Basic CIS question
 

I have been investigating the operation of the CIS system and I am confused about control pressure. I found this fuel diagram for an early motor and I think that I understand it.

My confusion is that as the control pressure (fiel pressure at the fuel distributer) increases why would the mixture lean. Shouldn't the mixture become more rich as the pressure increases?

Sorry, I forgot to attach the fuel diagram.
http://forums.pelicanparts.com/uploa...1153839919.jpg

That dosen't look like the CIS that is in my Audi. Was the 73 that much different?

There should be an air plate pushing up on the metering piston in the center of the diagram. I don't know what that throttle actuated valve is.

in a nutshell. the fuel pressure you are thinking about isnt really the same fuel that sprays into the cylinders causing ignition. control pressure, well "controls" the amount of resistance the plunger in the fuel distributor sees. the more pressure, the more resistance on the plunger. the more resistance on the plunger the LESS fuel gets sprayed into the cylinder. this happens because it will also require more air to lift the airsensor bar, that moves the plunger in the first place. phew, thank god i dont have a job as a techical writer. i would go hungry.

I don't know where that diagram came from. This is the CIS used in Porsches. http://forums.pelicanparts.com/uploa...1153843192.gif
In the middle of the diagram is the sensor plate.
As the throttle valve opens, controlled by the gas pedal, incoming air lifts the sensor plate and sensor arm.
Raising the sensor arm causes the fuel distributor to flow more gas through the injectors.
I couldn't tell you whether this is done by more pressure or just opening a valve in the fuel distributor.
You'll notice the idle mixture control.
By changing it's relative position to the plate lever you control the mixture.
Simple actually.
You can remove one injector. Place it in a jar. Turn on the ignition. Don't start the car. Then lift the sensor plate and watch the injector fuel flow.
Checks the injector and fuel distributor.
The idle mixture control screw is very sensitive to adjustment. Not over 1/4th turn per adjustment, usually 1/8th turn.
Easy to go too far.
Accessed with a 3mm allen wrench through a small hole next to the fuel distributor.
I hope this removes some of the mystery of CIS.

Doug,
GREAT explanation and diagram! Thank you!!

Best documentaion I've ever seen.

As soon as my alternator is installed and I've re-gaped my spark plugs, I'm diving back in.:)

Not my diagram. Not sure where I got it.
Thanks to whomever created it.

Doug,
Do you have any other diagrams like this? The combination "logical" and "physical" views really help me understand what's being documented.

Art,
Sorry, my library is still in it's infancy. Folks like Wil, Grady, and others have way more.
Is there something you are looking for?
If I've got it, I'll send it. With credit to the creators when possible.

First diagram was the CIS system used in the 1973.5 911T and the 1974 911 models.

That clarified what is involved. I have never had any issues with a CIS system other than vacuum leaks so I just considered it a "magic box" which when the plate moved caused fuel to be dumped in. Thank you for removing that "magic"

Thanks for the clarification Jim. I'm familiar with the CIS that used to be on my '76.
I'll be looking for you at Stinchcombs annual gathering.

Doug,
Not to hijack MIke's thread or turn this into yet another multipage CIS thread (is it possible for it not to?) but since you mention specific familiarity with your '76, it perked up my ears!

I have a relatively new-to-me (1-1/2 years ago) '76 that runs great. I know for a fact that a rebuilt fuel distributor was installed by a reputable shop right before I bought it. At the same time the electric distributor was also replaced with a brand new one ($$) plus wires, plugs, etc. The ONLY problem I have is that when overnight cold I have learned this:

Hold the key on "Start" an instant or two longer than feels normal. It fires immediately but will sputter momentarily and very slightly backfire before immediately idling fine. I'm talking 2-3 seconds sputter absolutely max if even that long. I can pull away fine too. I have no other symptoms of anything. Car runs strong in all other regards. The car does not idle higher until warm as I would expect it should, but still no driveability problems on cold start other than the start itself. Once in awhile for no apparent reason it WILL idle up higher at start up but again, only momentarily then settles back to normal idle. What does this sound like to you? WUR or AAR and in a nutshell (as if that exists with CIS) what's the fix? Any advice appreciated. I HAVE done innumerable searches and have not found anything that I think addresses this problem exactly.

