AFM Adjustment FAQ

[Clatter]

OK,
Making sure i haven't lost my mind here....

If you adjust the toothed cog wheel on the AFM (intake air sensor),
The running cruise mixture would be:

Richer if you turn the wheel counter-clockwise.
Leaner if you turn the wheel clockwise.

Right?
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Bus Motor Build

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[Joey]

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Joey

‘60 Kombi - '74 Bus - '79 Panel - '65 Beetle

[SGKent]

you better have marked it where it was before you began and also have a really solid understanding how it works plus an air to fuel meter hooked into that system with some trial runs before you began. If not a disaster is coming to a VW bus near you.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[merlinj79]

Also that picture shows how to adjust the big gear...that is the coarse adjust and it should NOT be necessary to adjust that unless the AFM was dissassembled or something.

For tuning, you want to use the fine adjust on the armature (the set-screw and little teeth.) There is supossed to be a jack-screw engaging the little teeth to allow discrete fine adjustments but mine is missing that so I just loosen the set screw and move it by hand.

Even the fine adjust is VERY sensitive...if you even think about moving it, you get a significant change.
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Rick
"Pumpkin II"
Stock 2L FI '77 Westy
CA Smog Stuff

[Clatter]

So Ratwell has the wrong information on his page?
(Scroll down, near the bottom)
http://www.ratwell.com/technical/Smog.html#fi

My bus has been modified; cam, heads, exhaust, compression, ignition...
It needs more fuel to keep it from leaning out up top.
Am using a wideband to tune, along with an adjustable FPR to add more fuel.

The numbers on the wideband are matching the info on Ratwell's site.
But info on this site says otherwise.
Trying to find the truth...

FWIW, it's been a loooooong time since i saw an AFM that didn't have the cover off at least once already.
If the cover has ever been pried off by someone else,
All bets are off....
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Bus Motor Build

What’s That Noise?!?

[Joey]

[SGKent]

If anyone tries this and they screw it up the damage is on them. DO NOT as some threads and forums suggest move the board or bend the wiper to get a better contact. Replace the AFM if the board is worn thru or the wiper bad. You can clean it with a high quality electronic cleaner designed to quiet potentiometers. Do not seal the door with regular RTV - it gives off acidic fumes that can damage electronic equipment. Use special alcohol based RTV if possible.

With the AFM out of the car make sure the door swings freely and smoothly. Set the door so it barely opens about the thickness of a piece of paper with a weight on the edge of between 59 and 61 grams. Then leave the spring adjustment alone. You will not reset it again.

Warm the engine to at least 180F oil temp. Make sure your timing is set per TheSamba specs to 30 degrees BTDC at 3500 RPM with the hoses disconnected. Put the hoses back on when done. Make sure you have no vacuum or exhaust leaks. Hook up your meter so that it cannot have any dilution from air outside the exhaust. Make sure the EGR is functional if you have one hooked up. The EGR tube in the plenum must be clean and not coked up. Your injectors must be clean and have good spray patterns.

Set the wiper so it gives you 14.7 to 15.0 with no load at 2000 - 2500 RPM.

Set the idle mixture to 13.9 - 14.1

Check the mixture at 2000 - 2500 again.

Do these steps several times until the numbers are stable.

Take it out on the freeway at 65 mph flat. The A/F mixture should be low thirteens like 13.1:1 13.3:1. Speed up to WOT and it should drop to around 12.5:1. If you are too rich or too lean from these numbers use the slider to adjust it until you get to them.

Drive down the street, flat and level in 2nd gear holding 25 MPH. The mixture should read about 14.7 - 15.0:1. Speed up to 35 mph in 3rd gear. The mixture should be about 14.7:1 - 14.8:1.

Please note - max power is about 13.5:1 to 14.5:1 however you will overheat the heads if you set the AFM to this at WOT.

