

Originally Posted by kupferbraun77

On addition to the steps above, also planning to film the sensor plate while driving. To see if any resonation occurs while entering the problematic RPM zone.

That would be pretty interesting, never seen such a video yet! Greatly appreciate it.
But - I don't suspect any benefits on your issue. Nevertheless - a shaking sensor plate is - if it's the case - pretty uncool for a CIS ...

A stumbling CIS engine - no matter if high or low rpms - is imho mostly caused by an inappropriate mixture for the particular driving condition.
This means the CIS is misadjusted due to one or mostly more issues. In short: It's tuned to hide some issues.
Which is good and bad at the same time. Good because you can get a CIS running anyhow, bad because of buying other issues with that.

In consequence you have to check almost everything on a CIS: Mixture, pressures, tightness, ignition timing, etc.

How many turns do you need to close the idle air screw? (reset it again after checking).

Thomas

[PeteKz]

If the engine runs well below and above the problem RPM range, then you can pretty much rule out an ignition problem. Ignition problems show up first at high RPM and high load. Of course, check that the spark advances with RPM, but you already did that, right?

If you can get a video of the sensor plate while driving, that would be very good info to add to the archives here. I've never seen such a video. In addition to transient behavior, I'm curious how far the plate opens at full throttle.

[kupferbraun77]

Originally Posted by Schulisco

That would be pretty interesting, never seen such a video yet! Greatly appreciate it.
But - I don't suspect any benefits on your issue. Nevertheless - a shaking sensor plate is - if it's the case - pretty uncool for a CIS ...

A stumbling CIS engine - no matter if high or low rpms - is imho mostly caused by an inappropriate mixture for the particular driving condition.
This means the CIS is misadjusted due to one or mostly more issues. In short: It's tuned to hide some issues.
Which is good and bad at the same time. Good because you can get a CIS running anyhow, bad because of buying other issues with that.

In consequence you have to check almost everything on a CIS: Mixture, pressures, tightness, ignition timing, etc.

How many turns do you need to close the idle air screw? (reset it again after checking).

Thomas

Hi Thomas,

Idle air scew is screwed out 2 full turns.

Roel

[Schulisco]

Originally Posted by kupferbraun77

Hi Thomas,

Idle air scew is screwed out 2 full turns.

Roel

Perfect!
This suggests that that there might be no vacuum leak. Good.

Next tasks:
1. Check if the dizzy advances as it should.
2. Pull out the Fuel Accumulator of the car, close the output with a pressure gauge of at least 5bar (not to overload it), then pressurize it compressed air up to 5bar max. Depressurize it suddenly by opening the hose at input. Does the piston follows the when the pressure reliefs? Or is there a hesitation or it is even stuck? It must follow immediately with no hesitation the pressure.
If not, it confuses the whole CIS with similar symptoms as you experience...

More info:
https://forums.pelicanparts.com/porsche-911-technical-forum/1119829-accumulator-pump-check-valve.html#post11704708

Thomas

[kupferbraun77]

Checked distributor advance, seems to be okay. 0tdc at idle and 35@6000rpm.

Made the video of the sensor plate. Plate seems stable also during the lower “bucking” rpm
https://youtu.be/4MlPi-cRCOk?si=uY7d2hVJHTHWZMa5

Next: accumulator check. Also wondering if my self-rebuild WUR could trigger this behavior. I think I did the rebuild as it should (fuel prssures within spec and holdong vacuum for enrichment). But… just an amateur 😅

[Schulisco]

The video from the sensor plate while driving is amazing!
It's very interesting to me as the senor plate doesn't move as the engine sound did and identifying when the engine stumbles. This let me conclude that the air flow into the engine is pretty even which is very good and let me believe that there's no issue with cam timing or sth. else. Also ignition timing seems from that perspective pretty normal (despite that a check of ignition timing is mandatory when chasing CIS issues) and I suppose - as I did earlier too - that the mixture is not correct! It may run too lean or too rich for the driving condition.

Yeah, first and foremost check the FA! It may be the culprit or - if not alone - ampliflies the phenomenom, when it cannot flatten the fuel pressure vibrations. I gave all the info how to test it (pressurized air from a compressor) above.
If it hesitates ab cannot move freely any more - replace it and test drive it.

