CIS Sensor Plate Height

[mikedsilva]

(update - I added to the title as additional things pop up that I am trying to understand)

Tried searching but so far, haven't found the answer.

Just acquired a 79 SC.
The engine was re-sealed by previous owner and he had the WUR and Fuel Distributor rebuilt.
Car starts well, but idles high when warm.

Tried adjusting the idle bleed screw, but I have it completely shut and it's still idling.

Assumed the airbox was blown but put a smoke machine into the brake booster line to check. I can see a whisper of smoke coming through the pop-off valve but no where else and quite a lot coming through the sensor plate; is that normal?

Ended up pulling the airbox (not a fun job, I will be pulling motor to put this back together).
CIS is not my forte, but the sensor plate, when resting, is open quite a bit.. I've attached a photo to try to give an idea.

Is this correct? I would have thought it should be fully closed at rest?

[E Sully]

No, there is a gap.

[Showdown]

When doing a smoke test you have to seal the sensor plate after removing the cover, colloquially called the peanut. You can stretch a rubber glove around the sensor plate housing and use a rubber band or zip tie to hold it in place otherwise it’ll let all the smoke out.

And, yes there’s supposed to be a tiny gap to let some air into the intake… but just a tiny gap.

[boyt911sc]

Mike,

It is next to impossible to do a sensor plate alignment with the motor installed in the car. The clearance between the sensor plate and the smallest diameter of the air funnel is so small that you need to use filler gauge (a strip of bond paper) or do a visual adjustment with a light source underneath the sensor plate.

For visual adjustment, you have to look directly above the sensor plate and with the motor installed it won’t be easy unless you have some optical tool. If you have the motor out, it would be simple.

Using a paper thin gauge is what I use for checking sensor plate alignment. Good luck.

Tony

[mikedsilva]

Originally Posted by boyt911sc

Mike,

It is next to impossible to do a sensor plate alignment with the motor installed in the car. The clearance between the sensor plate and the smallest diameter of the air funnel is so small that you need to use filler gauge (a strip of bond paper) or do a visual adjustment with a light source underneath the sensor plate.

For visual adjustment, you have to look directly above the sensor plate and with the motor installed it won’t be easy unless you have some optical tool. If you have the motor out, it would be simple.

Using a paper thin gauge is what I use for checking sensor plate alignment. Good luck.

Tony

ok, well the fuel distributor is out of the car, and looking at the pics supplied above, it seems that my sensor plate gap is HUGE. Also, the picture says that the plate should NOT protude into the funnel.. well, mine does... a LOT. I'll have a bit more of a look at the mechanism today, and also perform a smoke test on the airbox by itself, to see if it is indeed cracked as well.

Question: with the sensor plate gap so large, does this have anything to do with extra air entering the system, or do I still need to look for a large air leak that is allowing the extra high idle, even with the idle screw completely closed?

[47silver]

I lined cigarette rolling paper around the inside rim. It measures very close to spec. That was 10 years ago and no problems with throttle.

[mikedsilva]

Originally Posted by 47silver

I lined cigarette rolling paper around the inside rim. It measures very close to spec. That was 10 years ago and no problems with throttle.

Cig paper.. ok.. that is very thin.. maybe a couple of thou?

My other question relates to the movement of the plate. Lifting it is resistance free, till the halfway mark (approx)... after that, there is some resistance that feels like it is pushing against pressure. The assembly is out of the car, on the bench, and disconnected from fuel / pumps etc, when I do this.

Is this normal? Or should the plate move freely up and down for the entire movement?
Mike

['78 SC]

I just had a fuel distributor rebuilt by CIS Flow Tech. Here's their instructions for setting up the sensor plate:

[mikedsilva]

Originally Posted by '78 SC

I just had a fuel distributor rebuilt by CIS Flow Tech. Here's their instructions for setting up the sensor plate:

Thank you for posting this.
There are a couple of posts now that show the plate alignment like that.
Mine DOES protrude into the upper cone so clearly it is incorrect. I need to examine how it is aligned.

