CIS Running Rough

[DienstXIV]

Hi Everyone,

I have a 1975 Porsche 911S and I have been having some issues with my engine running rough. When I fire up the engine when it's cold the engine RPMs go up and down and it doesn't have a steady idle. if I rev it up the car revs up but won't go back to idle, it just dies. it does this for about 15 to 20 minutes

Once the car is warmed up the engine idles okay but when I rev the engine up the rpms stay at around 2000 RPMs and then slowly in about 5 to 10 seconds the rpms go back down to idle.

Does anyone know what I should check? I have replaced the fuel pump, had the WUR professionally rebuild, I replaced the cold start valve, the fuel injectors and seals and sleeves. I have also replaced all of the vacuum lines and replaced the intake runner seals. I replaced the plugs, wires, distributor cap, rotor and points but I did not adjust the dwell on the points I just tried to put the points in as close to how the old ones were as possible. I also adjusted the fuel mixture using the Gunson gas analyzer. I tried to get it to about 3% CO but when I took it to the emissions testing place it was 4.2%

Thanks in advance for the help!

Cheers,
Alex

[kach22i]

Sounds like you have been diligent on going though the usual suspects, "The List" found on the CIS Primer webpage.

You still have a vacuum leak it sounds like.

Most likely the one last hose on the back of the engine that requires a partial engine drop. It's a large hose that provides vacuum pressure to the brake booster and ties into a metal fitting located center and at the bottom of the firewall which forms the rear seat area (see post #6). If you have manual brakes with no booster ignore what I just said.

The other areas that could still be leaking are the airbox, check to see if the screws are raise up on the inside under the air filter.

Propane and Ether (starting fluid - have extinguisher on hand) methods sensing a increase in rpm are not always night and day.

Your boot may be shot, or the leak is on the underside and backside of the airbox where the spray is not getting.

I have a similar issue I'm chasing. A trick I did last year but not really needed this year is to let the car run for 5-minutes, then let it sit for 5-minutes before restarting. Somehow the little bit of heat generated by running the car has time to expand and seal the leak better than letting the car warm up for 10 minutes.

I just watched his cigar method video, interesting.

Porsche 911 CIS Leak Test using cigar smoke
https://www.youtube.com/watch?v=u78gJWYZgy4

Video below shows the screws that can be raised and an indication of a cracked box.

Part-1
How to Inspect your Porsche 911 CIS Airbox Fix Leaks & Install a Pop Off Valve Project Airkult EP23
https://www.youtube.com/watch?v=QC2GBB7nYBw

Part-2
How to Rebuild CIS Fuel Injection Porsche 911S! Teardown & Modification, Projekt Airkult Episode 24
https://www.youtube.com/watch?v=LNHasTH92Bg

EDIT:

I've also read that the distributor may have some vented lines coming in and out of it, don't want an air leak there either.

[IS300]

Ill be selling my cis soon

[DienstXIV]

Originally Posted by kach22i

Sounds like you have been diligent on going though the usual suspects, "The List" found on the CIS Primer webpage.

You still have a vacuum leak it sounds like.

Most likely the one last hose on the back of the engine that requires a partial engine drop. It's a large hose that provides vacuum pressure to the brake booster and ties into a metal fitting located center and at the bottom of the firewall which forms the rear seat area (see post #6). If you have manual brakes with no booster ignore what I just said.

The other areas that could still be leaking are the airbox, check to see if the screws are raise up on the inside under the air filter.

Propane and Ether (starting fluid - have extinguisher on hand) methods sensing a increase in rpm are not always night and day.

Your boot may be shot, or the leak is on the underside and backside of the airbox where the spray is not getting.

I have a similar issue I'm chasing. A trick I did last year but not really needed this year is to let the car run for 5-minutes, then let it sit for 5-minutes before restarting. Somehow the little bit of heat generated by running the car has time to expand and seal the leak better than letting the car warm up for 10 minutes.

I just watched his cigar method video, interesting.

Porsche 911 CIS Leak Test using cigar smoke
https://www.youtube.com/watch?v=u78gJWYZgy4

Video below shows the screws that can be raised and an indication of a cracked box.

Part-1
How to Inspect your Porsche 911 CIS Airbox Fix Leaks & Install a Pop Off Valve Project Airkult EP23
https://www.youtube.com/watch?v=QC2GBB7nYBw

Part-2
How to Rebuild CIS Fuel Injection Porsche 911S! Teardown & Modification, Projekt Airkult Episode 24
https://www.youtube.com/watch?v=LNHasTH92Bg

EDIT:

I've also read that the distributor may have some vented lines coming in and out of it, don't want an air leak there either.

