Mixture Creep

[Funracer]

When I check my mixture with an oscilliscope when the car is warm (10-15 minute drive) and check it again after an additional 10-15 minutes the mixture is richer. This causes the idle to creep up as well.

Once the WUR gets to its control pressure (in my case 3.5 bar in 4 minutes and does not change) what could cause the mixture to richen?

I’ve done several smoke air leak tests with a cold motor. May be time to do a hot air leak test.

[Tyson Schmidt]

Air temp/density
CIS doesn’t have compensation for this outside of the feedback system using the O2 sensor.

[Schulisco]

Originally Posted by Funracer

When I check my mixture with an oscilliscope when the car is warm (10-15 minute drive) and check it again after an additional 10-15 minutes the mixture is richer. This causes the idle to creep up as well.

Once the WUR gets to its control pressure (in my case 3.5 bar in 4 minutes and does not change) what could cause the mixture to richen?

I’ve done several smoke air leak tests with a cold motor. May be time to do a hot air leak test.

I suspect a misadjusted CIS among other issues. Idle adjustment is always a combo of ignition timing, CO setting and position of sensor plate. When vacuum leaks and/or other things also came along, the mess is perfect. The problem is: Almost anytime you can "tune" a CIS in a way that it will work anyhow, but never perfect under any conditions.

First step: I tend more and more often to ask people claiming idle or running issues on a CIS how many turns they need to close the idle screw. This shows pretty well if there may be one or more vacuum leaks because they are the top enemy of CIS.
So - what's yours? (Remember the setting to set it back).

More steps to come...

Thomas

[Funracer]

One full turn to close it.

But how would this affect a changing mixture? The longer my car runs the richer the mixture gets. I watched it today for 25 minutes FV went from 45% to 35% from start to warm. Maybe everyone’s car does this I just never noticed it before.

[Funracer]

Originally Posted by Tyson Schmidt

Air temp/density
CIS doesn’t have compensation for this outside of the feedback system using the O2 sensor.

So you are saying it is normal for the mixture to richen as the engine heats up?

Thanks

[Schulisco]

Originally Posted by Funracer

One full turn to close it.

But how would this affect a changing mixture? The longer my car runs the richer the mixture gets. I watched it today for 25 minutes FV went from 45% to 35% from start to warm. Maybe everyone’s car does this I just never noticed it before.

All I know - On CIS 911's the fundamental setting for the idle screw is two turns to fully close the idle screw. Half turn more or less is also okay to meet the individual settings. But more than that shows issues. This means - I suppose at least one or more vacuum leaks on your engine.

Vacuum leaks basically lean the mixture which may cause poor running, bad start, power loss etc.

Consequence:
Somebody noticed the lean mixture and "sorted it out" by enriching the mixture again, in simple way by turning the CO adjustment screw. All seems good, if the engine starts good and runs good. But probabaly fuel economy goes worse, power development gets worse on high rpms, the fumes increases etc. It varies from car to car.

You also can enrich the mixture by lowering the sensor plate. This causes the sensor plate (and therefore the lever pushes up the plunger in the fuel distributor) to travel more way and therefore the slots in the FD getting openend wider to give more fuel.

But then the basic adjustment of the CIS is out of spec. The car will run anyhow, but in details you'll notice this and that what the car doesn't made in the past...

I suppose sth in this way happened to your car also. In general the FV should have a duty cycle on idle roundabout 45-55% when idling warm. If it drops as seen on yours, this leads me to exact that scenario: Engine warms up, mixture gets richer, and the ECU reduces the FV duty cycle. So the mixture is too rich by the basic setting of your CIS, but not that rich that the engine will have a sweeping idle or sth like that because the ECU manages this by lowering the duty cycle. Otherwise when you lean the mixture on the warm engine to the FV duty cycle of 45-55%, the car won't start anymore or even harder than before, run with less power etc.

