patience

I've got a 1993 90 with S3 TB and DIY smallish bore MAF, 4 wire sensor... Just installed ARM1.

Thought, at first that MAF adjustment was not very sensitive. See earlier post. Poor luck using method eep offered. My MAF would not "play ball".

Now think MAF is darn sensitive, but ECM has sanity check logic. When MAF signal is out of "range" it ignores or discounts it. Sets fuel with best guess. ARM1 looks like ECM is open loop. Wanders a lot! Ends up too rich or too lean, depending on who knows what, but may be related to MAF somehow.

When I found the MAF trim range the ECM "liked" the ARM1 dropped in and respnded to MAF trim pot settings. I could choose a lean or rich idle. Choose middle of the range. Then, when driving car, ARM1 display made sense. Mid range at idle, lean at cruise, richer at WOT. Very lean when foot off pedal and coasting.

nothing

...you REALLY need a VAG-tool to set the CO pot correctly. Beg, <a href="http://www.steve-hall.com/cgi-bin/VAG-Locator.pl">borrow</a>, or steal one and monitor display field 8 while adjusting the pot. You are looking for a value of 128, adjust and then give it a while, the display has a delay. If you still haven't done it by June, I will be making an California road trip and may be able to swing by and help. Do you have to pass emissions testing every year where your car is registered? This adjustment must be dead solid perfect to pass.

On your A/F meter are you getting steady lights at wide open throttle? I am not, that is the problem I am having. The mixture should 'dither' rapidly from lean to rich at partial throttle, and be steady rich(ish) when you floor the pedal (open loop.) Mine keeps dithering at WOT, and no amount of pot adjustment helps. I haven't had time to play with it too much or make any further physical mods lately, so I don't have all the answers, yet, but I think some o-rings and further sampling tube modification will be the solution. Your current MAF (damn nice work btw) shouldn't have these problems, I think they are related to air turbulence from the plastic <a href="http://pictureposter.audiworld.com/70334/mafsensor.jpg">protrusion</a> and hopefully I'll have some solutions by the time you are ready with your bigger bore unit like mine.

patience

And, No, I am not getting dead steady rich on WOT. Am getting dither, in richer range than at cruise. Seems reasonable to me.

Why do you believe you must have dead steady? Someone told me that our ECM uses MAF input as "suggestion" until given facts by O2 sensor. Then it makes further changes accordingly. What I have been seeing seems to support that idea. In particular that engine ran "OK" with MAF grossly out of spec.

For what its worth, before Mike (Outstanding local Audi Wrench)noticed that my MAF was mis set and fixed it using VAG. (Pre MAF mod) I passed emissions test several times.

Am hoping Mance has sent me the hose kit.

nothing

I had my AF meter before I did my MAF mod, and it used to be steady rich at WOT.

patience

On the return trip, at WOT, AF was steady, richer than usual (first green, near yellow) LED.

Now some facts. When we hooked up the VAG COM, my MAF was at 140 or so. Could get it to 166 or zero with smallish changes in POT. Finally got it settle between 127 and 130, staying at 128 msot of time. At idle AF is showing richer than before. At cruise, AF is about the same as before... STOICH.

Butt O meter indicated more power. I'd been suspicious that the setting I had before was on the "wimpy" side, but did not want to believe it.

And some theory (math alert!)... Lets guess that the MAF sensor is designed to approximate a straight line relating temperature to air volume. Any straight line can be expressed as Y (air volume) = A * X(temperature of sensor) + B

A and B are the scale factors. The POT could be adjusting A or B, have not figured out how to be sure which, but suspect it is B...

So, when ECM gets a MAF reading, and it's at idle, it probably divides reading by some magic number and puts the result in field 8. When it is 128, things are in calibration. When it's too far from 128, it ignores reading and makes it's best guess. (I think)

Now, when we change the relative diameter of sampling bore and main air path, as we do with a modified MAF, the appropriate constant for A should change. Of course, we can't go in and patch the ECM. BUT, what we can do, is "fudge the MAF POT" or put an O-Ring in front of the sensor, which should speed up the air and reduce the temperature...

Remember that my current MAF bore is significantly smaller than nothing's and I incorporated a VS into my sampling tube. Total of all that is probably that my ECM is hearing closer to correct air volume. That may be why when I got it correctly calibrated, as nothing suggested now I get steady rich at WOT.

so, I bet if you stick the fattest O-Ring you have (from Mance's set) into your sampling tube and recalibrate, you will get steady rich at WOT.

Let me know?

By the way, when you change the O-Ring, if you don't have to change the POT much, we can guess it controls B. If it takes a large change, we can guess it controls A. Don't believe it makes too much difference which is true as long as we can get ECM to "listen" to MAF.

nothing

An S3 MAF is not usable on an mms200 car. I don't have time to search out custom sized o-rings at the moment, my job is currently sucking up way too much of my time. Your experience makes me hopeful that I will have success when I simply have more time to play with the CO pot.

patience

I very much doubt the POT is going to get you there alone. Try any good hardware store in the Oring bins. You want something about than .395 OD. That is just over 3/8" And I'm betting about 1/8 thick. Perhaps a bit skinnier? I'd buy all they had in the closest out side diameter. Should make a nice push towards the richer side of things.

Otherwise, once, if, I get rings from Mance with my S3 hose kit... and I figure out which one I need, I'll give you the rest. Since I have VS in front of my sampling tube, your size should be "fatter" than what I need.
Ciao'

patience

Re: I don't have a set of Mance's o-rings - he only sells them with a S3 MAF.