My car has manual brakes. I checked to see if I could find any cracks in my airbox without removing it but I couldn't see any. All of the screws in the airbox are nice and tight with no gaps. When I bought the car the airbox already had a popoff valve. The rubber boot on top of the CIS appears to be in great shape.

When you are talking about the distributor, are you talking about the fuel distributor or the ignition distributor? I couldn't find any vented lines coming out of my fuel distributor? I was thinking maybe the timing on my ignition distributor is off along with my points not positioned correctly because I don't have the tool to measure and set the dwell. I have decided to just buy the Pertronix electronic ignition. I replaced the vacuum line going from the distributor to the intake manifold.

I was thinking maybe my WUR vacuum line is installed incorrectly? I have a T fitting in the top of the decel valve but I think it is supposed to be connected to the bottom of the decel valve. I was told it doesn't matter but the hose on top is significantly smaller than the one on the bottom so maybe my WUR isn't getting enough vacuum?

Could my fuel distributor need a rebuild?

[Paulporsche]

Alex,

Even though your WUR was professionally rebuilt, it still may be out of spec. Which WUR do you have? See the chart and compare with the number on your WUR for the spec.

Sounds like your engine is running too rich, at least at startup. You should have your fuel and control pressures measured to see if you match the chart.

Your points setting may indeed be off which will affect your timing, which impacts whether you have the correct mixture. Either check the dwell (38degrees +/- 3) or install the Pertronix Ignitor 2.

See also the correct vac line hookup for a 75 in my next post

[Paulporsche]

Alex,

I can't save the vac connections diagram for you. If you go to the Porsche webpage and access the PET for your year, you will find all parts diagrams, incl vacuum connections for 1975-77

[DienstXIV]

Originally Posted by Paulporsche

Alex,

I can't save the vac connections diagram for you. If you go to the Porsche webpage and access the PET for your year, you will find all parts diagrams, incl vacuum connections for 1975-77

Can you send me the link to the Porsche website? I am not familiar with it.

[DienstXIV]

Originally Posted by Paulporsche

Alex,

Even though your WUR was professionally rebuilt, it still may be out of spec. Which WUR do you have? See the chart and compare with the number on your WUR for the spec.

Sounds like your engine is running too rich, at least at startup. You should have your fuel and control pressures measured to see if you match the chart.

Your points setting may indeed be off which will affect your timing, which impacts whether you have the correct mixture. Either check the dwell (38degrees +/- 3) or install the Pertronix Ignitor 2.

See also the correct vac line hookup for a 75 in my next post

My WUR part number is: 0438 140 009. I don't have the tools to measure the fuel pressure, I was measuring the CO percent to figure out if my car was running too rich or not. I took my car to the emissions testing center and it was at 4.2% CO what should the CO be? I have seen 3% but also 1.5% to 2.0%. The limit for the county emissions is 5% so I passed emissions but I still might be running too rich for specification. I have a Gunson gas analyzer but it is a little finicky. Will this method work or do I really need to get a fuel pressure tester? I am not sure a fuel pressure tester from O'reilly's or Autozone will work.

[boyt911sc]

Alex,

Use a CIS pressure gauge kit to measure your fuel pressures (system, control, and residual). You have control (cold & warm) fuel pressures. The warm control fuel pressure for vacuum assisted WUR-009 is measured 2 ways (without vac applied and with vac.).

It is mandatory to have a fuel pressure gauge for troubleshooting any fuel injection system regardless of type. Without one, you have too many unknown variables. Borrow or buy one. It will save you money and from aggravation.

Lastly, perform a smoke test to locate those hard to find air leaks in your CIS. Not being able to see any does not mean you don’t have one. A pressure test would reveal or confirm the integrity of your vacuum system. I would bet you have vacuum leak/s by the way your engine behaves. Keep us posted.

Tony

[kach22i]

Originally Posted by DienstXIV

My car has manual brakes. I checked to see if I could find any cracks in my airbox without removing it but I couldn't see any............

When you are talking about the distributor.....

Good on the manual brakes, one less vacuum hose leak to worry about.

I do not think either one of our cars are EGR equipment, those are on a little later cars with more smog protection, I only mentioned it because non-stock parts may have been substituted at some point in the car's life.

I too once looked for cracks too, but the kind from not having a pop-off valve that result in quite obvious ones. There are other possible breaches/cracks in places you cannot easily see and might open up under very specific conditions (very cold or very hot). These hard to find cracks are illustrated in the Cigar Smoke test video I posted earlier.

Screenshot below from about the 3:20 mark.

https://www.youtube.com/watch?v=u78gJWYZgy4


I got rid of the points in my first year of ownership, went with the Pertronix Ignitor magnetic pick up system, it has been great. However please note that your rotor under the distributor cap must now be a non-resistor style if I recall correctly. And I think it can be rev limiting, which is a great safety feature.