The next steps IMO: Check with a smoke generator for vacuum leaks and sort them out. Then check WUR control pressure and fuel system pressure for correct settings. Then check ignition timing on all rpms. Then check the sensor plate height and make the basic adjustment for the CIS ("injection begin" / start of fuel pump):





Thomas

[Funracer]

Originally Posted by Schulisco

You also can enrich the mixture by lowering the sensor plate. This causes the sensor plate (and therefore the lever pushes up the plunger in the fuel distributor) to travel more way and therefore the slots in the FD getting openend wider to give more fuel.

Where did your read this? I would like to learn more about this effect because I cannot understand it.

Thanks

[Funracer]

When you say you can enrich the mixture by lowering the sensor plate, You are talking about widening the angle between the sensors plate arm and the FD plunger arm with the 3mm mixture screw, correct?

[Schulisco]

Originally Posted by Funracer

Where did your read this? I would like to learn more about this effect because I cannot understand it.

Don't remember if I read it anwhere. Will post it here if I find some.
Mainly personal experience ... on my SC the sensor plate was way too low positioned...mixture was enriched due to several vacuum leaks: Airbox broken and glued several times by PO, some old robber hoses were broken, etc. Also a workshop failed in sorting out all issues. Cold start on ambient temp lower than 10°C still almost impossible...that was the startingpoint for me to deep dive into CIS ...

Bosch/Porsche does not point out the basic adjustment of the CIS for nothing...if you look onto the schematic drawing the sensor plate housing with the lever it's getting pretty clear that the CO basically is a fine tuning for the sensor plate as well:



#5 is a pivot where the lever/counterweight #10 is connected with the #9 CO adjustment screw lever. You can imagine this as a gear to translate the movement of the lever system around pivot #8 which holds the lever system in the housing. #8 is the pivot point of the whole lever system...

The impact of the sensor plate height is huge!

This drawing is taken from the Bosch Tech. instruction on "Gasoline Fuel-Injection System K-Jetronic": https://files.bmwclassic.nl/E21/K-Jet/Bosch_K-Jetronic_web.pdf
It also covers the lambda control used on the SC's from 1980-1983!

Originally Posted by Funracer

When you say you can enrich the mixture by lowering the sensor plate, You are talking about widening the angle between the sensors plate arm and the FD plunger arm with the 3mm mixture screw, correct?

Somehow ... yes.

When you lower the sensor plate it will travel more than before. This bigger angle the sensor plate travel will cause the plunger travel more in the FD and a wider opening of it's control slots for more fuel (and vice versa on higher sensor plate position).
All this makes no sense if the rest of the CIS won't be in perfect condition. If still issues persisting you hide / mask them without sorting them out and the car will never perform as it used to be...

Thomas

[Funracer]

Thanks so much for your explanation. I also read the K-Jetronic link you posted it was very informative.

8 is not a pivot point. It is a roller embedded in the FD plunger arm. Only 5 is a pivot.

There are only two mechanical adjustment points in the AFS mechanism. We all agree the 3mm CO adjustment screw (4) changes the angle between the sensor plate arm (6) and the FD plunger arm (9) in order to move the FD plunger and affect the amount of fuel delivered for a given sensor plate position.

The other adjustment available to us is the air sensor plate zero position shown in diagram A position 7. It can be set via bending a spring (early) or turning a screw (later).

The AFS has a static (zero) position and an active (floating) position.

Lowering the static AFS zero position will allow a bit less fuel available at the first instance of start because lowering the zero position lowers the FD plunger arm and thus the FD plunger while at rest (static condition).

Once the starter is engaged the AFS enters a non-static, active condition. It floats on a column of air moving through sensor housing venturi. Where the air plate floats depends only on the amount of air moving through the housing. Where it starts at rest, the zero position in the diagram (a) does not change the angle between 6 and 9 set by the 3mm CO screw. It might travel a bit more or less but it ends up at the same place in the air column.

Anyway these are my thoughts. The 3mm CO screw has huge effects on mixture. Changing the AFS plate zero position has not so much effect.

I do have a theory on why the AFS plate zero setting is so precisely described (level with the Venturi edge or a maximum .5mm below etc) but it has only to do with cold starting and not driveability.

