Sure sounds like a vac leak.

If you can't detect one in the obvious visible places -- intake runner boots, airbox, vac lines, etc. -- it is likely in an non-visible location. That seems consistent with your previous efforts to find a vac leak.

There are lots of connections on the backside of the airbox -- plenty of opportunities for leaks there and they are not visible unless you take all the CIS stuff out. (You may need a partial drop to have enough room).

There are links for tests for the AAR and AAV in the CIS Troubleshooting for Dummies thread. I am linking them here to make it easier.

http://forums.pelicanparts.com/porsche-911-technical-forum/740422-how-test-auxiliary-air-valve.html

http://forums.pelicanparts.com/porsche-911-technical-forum/112138-aar-good-bad.html

The decel value also has a diaphragm in it that can leak. I personally hate the decel valve. I removed the vac line to it and plugged it up.

If none of that works out you need a smoke test. You can do it at home. Some people have used cigar smoke. And then there is the "ghetto" method. Do a search on both.

And then there is the "full Tony" of the airbox.

When I fixed my vac leaks my mixture was 10 AFR at idle on the first start up! That is the equivalent of a "dope slap" as to what kind of difference vac leaks make to the normal function of the car.

I know it's frustrating but stick with it. You will have a new CDI and all the vac leaks fixed when you are done. I guarantee the car will drive better.

Thanks Tirwin, I will start down checking these. I just finished taking off the rubber boot on top (between the plate and bowl) and it was very gunky inside. My wife and I cleaned all the old crap off and reassembled, and she is doing much better.

She will start and idle when cold. After getting to really warm (takes 5+ minutes) she starts to have desires to stall out. If I keep the idle open (high) the hunting and wonkyness doesn't result in a stall out. If I start to lower the idle, her hunting gets worse until she stalls. I tinkered with the mixture a bit but couldn't get it right. Very small turns! I didn't notice the big change in idle I'd expect when removing the oil cap. So hunting a vac leak sounds like a good plan.

In my opinion, THE tool to have in tuning CIS properly is a wideband O2.

I bought the Innovate Motorsports LC-2 and gauge with the intention of installing it while I had the engine out. I didn't do that but I did wire it all up and connect it to a 12v power supply temporarily. I just swapped the narrowband out for the wideband while I tuned and adjusted the idle AFR to 13.2-13.5. Innovate makes a handheld unit too I think.

Here's why I'm saying this... If you are adjusting the AFR and idle to get the car to run, I am afraid you may be masking another lingering problem. Verify you don't have a vac leak or a CIS component failure first and then set timing, idle and mixture. It is always hard to troubleshoot over the Internet, but it sure SOUNDS to me like you have some other issue still in play.

I'm fleshing out the basics of how CIS works, in order to understand it in my own mind.

The central idea is this: air leads to gas. When the cylinders open their intake valves and move down, the draw an air vacuum above them. Air wants to fill this vacuum, so it lifts the sensor plate (in the top back of the airbox) and rushes past it. If the throttle is open, the way is even easier and more air rushes in.

When air lifts that sensor plate, it moves a lever. The flow of air corresponds to movement of the lever, and that movement allows a metered amount of gas to flow into the fuel distributor. By setting the mixture, we adjust how much gas is used for a given portion of air. So we have this basic equation:
Gas Volume = Mixture Constant * Metered Air Volume

If we have lots of gas and air, we get a lot of power. But we need the two to be in a proper ratio to one another.
Final Power = Total Air Volume * Gas Volume
Final Mixture = Total Air Volume / Gas Volume

In order to have the engine idle, we need a basic amount of air coming in even when the throttle is closed. This means we need to let a little bit of air in that isn't metered by the sensor plate. The idle adjustment screw gives us this a path to allow a fixed amount of air in.
Additionally, when the engine is just starting up it isn't yet pulling a good vacuum. And when it is cold, it needs more air. There is also a need for some air when you let off the throttle, but the engine hasn't yet slowed down.
Total Air Volume = Metered Air Volume + Idle Air + Startup Air + Cold Start Air + Deceleration Air

The Startup Air is provided by Auxiliary Air Valve. This is the little copper-colored thing on the right of the airbox which looks like two cymbals clapped together. It is open, allowing unmetered air, until the engine starts to draw it's own vacuum. If this is working, it closes right away. It doesn't care about engine temperature.

