Point of AFM idle screw?

[JustCallMeOrlando]

Hey guys,

This could come across as a stupid question, but hear me out. I've been pondering ways to kill all vane-type AFM's, but along the way, I digressed, and started looking into them a bit more.

This began with the other thread about the way they work, this one pertains to the idle screw on it....

...what does it do? Now I get that it bypasses air around the AFM's main sensor to allow for the idle, but why? What is the point of a controlled air leak?

Then I thought about the AFM not being able to open properly at idle, then smacked myself in the head. Of course it can. I know it can, I've looked at the output voltage.

On that note, what is the point of it? The only theory buzzing around in my head is about the spring tension varies slightly, and while under driving conditions this is fairly insignificant in terms of airflow, but when idling this can be enough to upset the idle and needs to be accomodated for. That's the only theory I've got, and there had to be a reason they put it there.

Ideas?

Cheers,
Nathan

[stradlater]

Wouldn't the Idle stepper motor let the air into the plenum mate? Not a AFM idle screw?

[JustCallMeOrlando]

There are 3 things to do with the idle on the 1G from the factory. There is the AFM idle screw, which lets air bypass the AFM itself into the intake piping. There is the TB idle screw which lets air bypass the main TB butterfly, and there is the ISCV which tries to control the idle by letting in various amounts of air from the intake.

Logcially, to me, there only needs to be the ISCV, and you let all air come through the AFM itself, not bypassed. Obviously, Toyota had other ideas when they came up with it, and thus added the other two.

The TB idle screw I can kinda justify, if someone wants to alter the ultimate idle rpm (although I still think the ISCV should do this), but the AFM one I see no point for.

[ra23celica]

Given that I have stuffed up my AFM idle screw myself, I can tell you that the only way, bar the numbers stamped on the body of the AFM, to set this up correctly is to adjust it while you have an emissions sniffer stuck up your exhaust pipe.
The position is set when you have the right % of gas on the meter dial.
So.....my uneducated guess here is that there are idle emissions tests results to be achieved for these engines and the way Toyota do it is to have an AFM idle bypass screw, which is adjusted at the factory as soon as the car rolls off the line, and then stamped onto the AFM body.
Make sense?

[ra23celica]

And while we are taking about "screws" - what is the best way to test/adjust the throttle body idle screw please mate?

[CrUZida]

Mitch, most AFM's have the screw blanked off from factory.

Whether this blanking happens after the car is tested for emmisions? I dont know.

[JustCallMeOrlando]

Surely the ECU could accomodate this rather than having to rely on a little screw, unless it's just not fine enough resolution at idle.

[ra23celica]

I don't doubt this mate, but on my AFM there is a rubber grommet on top of the tube that houses the AFM bypass screw, and '63' stamped next to this on the AFM body. I have a jap. 1G-GZE manual that shows in the pictures how to adjust it, along with gas %'s to achieve and a pic of the gas meter dial.

Edit: My comments follow from CrUZsida's post, JCMF is too quick with his typing!

[JustCallMeOrlando]

So your screw should be 16.3mm from the top of the tube?

[ra23celica]

According to the 'photocopied' BGB, yes.

[willwal98]

From my tinkering I see it this way. The AFM screw adjusts idle mixture to a certain extent and as stated before the TB screw adjusts ultimate idle. When I say it adjusts it to a certain extent, when it's all the way in it's set at maximum richness and as you screw it out it leans the mix. I guess it's because engines would differ with idle mixture and they use the screw for fine adjustment.

[The Witzl]

... just remember one thing guys....

Air is like water, it wants to follow the path of least resistance.
The vane, or "flap", in the AFM is resistance.... so that mixture screw does have a net effect on the whole rpm/load range - however in decreasing amounts as load increases.

Best option is if it aint right - get a good dyno tuner to play with it

[mrshin]

Another thing to remember - it's only old school stuff that has an adjusting screw on it. As years go on, then there are less things that can be adjusted (by hand) on cars. How many cars can you change the idle mixture, idle speed or ignition timing on now? (This isn't counting the wonderful adjustability of some factory ECU's with the correct hardware/software)

[camrygt?]

I've had the unpleasant experience of fixing a few cars which people have had a play with the air flow metres themselves.

As mentioned the screw is the idle mixture adjustment.
Most air flow metres have a spring loading on the "flap" which can be adjusted under the big plastic cover however when this is set correclty the idle is usually a bit off.

Because the engine draws very little air at idle it is difficult to get the mixture right as the flap is barely open.

Also adjusting the overall mixture with the big spring throws the idle mixture off considerably so the screw is used to bypass the flap to correct the idle mixture.
As the amount of air which can flow through this screw is very small it has vitually no effect on mixtures when the motor is under load.

To set it correctly a wideband o2 gauge is rather useful

[ra23celica]

Here is our answer - thank you camrygt?.