Regards Neil

[Schulisco]

Neil,

I appreciate the professional and objective exchange with you and all others here!

Originally Posted by Funracer

8 is not a pivot point. It is a roller embedded in the FD plunger arm. Only 5 is a pivot.

I have to admit - you're right...#5 is the only pivot where the lever system rotates. That's a misconception by myself. Thanks for clarification!

I personally didn't overhauled the air flow sensor housing by myself, at that time I sent it out to a specialist for overhaul. So I have to rely on other pictures too. The specialist also replaced the "ball" and put in a new plunger in the FD as well. The ball is a real steelball like those of ball bearings.

Here I found today a good explanation to service an CIS air flow meter unit:
https://rennlist.com/forums/928-forum/958637-refreshing-a-cis-kjet-air-flow-sensor.html
There you can see also the ball.

#8 is a roller which equals the movement of the lever and to make sure the plunger always rests properly on the lever.

Originally Posted by Funracer

There are only two mechanical adjustment points in the AFS mechanism. We all agree the 3mm CO adjustment screw (4) changes the angle between the sensor plate arm (6) and the FD plunger arm (9) in order to move the FD plunger and affect the amount of fuel delivered for a given sensor plate position.

The other adjustment available to us is the air sensor plate zero position shown in diagram A position 7. It can be set via bending a spring (early) or turning a screw (later).

The AFS has a static (zero) position and an active (floating) position.

Not only the angle, but also the gearing. The bigger the angle between both levers, the higher will be the mixture enrichment and vice versa.

Originally Posted by Funracer

Lowering the static AFS zero position will allow a bit less fuel available at the first instance of start because lowering the zero position lowers the FD plunger arm and thus the FD plunger while at rest (static condition).

Once the starter is engaged the AFS enters a non-static, active condition. It floats on a column of air moving through sensor housing venturi. Where the air plate floats depends only on the amount of air moving through the housing. Where it starts at rest, the zero position in the diagram (a) does not change the angle between 6 and 9 set by the 3mm CO screw. It might travel a bit more or less but it ends up at the same place in the air column.

Anyway these are my thoughts. The 3mm CO screw has huge effects on mixture. Changing the AFS plate zero position has not so much effect.

I do have a theory on why the AFS plate zero setting is so precisely described (level with the Venturi edge or a maximum .5mm below etc) but it has only to do with cold starting and not driveability.

Mostly agree.
This was also my first thought on this. But my personal experience showed different behaviour. I cannot explain it in detail but the CO screw changes the gearing between both levers that way that it enriches or leans the mixture over the whole traveeling way of the lever and therefore the mixture. I also won't believe it but my experiements proved it. I wondered too about this.
So my conclusion is - the lever system together with the design of the FD is pretty exact fitted to the car. The CO mixture screw also affects the mixture on higher rpms noticeable. The higher the rpm the less the impact of the CO screw setting, but it's still noticeable. The most significant effect is on idle together witht the idle screw, no question.

Thomas

[E Sully]

Originally Posted by Schulisco

I have to admit - you're right...#5 is the only pivot where the lever system rotates. That's a misconception by myself. Thanks for clarification!

I personally didn't overhauled the air flow sensor housing by myself, at that time I sent it out to a specialist for overhaul. So I have to rely on other pictures too. The specialist also replaced the "ball" and put in a new plunger in the FD as well. The ball is a real steelball like those of ball bearings.

Here I found today a good explanation to service an CIS air flow meter unit:
https://rennlist.com/forums/928-forum/958637-refreshing-a-cis-kjet-air-flow-sensor.html
There you can see also the ball.

#8 is a roller which equals the movement of the lever and to make sure the plunger always rests properly on the lever.



Not only the angle, but also the gearing. The bigger the angle between both levers, the higher will be the mixture enrichment and vice versa.