The Cold Start Air is allowed in via the Auxiliary Air Regulator. The AAR allows this unmetered air into the engine when it is cold, but as the engine warms up it closes off this extra air source. The AAR is the lead-colored finger-shaped thing on the right side of the engine.

The Deceleration Air comes in when the engine vacuum jumps. If you have an open throttle and suddenly let off, you shut off the air to the engine but it is still running fast. It needs additional air to avoid cutting out, so the deceleration valve sees the high vacuum and lets some air sneak past. The deceleration valve is hiding behind the air box on the left. It is the copper-colored rocket-ship looking thing with 3 hoses on it.

When the engine is all warmed up and idling, the only source of air should be the idle air and a little bit of metered air coming past the sensor plate. In equation terms, the idle adjustment is the y-offset of our engine power equation, and the mixture is the slope. But if extra air is sneaking in via one of the other systems, it will throw off our entire calculation.

As described above, gas is metered out based on air flow past the sensor plate. However a little extra gas is needed at startup, when the engine is cold. The Thermotime Switch is inset into the timing chain cover on the left of the engine, just inside the bumper. It senses the heat of the engine and turns on or off the Cold Start Valve. If the Cold Start Valve is open, the engine gets extra gas that isn't metered off the sensor plate airflow. The Cold Start Valve lives on the back of the air box just around the corner and under the idle adjust screw.

I am still working out exactly how the WUR/CPR works. I believe it senses the level of the vacuum and and somehow tells the fuel distributor to modify it's mixture. The WUR gets it's measure of the vacuum via a hose which runs past the Thermal Valve. The Thermal Valve keeps the WUR from getting a vacuum reading for about 30 seconds after startup. Then the valve opens up and lets the WUR do it's own thing.


This is my current understanding of CIS. I am still learning a lot and I think there are probably errors in my initial take. However I am getting closer to knowing what is going on in there. And, in time, I should be able to apply this knowledge to make things work properly.

Probably off the wall..., but is the throttle cable letting the throttle plate fully close...?

Yes, when I had it open and was cleaning the throttle plate she turned as close to 90 degrees as I could see.

So I started her up, moved the idle up, and rev'ved her a bit until she got warm. I have to put the idle screw at 3 and 5/8ths turns "out" or she won't idle. At that setting, after warmup, she idles at about 2K.

However the idle is punctuated with "whoomp" noises that feel like precursors to a backfire. I'm not sure if that is a sign of too rich mixture, too lean, or something else. When she whoomps, the whole car vibrates a little.
After she is truly warm (nearly 30 minutes of running with me revving from time to time) I presume all the cold start related stuff is in its turned-off position (assuming it is working properly). If I lean her out (mixture turned counter clockwise) she wants to die and stop idling. If I make her richer, she keeps idling but continues to whoomp-whoomp at irregular intervals every second or two.

I did confirm that taking the oil cap off does result in a change in the idle. When she was still warming up (maybe 5 minutes after startup) she had a hunting idle. Taking the oil cap off smoothed that out a little, and also brought the idle down slightly. She stopped hunting as she warmed up and I turned up the idle to avoid her dying before she got truly warm.

If I turn down the idle, the whoomp-whoomp begins turns into a erratic idle that threatens to stall/backfire unless I rev her right away.

I think the deceleration valve is working, because she falls off smoothly after I rev and drop throttle a big drop. I am thinking about checking function of the AAV and AAR by taking off those hoses after she is warmed up. That shouldn't have any effect if they have properly closed.

I am still working out exactly what the WUR is doing, hopefully a better understanding of that will help me find the problem.

