Perplexed by CSI Air Flow Sensor

[jlynch1960]

Picked up an inexpensive 3.0L engine for kicks. Figured I'd take it apart learn a thing to two and decide where to go from there.

Got the Probst book on Bosch systems and started disassembling/cleaning the CIS. I'm puzzled by the location of the air sensor plate which sits beneath and is sprung against the lower edge of the air funnel? I know that some systems use a downdraft mechanism, but the throttle is downsteam of the air sensor so it looks like there would be no movement of the air plate with the throttle opened up.

What am I missing?

[theiceman]

not to oversimplify but when you open the butterfly in the throtle body, it instantly draws more air ... and since the air has to go by the air plate it lifts it .. it is set i think for a 1mm gap even at rest so some air can get by.

[47silver]

As was stated, as the throttle opens, the vacuum from the cylinders draws in air, The air crosses the plate and raises the arm to allow
More fuel to flow.
I used cigarette rolling paper to set the gap by affixing the paper around the inside of the plate. Worked for me.

[Bob Kontak]

Originally Posted by jlynch1960

I'm puzzled by the location of the air sensor plate which sits beneath and is sprung against the lower edge of the air funnel?

Is sits beneath but raises up into the air funnel as the TB calls for air. It is NOT sprung against the lower edge.

[jlynch1960]

Bob Kontak from Canton wins the reading comprehension test.

The "default position" of the air plate is below the funnel. I undid the bolt that holds the air plate and tried to move the plate into the funnel, but all of the range of motion of the lever to which the plate is attached is below the funnel?

Below the funnel is the air box where I would assume the filter goes. Above the funnel is a rubber chamber that directs the air toward the throttle which then sends it to the lower section of the airbox and out to the manifolds. Someone will probably ask to see a pic.

[javadog]

Got pics?



In absence of any useful info, I can only speculate. The sensor plate does move upwards on a 911. Perhaps yours won't move because the fuel distributor is still attached and the plunger is stuck.

Or, maybe the sensor plate is not centered and is binding.

Not sure what bolt you removed...

JR

[boyt911sc]

Originally Posted by jlynch1960

Picked up an inexpensive 3.0L engine for kicks. Figured I'd take it apart learn a thing to two and decide where to go from there.

Got the Probst book on Bosch systems and started disassembling/cleaning the CIS. I'm puzzled by the location of the air sensor plate which sits beneath and is sprung against the lower edge of the air funnel? I know that some systems use a downdraft mechanism, but the throttle is downsteam of the air sensor so it looks like there would be no movement of the air plate with the throttle opened up.

What am I missing?

jlynch,

I have attached pictures of metering units with sensor plate at rest, centered and aligned with just air gap between the funnel body. The sensor plate goes up when the throttle body opens up accordingly. It is very easy to understand the mechanism of the metering unit in conjunction with the fuel distributor's plunger.







Could you post a picture of your metering unit showing the top of the sensor plate? Maybe it is improperly set and not swinging freely as it should. Notice the strings keeping the sensor plate from swinging? Keep us posted.

Tony

[jlynch1960]

Thanks for the help. I suspect the control plunger is not working properly and is stuck - someone must have installed the plate incorrectly. Here are some pics. First shows the air plate installed below the funnel. Second shows the slight travel downward of the plate - about 1/4" at best.

As I mentioned I took the plate off of the lever and tried to install it inside the funnel, but there's not enough travel to allow it to move upward and I didn't want to force it and possibly break something.

Btw, what is the normal travel of the plate when it's working properly?

[javadog]

You've got it backwards. Up is down, down is up.

JR

[Jdub]

[javadog]

Maybe I'm confused by hat you are trying to show. The normal resting position of the sensor plate on a CIS car is a few thousands of an inch below the bottom of the funnel portion of the metering unit. Your first picture looks fine, as far as I can tell. The sensor plate should move towards the camera when in operation, as the air flows from the airflow unit to the throttle body. My assumption is that your fuel distributor is still in place and your plunger is stuck, preventing the sensor plate from moving normally.

JR

[jlynch1960]

Yes fuel distributor is still attached. I'm certain now that the plunger is stuck and it's clear that someone at some point attached the plate incorrectly.

Guess I should have waited until I had it fully disassembled before posting - it just seemed so wrong based on my readings in the Probst book. Again, thanks for setting me straight.

[boyt911sc]

Originally Posted by javadog

Maybe I'm confused by hat you are trying to show. The normal resting position of the sensor plate on a CIS car is a few thousands of an inch below the bottom of the funnel portion of the metering unit. Your first picture looks fine, as far as I can tell. The sensor plate should move towards the camera when in operation, as the air flows from the airflow unit to the throttle body. My assumption is that your fuel distributor is still in place and your plunger is stuck, preventing the sensor plate from moving normally.

JR

jlynch,

Jeff (javadog) has a very good observation about your problem and you should follow this up. I don't see anything wrong with the sensor plate in the first photo. Have you tried to lift the AFS (air flow sensor) lever from beneath? Assuming that the plate is not binding against the funnel body and you are having heavy resistance to lift the AFS lever, your problem is a stuck FD plunger.

Take a look at the pictures I posted showing the air metering unit. The FD is held only by 3 small screws (5-mm). Pull out the FD and inspect the plunger. To play safe, assume the plunger is not stuck when removing the FD. Care should be taken to prevent accident with the plunger.

With the FD removed, the AFS lever should now move up and down with minimal resistance except its own weight. If you are looking for some excitement in CIS troubleshooting, FD would keep you on your toes. Keep us posted.

Tony

[jlynch1960]

Yes, it's stuck.

Allen keys in the car which is in a garage 20 blocks away. When the temp gets above 10 I'll go get them and take the distributor off.

Thanks again.

[T77911S]

the OP is thinking the funnel resting place or starting point is at the TOP of the AFM. it is not.
its idle or starting point is where it is resting in the pics. your finger is actually pushing it about 1/4 inch below its starting or resting point.

as the RPM's go up the plate goes up into the bowl. you will see what looks like 3 sections of the bowl. one is idle, one is cruise and at the top is for upper/WOT operation.

as tony said, the AFM and plate look fine

this is an up draft AFM. so as the RPM's go up, the sensor plate goes up. the 930 uses a down draft AFM. the bowl or funnel is up side down compared to yours and the senor plate moves down as RPM's go up