If the above is incorrect, please someone chime in, I'm going off really old memories.

I also recall at one time some of the rotors had to be filed down and about 1/16" shaved off the bottom to get it to seat properly (too tall) with the Pertronix Ignitor, but last few rotors I've used fit without modification.

[DienstXIV]

Originally Posted by boyt911sc

Alex,

Use a CIS pressure gauge kit to measure your fuel pressures (system, control, and residual). You have control (cold & warm) fuel pressures. The warm control fuel pressure for vacuum assisted WUR-009 is measured 2 ways (without vac applied and with vac.).

It is mandatory to have a fuel pressure gauge for troubleshooting any fuel injection system regardless of type. Without one, you have too many unknown variables. Borrow or buy one. It will save you money and from aggravation.

Lastly, perform a smoke test to locate those hard to find air leaks in your CIS. Not being able to see any does not mean you don’t have one. A pressure test would reveal or confirm the integrity of your vacuum system. I would bet you have vacuum leak/s by the way your engine behaves. Keep us posted.

Tony

Hi Tony,

Will this fuel pressure tester kit work?

https://www.amazon.com/Tool-Aid-33865-K-Jetronic-Injection/dp/B0002SR5NU

I have a smoke machine I can use for this but the tough part is that the seal between the airbox, air filter and air filter cover is warped so I am going to have to tape off the whole airbox. I really hope I don't have a crack in the airbox!

Cheers,
Alex

[boyt911sc]

Originally Posted by DienstXIV

Hi Tony,

Will this fuel pressure tester kit work?

https://www.amazon.com/Tool-Aid-33865-K-Jetronic-Injection/dp/B0002SR5NU

I have a smoke machine I can use for this but the tough part is that the seal between the airbox, air filter and air filter cover is warped so I am going to have to tape off the whole airbox. I really hope I don't have a crack in the airbox!

Cheers,
Alex

Alex,

I recommend S&G tool 33800 for less than $100. This is the same pressure gauge you posted but comes in an ordinary carton box not in a plastic carrying case shown in your attachment. The plastic carrying case is $50 extra.

There are several ways to do an effective air leak test with a CIS motor. The key is to isolate the CIS airbox from atmospheric condition for the smoke to locate the hard to find sources.

Tony

[DienstXIV]

Okay everyone it looks like the biggest vacuum leak is through the mixture adjustment hole. is there supposed to be a plug that you put in and take out every time you adjust the mixture or is this something I'm just supposed to plug up as well?

[boyt911sc]

Originally Posted by DienstXIV

Okay everyone it looks like the biggest vacuum leak is through the mixture adjustment hole. is there supposed to be a plug that you put in and take out every time you adjust the mixture or is this something I'm just supposed to plug up as well?

The access hole to the 3-mm fuel mixture set screw is at atmospheric condition. It is located outside the air box where the air filter is located near by. Get some reference manuals and get to know more about your CIS.

Tony

[kach22i]

Originally Posted by DienstXIV

Okay everyone it looks like the biggest vacuum leak is through the mixture adjustment hole. is there supposed to be a plug that you put in and take out every time you adjust the mixture or is this something I'm just supposed to plug up as well?

I did a search and found this 2018 thread with similar results. I'm reading it now and learning new things.

Vacuum leak at mixture adjustment screw - 83 911SC
Vacuum leak at mixture adjustment screw - 83 911SC

Originally Posted by 75 911s

That set screw is part of the air sensor arm. The arm is basically a teeter-totter with one end on the air intake to the throttle body and the other directed from the fuel distributor - and simply pressing down on the screw will open the air intake slightly. That's why adjusting the 3mm set screw is fussy because if you apply downward pressure, you move the arm a bit.
Tony is right of course, that set screw is on the atmospheric side. So smoke emanating from this area is not a problem in your test.

Over rich by how much? What does the tail pipe look like and what do the plugs look like? There is an amazing CIS thread - I'll see if I can find it and link it here.

Another thread, Post #25 (partial)

CIS warm idle goes lean
CIS warm idle goes lean

Originally Posted by boyt911sc

Do not tinker or adjust the mixture screw setting until you are 99.9% sure that you don't have any significant air/vacuum leak/s in the system. If tests have confirmed and verified the absence of significant air leak/s in your engine, then it is time to go play with the mixture.

There is no logical reason to check or even set your air mixture if you have undetected air leak/s. Since you don't know if your vacuum is compromised, you are simply hoping you don't have any. In summary, you are guessing!!!!!...............