"WHOOMP.." Ignition of fuel in the exhaust...?

Lots of miles, timing chain slipped a tooth or 2....??

New chain tensioners ~60K miles ago after total engine rebuild.

OsoMoore,

I will never claim to be a CIS expert. There are others far more knowledgeable than I. All I can speak from is my own experience and layman's understanding.

You are mostly correct.

I like to think of CIS as a the stepping stone from carbs to "real" fuel injection where the ECU has timining maps, fuel maps, etc.

CIS is largely about two things:the relationship between fuel pressure and air.

The name "WUR" is a bit of a misnomer because it leads people to think it is only in play during startup. Not true. It regulates the flow of fuel at all times. If the warm control pressure is too low, the mixture will be overly rich. Too high and the mixture is overly lean. Changing the idle mixture cannot overcome a fuel pressure that is significantly out of spec. This is why you start by verifying fuel pressures.

As you noted, having the proper air/fuel ratio is important. That ratio should be different during different conditions. You want a richer mixture at WOT for maximum power. You want closer to stoichiometric at idle for better emissions. "Real" EFI lets you carefully adjust the parameters. CIS isn't that sophisticated. Baseline fuel pressure is off -- everything else suffers.

Now as far as air the challenge with CIS is not that it can't work well -- it does. The challenge is that there are so many possible points for vac leaks to occur. The thing that the AAR, AAV and Decel Valve all have in common is that they can bypass air around the throttle body. You want that at cold start. You do not want that under normal operation. Look at the pipe on the back of the airbox and you will see they all branch off of it.

An example to illustrate the point: WUR operates correctly and enriches the mixture on cold start, but AAR fails closed such that you're getting more fuel and not enough air. Result is overly rich cold start. As the engine warms up you want the WUR to lean mixture to normal and the AAR to cut off the extra supply of air at the appropriate time. (I don't think the AAR failing closed is all that common -- I would expect the reverse. It has a bimetallic strip similar to the WUR and gets 12v power which heats the element and causes the valve inside the AAR to close. If the AAR does not get power or the internal valve sticks it will stay open which has the effect of being a vac leak. If the valve will move then it will only shut off once it heats up due to engine heat soak.)

I hope that helps.

And it is definitely a lower-case "whoomp" - someone who isn't listening probably wouldn't pick it up easily. :)

Mmmmm CIS 101.

Does the throttle adjust screw bypass air past the sensor plate, or just the throttle plate?

Took apart the throttle late area of the CIS and all it's 6 wondrous hoses. Cleaned, and put in new hoses to the decel valve, thermovalve, WUR topside, and WUR side-side. Still no luck.

Definitely sounds like there is a vacuum leak in there somewhere though. I am about to call a local shop I was recommended too... I don't have enough summer to spend another week pulling off parts when I don't have the proper tool (a smoke machine).

Try blocking off the line to the power brake booster by capping the connector on the air box. You may have a cracked line to the booster.

I'll isolate that hose and give it a try.

Here are some sweet pictures of the throttle body. Description of orientation are based on your view looking into the engine bay from behind. See the last two pictures (next post) for the connected setup.

Front Left. The unstoppered nipple goes to the top of the WUR. Needs a fitting to change sizes along the way.
http://forums.pelicanparts.com/uploa...1434546296.jpg

Back Right. The little nipples go to other hoses that aren't part of the throttle body or deceleration valve. I believe they go to the AAR and AAV, but I'm not sure. I just know which hoses go to which spots based on how I disassembled it.
http://forums.pelicanparts.com/uploa...1434546307.jpg

The back side.
http://forums.pelicanparts.com/uploa...1434546344.jpg

The left side.
http://forums.pelicanparts.com/uploa...1434546357.jpg

The front side.
http://forums.pelicanparts.com/uploa...1434546370.jpg

The right side.
http://forums.pelicanparts.com/uploa...1434546384.jpg

Looking in the top of the body at it's front interior side. You can see the hole for the WUR, which lies above the throttle plate.
http://forums.pelicanparts.com/uploa...1434546398.jpg

