I think that I need to increase the airflow reading from the MAF to push my ECU into open loop (even though I still think it should go open loop based on throttle position alone.) I am going to add an o-ring to smooth flow around the plastic protrusion in the sampling tube and increase velocity over the wires themselves. If that doesn't do the trick, I will reshape the entrance to the sampling tube into a velocity stack to get more air, going faster through there (that will have to wait a while due to long hours at work right now.)

None of this will hapen until at least next week, so my ECU will have some more time to adapt. Thanks to Craig (Patience) for some good ideas.<ul><li><a href="http://forums.audiworld.com/12v/msgs/25498.phtml">Original thread.</a></li></ul>

Hey, I just got home (almost 1am ... *yawn*) and it's too late for me to have things like coherent thoughts about this, but just something to think about until tomorrow: remember that the MMS200 doesn't learn so much as the MMS300+ ECUs, which is why you have the CO adjustment screw. Did you try fiddling with that a bit? Most of what 3-wire MAF owners do by driving, you have to do by giving the screw a little twist and seeing what happens. The CO adjustment was set with a very different amount of air crossing the MAF's wires.

I wouldn't try increasing the air speed over the wires for now - you've already done that. I think you need to concentrate instead on getting the ECU to understand the new signals it's getting.

Anyway just a thought; I'll try to think on what the ECU wants to go open loop. You might also try slowly twisting the throttle cable arm on the throttle body (engine off) and listening for the "click" at idle to make sure that the throttle position sensor is aligned properly.

Won't it move slower?

Before I go on, let me say that I like fooling with the CO pot. It probably adjusts the scale factor of the sensor and sounds like a great idea.

I think slower moving air is what happens in a 1993 90 when we change from a MAF with a 1.9" bore (stock) to the 3" bore in the 471 MAF that Nothing used.

Indeed, more air can get to the TB now and my car with Mance's Stage 3 TB has more power, but the air past the sensor wires is going to be moving slower for that air volume than it would have before changing the MAF. While more air is moving the sensor wire is seeing less velocity (cooling) so it "thinks" less is coming.

I think that is why Mance's O-ring solution works. They restrict the bore of the sampling tube and force the same volume of air to pass over the sampling wires at a higher speed.

&gt; Won't it move slower?

I'll admit to sleeping through fluid dynamics when they explained this sort of thing (... and a few other things ... actually I think I slept through most of fluid dynamics) but I thought that more air was moving across the wires because obstructions behind them were removed. Right?

At any rate, the conditions are different and you can fix it by using the o-rings, adjusting the CO trim, or (preferably) both.

The CO potentiometer is basically a voltage divider connected in parallel with the output of the MAF sensor:

http://www.12v.org/engine/index.php?section=hw&amp;ver=2&amp;comp=g74#list

illustrates the internal connection. The order of pins in the picture is 3 - 4 - 1 - 2.


With the engine idling, the voltage between pins 1 and 2 of the MAF should be between 0.3V and 1.1V; at 4000RPM that should jump to around 1.5V and 3.4V; you might see if yours are off from that. Granted, that's not the proper way to check the CO trim but it might do the job.

Having two measurement points hints at CFM to "cooling" relationship adjustment and certainly gives something to look at. The wide range of OK voltages at 4000 RPM makes adjusting that trim a bit hit and miss...

I still suspect we need to speed up the air over the wires to bring things into adjustment range.

I will be able to test when I have Mance's S3 hose kit so I can connect 3" MAF to S3 TB. Right now am limited to accordion hose which appears to be about 2.5" max dia. So am using my first MAF mod which has measurement bore with velocity stack in front of it.

This setup has run strong for 900 miles with no CEL or bad manners. Mileage is better than before, but only tested at higher than normal speeds with AC on.

Will that hose stretch without tearing?

Turning the CO pot definitely affected my mixture, but my ECU still does not go into open loop. I'm sure it is due to turbulence in the sampling tube, my A/F meter readings at WOT can be quite erratic, especially around 4000 RPMs and near shifts. I need some o-rings (I have no idea where to get custom sizes, and no time to seek them unfortunately - 12 hours a day, six days a week of work atm) and/or the VS I ordered from Mance.