Mostly agree.
This was also my first thought on this. But my personal experience showed different behaviour. I cannot explain it in detail but the CO screw changes the gearing between both levers that way that it enriches or leans the mixture over the whole traveling way of the lever and therefore the mixture. I also won't believe it but my experiments proved it. I wondered too about this.
So my conclusion is - the lever system together with the design of the FD is pretty exact fitted to the car. The CO mixture screw also affects the mixture on higher rpms noticeable. The higher the rpm the less the impact of the CO screw setting, but it's still noticeable. The most significant effect is on idle together witht the idle screw, no question.
Thomas

It is a good idea to clean and if needed replace the ball bearings and spring. According to the manual, it should be lubricated with silicone (dielectric) grease. Mine was in fair shape, but the roller on the arm was sticky. It is important to get the plate centered and that the arm moves freely. I did not find it difficult.


The replacement of the plunger in the fuel distributor is concerning. Everything I have read is that it is originally fitted to such high tolerance, it cannot and should not be replaced. A different plunger could lead to either seepage around the piston or binding, dragging of the plungers movement. This is also true of the system pressure regulator piston, only the shims and O rings are replaceable. The Service Manual says to replace the fuel distributor if either is damaged (fitted part). I guess you got lucky.
I'm in agreement with Funracer as far as using a lower than specified plate position, I don't think it would be effective. When the car is running at idle the plate will be in a certain position due to air flow no matter what the plates starting position, and the mixture screw would still have the fuel distributor piston in a certain position to get the correct mixture. The only thing I see it accomplishing is having the fuel distributor piston start at a lower point than Bosch engineered it for.

[Funracer]

Originally Posted by Funracer

I’ve done several smoke air leak tests with a cold motor. May be time to do a hot air leak test.

My last smoke test was about a year ago. Any small leaks were addressed at that time.

I watched this today: https://youtu.be/j80k3n_IKkQ?si=OjLF5Kc5rmlHhdmK
He does the smoke test cold. I drove the car for 20 minutes so it was quite warm, then followed the video.

My car came with a pop off valve already installed. It was leaking like mad today, I put a screwdriver tip under the edge of the pop off and it jumped right out of the hole.




Lots of different sealants colors there. New pop off valve on order.

It will take a few days to get it and install. I will report back then.

Thanks

[Funracer]

Purchased my replacement pop off valve on June 29th and received it July 8th!!
Installed today. To prep I chipped off the old epoxies and sealants and used alchohol to clean up around the opening. Also lightly sanded the outside ring on the pop off valve.

After reading a lot about which glue to use I went with this JB Weld product:

It is noticeable stickier than regular grey JBW. It says it sets up in 15 minutes and they are not kidding. I was done using it in about 10 minutes and not sure it would have been spreadable for another 5 so be ready to go after you mix the two halves.

I have read that with JBW more is better so I gooped it around pretty heavily with the supplied popsicle stick and my finger.
Installed the new valve and tapped ii in lightly with a rubber mallet until it seated.

Would have preferred the epoxy was black but it dries tan.
If I had it to do over again I would have been sure to put more JBW on the spring side of the valve. hard to get to afterwards and it looks a little light there in the picture.

I went back and added some to that right corner just to be sure.

Says its ready in 30 minutes but I am going to wait over night before heat cycling the epoxy.

[Schulisco]

Hopefully you ensure that no dust fell into the hole...if not this would become pretty bad .... I'm a pedantic guy, I would vacuum the lower chamber of the airbox for sure.
I used a specialized 2 component plastbond adhesive from Petec, no. 98325, which is specified for most plastics and metal for strength, flexibility, durability, fuel resistance and higher temperatures up to 140°C / 284°F
Thomas

[Funracer]

Originally Posted by Schulisco

Hopefully you ensure that no dust fell into the hole...if not this would become pretty bad .... I'm a pedantic guy, I would vacuum the lower chamber of the airbox for sure.

I left that part out. Had a rag stuffed in the hole all week while I waited for the new part.
After chipping off the old epoxy I shop vac’d everything. I don’t really think a bit of dust would do much damage but its good practice to keep everything clean for sure.

Not sure I can get back to it tomorrow but in a day or two I will reset the mixture as required and see if this repair has an effect on my warm idle creep.

Thanks