If this is proved okay and symptom is still there, commence with this:
Either you check the mixture with an AFR gauge while driving or on a test stand. AR should be ~14,7 ideally while cruising with engine warm, and ~12,5 on WOT/accelerating. I bet it's way out of these values ...

If you cannot lend a AFR gauge for a test drive then I strongly recommend to do the rest of the basic adjustement of the CIS as follows:

Here I linked the procedure from KlassikATS in his YT video:
https://forums.pelicanparts.com/porsche-911-technical-forum/1119098-911-sc-cis-residual-pressure-2.html#post11699451

Bosch / Porsche itself recommends this way in their workshop manuals:
https://forums.pelicanparts.com/porsche-911-technical-forum/1108835-cis-sensor-plate-height-2.html#post12201501
I prefer to follow this way. Why? Kurt from KlassikATS is one of the most experienced Porsche specialists in the web and probably also in the real world. But I disagree with him in this point as opening a single fuel line behind the fuel distributor may change the pressure situations for the others. And this may confuse / distort the adjustment. Never proved by myself. But my experience told me that it will as pressures in fluids distribute physically. The injection valves will make a pretty unique whine sound when they start spraying fuel:
https://www.youtube.com/watch?v=4iWZdvOow0g
When you hear that noise (without lifting the sensor plate!!) then turn the mixture screw 1/2 back to shut down the injection. That's the point you can start the engine (if all works fine) and go for a drive directly to a certified gas tester to finally adjust the mixture as Porsche described to the given CO values. Then you're done. If the stumbling still persists then it's still one or more another issues but don't ever touch this basic adjustment again!

Thomas

[Mike Czech]

I also have a '77 911S and have/had similar stumbling around 2,000-2,500 rpm in all gears after the car warmed up. When it was cold, I didn't have stumbling and the car ran great throughout the rpm range. There is another thread here on Pelican which addresses this issue, and it was mentioned that this was an issue from the factory - a long-time Porsche dealer mechanic mentioned this in the thread.

I happened upon a partial solution for my stumbling. I needed to fix a cam box oil leak and in the process I discovered that my cam timing on both sides was WAY off spec. After I replaced the gaskets, re-timed the cams per factory spec, most of my stumbling went away. I only have a very very slight stumbling now. If you can't diagnose the problem elsewhere, your cam timing might be something else to check.

[Mike Czech]

Also to add what I mentioned in my previous response, I have a rebuilt WUR with factory spec pressures, a rebuild fuel distributor, new injectors, and I'm pretty sure that I don't have any air leaks in the air box. I haven't measured my CO2 exhaust, though.

[kupferbraun77]

Shot a video of fuel pressure gauge while driving. At idle pressure is at 4. When accelerating the vacuum in WUR drops which sets control pressure to 3.2Also under constant throttle… this is when the stumbling appears.
https://youtu.be/LQtFhHxNhQ4?si=g4XJdpw4C1BTTpri

Any toughts?! Is there anything off here?

[Schulisco]

Way too low. Must be consistant higher than 4,8 bar as Porsche tells in the specs. Check out fuel pump, check valve, fuel filter and fuel accumulator. If they're tested and proved good check the fuel pressure regulator in the fuel distributor.
Fuel pressures and vacuum tightness are almost everything on a CIS.

[kupferbraun77]

@Schulisco are you sure? I placed the gauge between fuel distributer and WUR. This should maesure the control pressure and not the system pressure. System pressure is 5 bar

[Schulisco]

Originally Posted by kupferbraun77

@Schulisco are you sure? I placed the gauge between fuel distributer and WUR. This should maesure the control pressure and not the system pressure. System pressure is 5 bar

Okay. My fault, thought it was the system pressure. General seems to be ok basically.
But the same here - the control pressure must not vary this large...it should be also mostly constant/steady and slow rising and not synchronized with the throttle. Are you sure you git the right WUR for the car? Does it has a vacuum connection? If it has one and should have a WUR with vacuum connection check the vacuum connection. I suppose that it mixed up with the ignition distributor vacuum advance...

Here are the tech specs for the 1977 911(s) out of the Porsche workshop manual:





Crosscheck the part numbers for WUR, FD and distributor if they fit to the car too.

Also make the following test: Close the vacuum connection to the wur and check the cp if it's now consistant to the temperature only. Then make a short test drive. If the behaviour is better, check the vacuum lines meticously.

Here you find a vacuum diagram:
https://cis911primer.com/pages/descr_midyear.html

Thomas