It took me two years to figure these things out and numerous AFM's including rebuilt ones. Colin spent years perfecting it too. My suggestion is, and it stands that working on an AFM is the most dangerous thing one can do to their engine other than perhaps trying to time an engine by ear. I would suggest hiring Colin to do this for you rather than taking a risk with your bus engine. All other things must be correct or one is adjusting the AFM to hide other problems.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[Clatter]

Wow!
What an eye-opener!
Thanks SO much SGKent (and Colin) for taking the time to do these things.
After reading this post, I had to set up and check out some AFMs.

One came from my bus, and it was screwed down much tighter than your spec.
The other two were an OG Bosch (made in France(!?)),
and a rebuild from RAM products in Northvale NJ.
These two appeared to have never been opened, so I pry, pry, pried them open!
before monkeymonkey-ing with the cog wheel,
I tested the 60g rod on them.

Both were the same in that they opened the door about 1/4 inch from closed, measured at the far edge where it would sit at rest.
The RAM one didn't close to seal, and had a bypass hole drilled in it, so it was probably junk.
One can guess that the spring would lose a bit of tension after 35 or so years, so this all seems right on track with the 60g spec.

So, should i give it a good hosing down with that spray in the picture?
It's pretty dusty in there.
Also,
Is it a good idea to smear a tad of dielectric grease on the track and wiper?
Seems like it would help the track last longer.
(An old-timer told me to do that once)
Maybe just hit it with some compressed air?
Where do you get the special silicone for the lid?

So many questions,
But getting this dialed in really feels great!
Thanks again
And again...
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Bus Motor Build

What’s That Noise?!?

[merlinj79]

I have a rebuilt AFM on the shelf (from BD). I opened it up and the wiper contact surface is worn. Not worn through, but clearly the rebuild did not include the contact surface.

My rebuild looks more like it was "cleaned" rather than "rebuilt"

Are any rebuilds available that include a refurbished (vice cleaned up) circuit board, or is that even possible?
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Rick
"Pumpkin II"
Stock 2L FI '77 Westy
CA Smog Stuff

[SGKent]

lots of good questions. I don't know about the dielectric grease. I used just electronic contact cleaner that I got at Fry's a high end product to wipe the board. I used light compressed air to blow out the insides and made sure the 4 screws were firm but not so tight they would break the board. I used a special Electronic RTV to seal it but GE Silicone II is supposed to be better than automotive RTV if you can't find or afford the electronic kind.

When I began the AFM quest there was one AFM that was worn and a firm in Livermore gave me a new board to replace a bad one in a unit. They were great people and they provide many rebuilt units to the industry. Even with a new board I could never get this one unit to work right and after studying the design I understood why. To understand the AFM one has to understand how it was made.

The board is not a linear resistor. It has multiple outputs not just one. As the wiper moves across the board it reads both the X and Y position of the wiper, and then it relays where the wiper is through different pins. So, if you move or replace the board to correct say a worn spot, you are moving the curve of the X and Y axis, and not just looked at across the board. The ECU is watching a football game as all the fans in the stadium, not just the end zones. Move the ball in any direction and some fan is closer and some farther. That wiper movement changes how it performs. What Bosch did was put that AFM on some kind of special machine and then use a small laser tool to burn away a tiny bit of resistor on each section to get the X and Y axis to read just right as it moved down the board. You can see the small burn marks if you look for them.

My suggestion is to keep an eye on e-bay for NOS ones that come up from time to time. Make sure it is the correct number. 018 and 020 are interchangeable per FIC. We paid about $250 each for the two NOS ones we found on fleabay and they were easier to adjust than you can imagine. I used the same settings to get a NOS one just right and sent to Chris up in Eureka. His cam is a little less aggressive than mine and he reported it was a tiny bit rich but close. His cam may pull more vacuum at idle than the Web142 so that would make sense. But moving the slider a tiny bit lean would solve that issue and make it perfect.

