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?