[DienstXIV]

It was really hard to find any vacuum leaks with smoke billowing out of the mixture adjustment hole and it was really hard to plug it up. I think tonight I will remove the rubber boot that connects the throttle body to the metering plate and just connect the smoke machine to the throttle body, does anyone know the best way to make a good seal? I was using duck tape last night and smoke was leaking out of the duck tape pretty bad.

@Tony I have been doing some research on my CIS system and while I am waiting for my fuel pressure tester kit to come in I think my decel valve may be defective. I am able to get my engine to idle very well at approximately 950RPMS when everything is warmed up but when I rev the engine up the RPMs hold at around 2,000RPM. sometimes they sit there and don't go down and sometimes they eventually go down. Sometimes if I rev the engine up that will make the engine go back down to idle speed but sometimes the engine stalls. Could there be an issue with the decel valve?

Also the electrical connections to the WUR and Cold Start Valve have some blue corrosion on them, I got the smallest files I could find and tried to clean them up but they are still pretty bad. That electrical connection cleaner didn't work either. Anyone else know a trick? I hear salt and vinegar in water and then baking soda and water can help, anyone ever try it?

Thanks everyone for all the input I really appreciate it and I hope I can get this car running well soon!

Cheers,
Alex

[dfhtrhjn]

Sounds like your WUR. I experienced the same behavior, exactly the same.

I modified my WUR to be adjustable. I then tapped that adjustment pin in too far, and the mixture became slightly too rich. It did the same thing yours is doing now until I leaned it out just a little and now it runs perfectly.

Your rebuilt WUR can be off by a really small amount and cause the car to be undrivable. If you're handy you can mod yours to be adjustable, then you will know for sure, and make adjustments later if need be. I modded mine in about 30 min with no experience. Instructions below.

Made an Adjustable WUR- Step by step, got Pics!


Edit: Your WUR has a little vacuum line at the bottom on the side. My rubber line was old, and I put a tiny hose clamp on it to make sure I'm not getting a vacuum leak there.

[boyt911sc]

Originally Posted by DienstXIV

It was really hard to find any vacuum leaks with smoke billowing out of the mixture adjustment hole and it was really hard to plug it up. I think tonight I will remove the rubber boot that connects the throttle body to the metering plate and just connect the smoke machine to the throttle body, does anyone know the best way to make a good seal? I was using duck tape last night and smoke was leaking out of the duck tape pretty bad.

@Tony I have been doing some research on my CIS system and while I am waiting for my fuel pressure tester kit to come in I think my decel valve may be defective. I am able to get my engine to idle very well at approximately 950RPMS when everything is warmed up but when I rev the engine up the RPMs hold at around 2,000RPM. sometimes they sit there and don't go down and sometimes they eventually go down. Sometimes if I rev the engine up that will make the engine go back down to idle speed but sometimes the engine stalls. Could there be an issue with the decel valve?

Also the electrical connections to the WUR and Cold Start Valve have some blue corrosion on them, I got the smallest files I could find and tried to clean them up but they are still pretty bad. That electrical connection cleaner didn't work either. Anyone else know a trick? I hear salt and vinegar in water and then baking soda and water can help, anyone ever try it?

Thanks everyone for all the input I really appreciate it and I hope I can get this car running well soon!

Cheers,
Alex

Alex,

You could bench test your decel valve. You need a hand operated vacuum pump like a Mity Vac. I might have some pictures of the test. Normally, the decel valve is closed at 17”~18” Hg and opens up 18”~19” Hg.

So you need to verify if your decel valve is not opening too prematurely (16”Hg and below). You could adjust it accordingly.

Tony

[DienstXIV]

Originally Posted by boyt911sc

Alex,

You could bench test your decel valve. You need a hand operated vacuum pump like a Mity Vac. I might have some pictures of the test. Normally, the decel valve is closed at 17”~18” Hg and opens up 18”~19” Hg.

So you need to verify if your decel valve is not opening too prematurely (16”Hg and below). You could adjust it accordingly.

Tony

@Tony thanks for the help, some pictures would really help in how to set up the bench test. My strategy for this weekend is as follows:

1.) Make absolutely sure that there are no vacuum leaks in my CIS
2.) After reading through the troubleshooting section in CIS Primer they said to plug the top vacuum line on the decel valve to see if the engine's high idle improves. This essentially deletes it I think. Reading through the forums some people have removed the decel valve all together, so that could be an option but I am not too excited about it. If the idle does improve on to number 3.
3.) Bench test my decel valve and adjust as needed. I really hope it's not broken because I looked them up and a new one is very expensive!

Cheers,
Alex

[Paulporsche]

Alex,

John Walker recommends 3 - 3.5%. I would think this would be without a cat, which you wouldn't have on a stock 75

Try this for the PET:

https://www.porsche.com/international/accessoriesandservice/classic/genuineparts/originalpartscatalogue/

Click on 911, 1974-77