In the interim... here is a video I just made of putting a smoke machine into my airbox. The orignal reason for pulling it, was to check for leaks, as the idle is VERY high and the airbleed screw is completely shut.

Is there also a spec for how far closed the throttle plate should be?

https://youtu.be/H_SAcKXJlL0

[psychoideas]

Is your sensor plate out 'a lot' while you have fuel control pressure?
I know it's not in the car but the test sequence states that you should have control pressure before checking the sensor plate height.

[mikedsilva]

Originally Posted by psychoideas

Is your sensor plate out 'a lot' while you have fuel control pressure?
I know it's not in the car but the test sequence states that you should have control pressure before checking the sensor plate height.

Will put the motor on my test stand soon, and double check.
The sensor plate is not even centred on my setup; The arm doesn't even rest on the clip/screw adjuster. The edge of the sensor plate hits on the funnel and that is what stops it from lowering further. Not sure how this affects things...
I've loosened the bolt, laid some thin paper around the edges, and centred the plate. It now closes somewhat more normally.

[psychoideas]

Originally Posted by mikedsilva

Will put the motor on my test stand soon, and double check.
The sensor plate is not even centred on my setup; The arm doesn't even rest on the clip/screw adjuster. The edge of the sensor plate hits on the funnel and that is what stops it from lowering further. Not sure how this affects things...
I've loosened the bolt, laid some thin paper around the edges, and centred the plate. It now closes somewhat more normally.

Sounds like you're making good progress. CIS can be stressful to diagnose. You need the proper fuel pressure test kit and start from the beginning (the fuel pump).

Good luck

[psychoideas]

https://jimsbasementworkshop.com/CIS/pages/comp_page.html

[targa80]

Yep the video definitely shows a blown air box and that is repairable. See the attached link on how I repaired my blown box. You will still need to smoke check the CIS system once it is reinstalled on the engine. You might find leaks on the rubber seals between the box and manifold intakes, at the fuel injector seals or any other vacuum hose connections.

https://forums.pelicanparts.com/porsche-911-technical-forum/1081238-blown-air-box-repair-upgrade-spider.html

[SkiVT]

In addition to the Jim’s Basement link, this guys YouTube channel has two very good CIS videos you may find helpful.

https://youtu.be/Ii5k773lwKs

[HaroldMHedge]

Originally Posted by SkiVT

In addition to The Jim’s Basement link, this guys YouTube channel has two very good CIS videos you may find helpful.

https://youtu.be/Ii5k773lwKs

Klassik Automotive Training School videos are great.

['78 SC]

Mike,

Nice video of using a smoke machine to find the leaks. Much more effective than pressuring up the intake and spraying soapy water. I would repeat the test after the intake is assembled (seal the exhaust and put a rubber glove over the throttle body) to confirm no leaks all the way to the heads.

Clearly you have massive air leaks (apparently enough to support 1200 RPM ). Likely the air sensor plate rest position was manipulated to supply enough fuel to match all that un-metered air so it would idle. Not a good thing, since that means fuel would flow anytime the fuel pump was energized. Did you ever try jumpering the fuel pump relay and listening for a squealing noise from the injectors? (Be careful of flooding the cylinders with fuel.) That would indicate the control plunger in the fuel distributor is too high and allowing fuel to flow when the sensor plate is at its rest position.

Clearly you're going to need to work through the whole system checking for leaks, verifying mechanical set up (metering plate position) and setting correct system and cold/warm control pressures (with and without manifold vacuum). The good news is the 78-79 CIS is simple and actually quite reliable.

Answering your question, the throttle plate should be fully closed at idle. The idle air bypass can supply more than enough air.

[boyt911sc]

Mike,

That was a good test. I stopped using pressurized and soapy water to look for hard to find vacuum leaks after using a smoke tester for several years now. While you are at it, repair the POV and have it facing towards the front of the car. That is a common mistake people committed that limits the opening of the valve during a backfire.

You cracked air-box could be repaired but will require a lot of your time. Your AFM unit also needs some work too. The AFM sensor plate is simple to align but the arm should be able to swing up/down freely. You have to free the restriction at the pivot or fulcrum point. Good job.