Looking in the top of the body at it's back interior side. You can see the gaping hole for the idle bypass, the large hole for the deceleration valve intake, and the small holes for the two little hoses that go in the AAR/AAV direction. The small holes are just barely above the throttle plate's closed position.
http://forums.pelicanparts.com/uploa...1434546413.jpg

Looking into the bottom of the body at it's back interior side. You can see the gaping hole for the idle bypass and the large hole for the deceleration valve exhaust. You can see the small hole for the "signal" hose to the deceleration valve and the WUR side connection. On the way to the WUR, it goes through the thermo valve.

http://forums.pelicanparts.com/uploa...1434546470.jpg

Looking into the bottom of the body at it's front interior side. You can see the little hole leading to the plugged nipple on the forward side.
http://forums.pelicanparts.com/uploa...1434546454.jpg

Reassembled with new hoses. I used about 4 feet of thin hose and 1 foot of thicker hose. Don't remember the actual sizes I bought. I brought my old ones to BumperToBumper and used my micrometer to find the closest matches. If I'd had more time, I'd have just ordered cloth hose from Pelican. Getting the old cloth hoses off was a very difficult task, and destroyed the ends of several of them.


http://forums.pelicanparts.com/uploa...1434547545.jpg
http://forums.pelicanparts.com/uploa...1434547557.jpg

I was looking through Jim's Basement again, and realized I have an EGR valve on my machine. Perhaps that is the source of my leak. I'm going to try blocking it off.

Must be a CA car originally with an EGR.
Definitely a possible source.
Especially where it enters the bottom of the Airbox in the back.
Get rid of it and plug all the holes if you don't need it in WI.

Yes, bought by a lady doctor in California, but there is no smog testing in WI. She put 200K on it before selling it to Leland. I'll read up on EGR's. I presume there is more to plug than just the throttle body connections, right?

EDIT: Some EGR posts for my reference:
http://forums.pelicanparts.com/porsche-911-technical-forum/375741-egr-where-1979-sc.html
http://forums.pelicanparts.com/porsche-911-technical-forum/842902-what-egr.html
http://forums.pelicanparts.com/porsche-911-technical-forum/367288-cis-problem-tested-have-results-opinions.html

Looks like I may need to pull the CIS off to plug the airbox connection.

Oh my gosh - Leland's post on this same system on this same car: http://forums.pelicanparts.com/porsche-911-technical-forum/5418-can-smog-bs-removed.html

It is DONE!

http://forums.pelicanparts.com/uploa...1434829263.jpg
http://forums.pelicanparts.com/uploa...1434829278.jpg
http://forums.pelicanparts.com/uploa...1434829296.jpg

I have two weeks until I can get her into the Porsche shop, so I broke down and pulled the CIS out. I knew it was hard without dropping the engine, but I finished in about 4 hours. Pull off the rubber conduit between the air sensor plate and the throttle body and it fits out easily enough.

The real challenge was getting the EGR unbolted so you can reach cylinder 6's back intake manifold bolt. I spent about an hour on just those two things. I HATE EGR!!! Also, getting the last bolt required no less than 6 things attached to my socket wrench. Evil nasty corner!

Finally, whoever's grand idea it was to use non-ferrous washers on the intake manifold bolts can go jump in the lake. I couldn't get most of the stinkin' things off before I pulled the manifolds off. And, of course, cylinder 6's back washer slipped off and teetered on the edge of the hole to the valve. I couldn't snag it with my magnetic pickup, and I couldn't reach it with my pliers. I gently nudged and....
it fell in :mad: Fortunately, that valve is closed so I can get it out later with a claw or gum on a stick.

Anywho, no obvious cracks in the airbox. I'm going to clean it all out, replace the rubber, and generally put it in good shape. Hopefully that fixes the problem. If not, there's also CS Motorsports two weeks from Monday.

So besides the bolts, rubber manifold sections, and vacuum hoses, what should I replace while this is all off?