P.S. I have been tempted to buy one of the generic fuel injection test rigs from Harbor Freight but not sure I would know how to hook it up, much less what it was telling me! Since my problem is relatively minor and of no other impact than the start I hesitate to buy one of the $100 test rigs here.

Dan,
I'm not an expert on CIS at all. I know the basics. I'm sure there is someone on the board that could give you a better answer than anything I could come up with.
My experience with my CIS was setting the idle mixture screw properly after I got it too rich and flooded the motor.
Discovered that once I put the injectors into a jar and noticed flow without cranking the motor. Backed off the mixture and stopped the flow. Problem solved.
I went to Webers a couple years ago since I basically race the '76. The Webers have been more of a challenge to get right than the CIS ever was.
I would post your question and see what comes up.

Thanks Doug.

Dan,

From the sounds of it your problem is not likely to be in the WUR or the AAR as these are timed mixture adjustments and would work over minutes, not seconds. When they fail they should either make the car run pooly when it is cold, or hot depending on how they fail. I would expect that your problem is due to either the mixture setting, or the cold start injector sticking open too long. Obviously I am not an expert, but that would be my best guess.

Dan, you really ought to spring for a CIS gauge set. It's the foundation tool for working on our cars' fuel systems. You'll recover the investment (no matter where you get the gauge from) the first time you use it. There's tons of info here and elsewhere on how to hook it up and interpret the numbers. You'll learn a lot and will have more confidence in diagnosing any future startup or driveability issues.

Brian

Ahhh! My father has come back to life!:D Brian, I know you're absolutely right of course. I have told myself that a few times. "Just bite the bullet and send for the damned thing". I'm sure once it and the instructions were in my hands (and a printout or two from here) I'd figure it out.

Mike, I like your diagnosis. The mixture idea doesn't sound quite right to me as I have no symptom other than that initial momentary start problem (though I suspect I might be running a tad rich overall). But the cold start valve or injector as you called it sounds right. I don't know EXACTLY its function but the sound of it is a temporary richening of the mixture at start up. If I were getting an intermittent failure (mostly NOT working but occasionally operable) it might explain my symptoms. Anyone know if this is an adjustable valve/injector or is it a part you remove and replace? Other considerations with it? and does anyone have a picture of its location and ease/difficulty in R/R'ing it?

It is a solenoid actuated valve that should only inject when the starter is turning. Unforunately it can not be adjusted/rebuilt, but you can test to see if it is sticking. Just pull it out (located at the back of the engine) and put it in a glass jar while someone else turns the key. It should start and stop instantly with the starter turning over. If it does and does not leak when closed and it has a good spray pattern while the engine is turning over it is working correctly.

Dan,

Sounds like your cold control pressure may be too high, giving you a too lean mixture @ startup. This is WUR. It's also possible your AAR is off a little, which is why your revs aren't high @ startup, although this may not be too much of an issue in So Cal. It's also possible your mixture is set just a little lean.

JCWhitney has a CIS .fuel pressure ga

Mike and Paul, Thanks to both of you. The most on-the-mark advice I've gotten so far. I went through another search in the last half hour and read that getting to the cold start valve is difficult as its at the back of the engine. If so, is the wiring to the valve long enough to be able to do the "pull-and-put-it-in-a-jar test, Mike? Have you done this?
As for the WUR - I remember reading something about tapping a plug in it(?). Guess I'd better got back to searching or this really will be a multipage hijack. I may post (yet another) CIS thread.

Yup..what Dan and Paul said. Couple of points to add here. Think of the WUR as a transitional device that is used between the cold start valve firing and the engine being at operating temperature. If the car fires right up and then stumbles, you most likely have the WUR set too lean. If it starts right up and then the rpm's hunt [high / low, over and over- like 17 year old sitting at a traffic light] you have the WUR set to rich. Adjustment requires tapping the pin in to make it rich and out to make it lean. I highly recommend that you make the WUR adjustable because it makes the process very accurate..a couple of PSI can make all the difference and whacking it with a hammer ain't my idea of fine tuning.
The WUR and the AAR work together during this transitional period. The AAR is providing the air and the WUR the fuel..hence, the warm up mixture. If one is not working properly then it will affect the operation of the other. Once the engine has reached operating temperature the AAR is out of the picture and the WUR is left to provide off idle enrichment via vacuum connection [at least thats how my SC works].
The AAR should be slightly open on a cold engine and full closed after a few minutes.