Tony

[mikedsilva]

Originally Posted by boyt911sc

Mike,

That was a good test. I stopped using pressurized and soapy water to look for hard to find vacuum leaks after using a smoke tester for several years now. While you are at it, repair the POV and have it facing towards the front of the car. That is a common mistake people committed that limits the opening of the valve during a backfire.

You cracked air-box could be repaired but will require a lot of your time. Your AFM unit also needs some work too. The AFM sensor plate is simple to align but the arm should be able to swing up/down freely. You have to free the restriction at the pivot or fulcrum point. Good job.

Tony

Thanks for the encouragement guys.
Here is the sensor plate after centering it.
I am not sure how to describe the resistance I was sometimes feeling, when raising the sensor plate. Almost like it was "pumping" against something.. however in this video, it is totally free.

[mikedsilva]

Originally Posted by '78 SC

Mike,

Nice video of using a smoke machine to find the leaks. Much more effective than pressuring up the intake and spraying soapy water. I would repeat the test after the intake is assembled (seal the exhaust and put a rubber glove over the throttle body) to confirm no leaks all the way to the heads.

Clearly you have massive air leaks (apparently enough to support 1200 RPM ). Likely the air sensor plate rest position was manipulated to supply enough fuel to match all that un-metered air so it would idle. Not a good thing, since that means fuel would flow anytime the fuel pump was energized. Did you ever try jumpering the fuel pump relay and listening for a squealing noise from the injectors? (Be careful of flooding the cylinders with fuel.) That would indicate the control plunger in the fuel distributor is too high and allowing fuel to flow when the sensor plate is at its rest position.

Clearly you're going to need to work through the whole system checking for leaks, verifying mechanical set up (metering plate position) and setting correct system and cold/warm control pressures (with and without manifold vacuum). The good news is the 78-79 CIS is simple and actually quite reliable.

Answering your question, the throttle plate should be fully closed at idle. The idle air bypass can supply more than enough air.

I now have the motor sitting on my engine test stand. I've never run an SC motor on this before so I had to figure out the wiring side... thanks to a few members here, it turned out to be a lot easier than I thought.

After getting the motor started, it seemed to run nicely. I had not powered up the AAR or WUR yet and forgot to plug in my AFR gauge..
It soon stalled and I guessed it might have been 'lean' on the sensor plate adjustment screw.

I turned it clockwise to richen a tad while I had an injector removed, and had the pump running. I got to the point where a bit of fuel dribbled out, then I leaned it a little more.
Reinstalled injector, and got the engine started. It was showing as quite rich, so I leaned a little more.

I just wanted to get the motor stable enough to continue to run while I checked things.
Here is a video showing my clambering around and disorganised!

The idle speed is still a little high. With the idle speed screw completely in, it still idles a little high. I don't have a tach hooked up yet, but it is high.
Somewhere in its life, the throttle body had been adjusted as the yellow paint mark was not on the locknut anymore. So I decided to loosed the locknut and close the buttefly a little. It didn't take much to get the idle to a normal sounding speed... and the idle speed screw when fully closed, now drops the rpm a little too low. However, it still runs. So I am not sure if that means I still have air leaks somewhere?

I bench tested the AAR with heat and also using 12V. The plate closes fully after only a few minutes. Interestingly, the plate in my AAR has a small square 'notch' cut out in the edge of the disc... but it still closes fully... not sure what the notch is for or if anyone else has seen it?

I bench tested my decel valve: the one that looks like a little bell on the left side of the motor and and has a vac line running to the WUR. I cannot blow through it with no vac.. but when I use my brake bleeder tool to pull some vac, I can blow through it I figure it must be working? The idle would often 'hang' way too high though, and if I removed it, and plugged the line, the rpm seemed to return nicely. I did not try to loosen the nut to adjust it; perhaps I should.

So the question is, do I still have a large air leak? Should the idle speed screw be able to stall the motor by starving it of air?
At the moment, if my idle screw is only a half turn out from fully closed, the engine runs nicely as you can see in the video.

Open to suggestions and ideas.
Thanks, Mike.