The reason I advise folks to stay out of them is that one cannot adjust it without first making sure everything else is perfect and then using a good air to fuel gauge to see what they are changing. However once you do get them done the bus will run like a charm. I was on the freeway the other day. It was 104F outside and I was going 65. The head temp was 345F and the oil 180F. Doesn't get any better. The rebuilt AFM I put on it originally yielded 475F head temps in a similar condition so you can see how much cooler the engine runs when adjusted right. And, the car passes smog perfectly like a new one.

Re the EGR and question of how the door responded on the used one. The used one I had opened about 3/8" so yes I would guess that the spring weakens with age. I used a NOS one to get the 60 grams off and it worked for several others I set up so it is better than "make a guess." One must have a starting point. Before I had the NOS one to start taking numbers off, when I first began this AFM quest I tried monkeying with the spring on a couple of used ones and could never fine tune it. The cause is simple. After 40 years who knows how strong that spring is and what the PO has done. Since we all have a 2.0L motor, the air going in to run that motor is the same for all of us so any given engine speed should send the same curve of the door opening on an AFM in any of our buses. What is different is the compression, timing, cam etc and that is where one has to play with the wiper to fine tune the AFM. Richard Atwell got it right when he said to use the wiper and not the spring.

I might add - on a L-jet engine, the amount of fuel your engine gets is really determined by the spark from the engine. The coil sends a pulse to the ECU which uses that pulse as a trigger to know how much fuel to send back to your engine. The signal from the TSII tells the ECU to add more fuel when it is colder. The AFM tells the ECU how to fine tune the mixture and the speed the door is moving open or closed lets the ECU anticipate whether to give it an extra shot of fuel like an accelerator pump does. So when someone changes that door spring randomly to see what they get, they are changing more than just the overall fuel mixture. As to the EGR since that question comes up. The EGR Filter cools the exhaust gas before it gets to the plenum. When that gas is injected, it is injected thru a tube in the plenum evenly for all 4 cylinders. That tube must be clean and they do carbon up. That gas has no oxygen or fuel left in it. Effectively it is CO2, Nitrogen, Argon etc. That reduces the amount of oxygen in the cylinder just a little which slows the burn and lowers the combustion temperature. A racer would not want that. However in a prone to overheating air cooled motor moving a 4000 pound brick, we can use all the cooling we can. If you look at the gasket for the EGR there is a small metering hole in it so only a little inert gas can be added. At WOT the EGR closes so you have full power at WOT when you need it. But at cruise the combustion temp drops. A little less efficient but cooler. Same for the best air to fuel ratio for cooling. It is less efficient than say 14.2 or 14.5 but cooler. The perfect mixture for a bus engine if you want power and fuel mileage is simply using an O2 sensor. It will keep you at 14.7 which gives you power and fuel efficiency. However it is too much heat on a 2.0L engine and I would not be happy melting heads.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[Joey]

Great info SGKent. The AFM in my '74 ('79 engine) has never been open. The one in my '79 has. A friend of mine borrowed Randy in Main's LM-1 a few years ago and I adjusted the large cog wheel a tooth or two to correct things. I'll have to try the 59-61 grams thing on the AFM door to see where how close it is.

With a bone stock FI engine with no vacuum leaks, good compression, etc. I would think one should only have to adjust the large cog (due to the aging of the spring) using the 59-61 grams on the door to bring things back into spec.
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Joey

‘60 Kombi - '74 Bus - '79 Panel - '65 Beetle

[SGKent]

[Clatter]

OK, I got it WhooHoo!!!!!

First, look at that picture that Joey posted up...
Has anybody ever looked at it before?
I think NOT!
Unless the clocks fun backwards in Nova Scotia....?
-Seriously- Look at it.
It says the wrong information in words:
Clockwise to richen,
Counterclockwise to lean.
But if you look at it, the arrows show the exact opposite.
Ratwell was right! (again)...
The thing works like a fuel screw!
CW to LEAN
CCW to RICHEN
!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!