I have not run this test on a Porsche, so I do not know as I bought a roller and have not installed the cold start valve yet.

Ron/Paul- I thought that the WUR/AAR worked of of a thermal proscess using a heating element to adjust the control pressure which should not make any significant adjustment over a 2-3 second time period (total length of time the engine sputters) unless there is a stick valve that loostens quickly.

I've become intimate with the CIS in my 78SC. So I want to be careful about what I say about the way CIS works on other cars but it is my understanding that the basics are pretty much the same. When you crank the starter the cold start valve shoots a healthy amount of fuel into the intake and the car fires up. Once the starter is released that fuel source is cut off and the engine relies on the WUR to feed it. My gut feeling is that the WUR must provide an almost equal amount of fuel to allow the transition to take place smoothly. On my car I found that the stumbling after the initial start up was from my WUR being set too lean. A whack of the plug corrected the problem. Unfortunately, the next morning I had the oscillating idle because I went a little too rich. This time when I pulled the WUR I made it adjustable and was really able to zero in on the control pressure.
Back to what you asked..yes, the WUR and the AAR are temperature controlled devices but if you notice the vacuum connections on a 78SC there is a thermotime valve that prevents vacuum from being applied to the WUR for the first few seconds of start up. This helps keep the control pressure on the rich side for a few more seconds. I'm quessing that the factory hung this thing on there for a reason. Once, the engine has stablized and the thermo valve opens, the vacuum leans the WUR.

Ron, Thanks for the info. I suppose I DO need the CIS test setup and just "whacking" that plug in the WUR blindly would be ill advised.

Still, if I test it or not and decide to try the "whacking" method (you realize what a non-Porschephile stumbling into this thread would think?!)how do you measure the depth of change to the plug you are hitting? A tire tread depth guage seems like it would work.

Guess I'd better do a search for the thread on making the WUR adjustable and try those disconnected cold start valve, and in-the-bottle tests.

If you look at the cross section of the WUR you will see the plug, the arm, etc and the diaphram that is actually controlling the pressure. Although I enjoy the humor in the image of fine tuning a 911 with a hammer and a punch, I've come to realize that 'WHACKING THE PLUG' is really too coarse an adjustment. Its too hit or miss even with gauges. Once I made the WUR adjustable I set it rich, with the plug much lower then it should be. I attached the gauges and slowly tightened the nut to pull the plug up and lean it. I was amazed how sensitive this adjustment was. I also began thinking in terms of PSI rather then BAR. A couple of PSI makes a big difference.
http://forums.pelicanparts.com/uploa...1154005721.jpg

The exchange between Mysterytrain and Dan in Pasadena was briefly entertaining but it’s a classic example of what I call "1/4 turn tuning". Guys, you've bought a Porsche, either get someone who knows to fix it or get the right tools to do it yourself. For a CIS the tools are the pressure tester and an exhaust gas analyzer (A/F meter). Sadly the CIS has gotten an undeserved reputation because it can relatively easily adjusted by someone who doesn't know to a state where it doesn't work.

As Jim Sims said, early 911 CIS is as originally shown and includes a throttle actuated control pressure valve. This enrichens the mixture (drops the control pressure) at full throttle. Later CIS accomplished the same with a vacuum controlled WUR.

Hladun..I'm not sure I understand what you mean by entertaining. I'm not recommending that anyone adjust the WUR without a set of gauges. If anything I thought I was offering up 'my' experience with 'my' system and the interaction of the WUR and AAR.

Ron,
Started to write a response to Hladun's advice and "tone", I decided better. Thanks for the discussion and friendly advice. THAT kind of advice is sincerely appreciated.