Anyhow,
After going through everything in the Bentley again this morning,
I got pissed off.
Went in there and started wacking that damn AFM cog gear CLOCKWISE.
Like two whole turns.
Eureka!
No more rump rump rump idle,
No more lurching/bucking
No more fuel stink and eyes burning.
Idle AFM is right at 15:1.
cruise is between 12.9 and 13.4 depending upon the hills and accel/decel.
WOT goes down to the low 12s , like 12.1 but climbs a bit as it winds up.
My car is 79 CA, so it has a WOT switch.
So, on the switch gets it a bit rich,
but if it's OK with VW, it's OK with me.
My understanding was that switch was _full_fuel_, right?
It goes to high 11s lows 12s on decel, which is a bit disturbing.
Shouldn't it be lean on decel?
The decel valve tests OK.

Anyhow,
I put the AFM on the bench and tested it for spring tension.
121 grams when measured at the edge of the flapper door as SGKent shows above.
Because my motor is like a Raby "Camper Special", (not real one) it is bound to run a bit differently than a stock one.
Maybe this can help others who build a similar combo.

So,
In conclusion the Samba shows it's awsomeness and it's ugliness again...
Wise wizard SGKent shows a well-researched way of resetting an AFM,
and Joey shows a rediculous diagram that contradicts itself....

Thanks for helping regardless,

It really is the thought that counts.

Anybody else going to the Strawberry Music Festival?


_________________
Bus Motor Build

What’s That Noise?!?

[airkooledchris]

[SGKent]

121 grams is really stiff. You may have a bad TSII and someone compensated by leaning the AFM.

Re- L-jet

Rule 1) ECU controls how much fuel gets sprayed by how long the injectors are open and when. All 4 fire together. FPR controls fuel pressure based on manifold vs ambient pressure.

Rule 2) Pulse from coil tells ECU how often to spray fuel. RPM and pulse width set baseline.

Rule 3) ECU never subtracts fuel - it only adds to the baseline based on input from TSI, TSII, AFM and O2.

This is all in the Probst book on Bosch FI systems including L-jet.

You are essentially telling the ECU not to add fuel by tightening up that door. That implies the ECU is getting signals from somewhere else to go rich. Most common culprit is the TSII. TSI is in the AFM. TSII is on the 3-4 head.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[Clatter]

TSII and TSI both tested perfect.

Seriously, i spent almost three days on this.
Been collecting L-Jet FI parts for 15? (!) years now.
It all came down to tightening that dang spring.

My other AFM behaves the same at the same setting.

WOT switch, you can see it on the LM-1.
You can hear it, too.
One if the reasons i converted to the 79 CA set-up.
VW went back to it. Might say something about it?

Jake said you needed to tweak the flapper door spring to get his combo to run right at cruise.
If his set-up is more efficient, it stands to reason that it would be moving more air, and burning it, and needing less fuel.
Right?

I'm super stoked to see this method with the weight on the door.
Everybody just says how the AFM is this mysterious device to be left alone.

Regardless of what is happening, the cruise AFM is right.
And the CHT numbers are super cool.
If something is wrong, hopefully it won't 'fix itself' in the next 10 days.

So, who is the person that made that dorky diagram that Joey posted?
I have a package for him.
Anybody have his address?
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Bus Motor Build

What’s That Noise?!?

[SGKent]

[Joey]

[SGKent]

[Clatter]

Here's another clue why my bus wants the AFM spring so tight...
Re-reading the troubleshooting manual, it says the injectors should always be blue.
When i bought mine, they were white.
Maybe they are a (bgger) set for Digifant or something?
They got painted black at the shop who did the re-build on them.
What color are yours?

Dang, Joey, I still can't get over how i fell for that prank you and that guy from VA pulled.
Got me good!
That's a great one...
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Bus Motor Build

What’s That Noise?!?