20VT90q

I've read here about how to adjust an MAF after a large bore swap (with and without vag-com). However I've never read anywhere that you need to disconnect the 02 sensor first before doing the test. Does this make sense? I read this at Motorgeek.

"If your maf is adjustable and you ARE worried about its state of tune. it is VERY easy to get it tuned correctly.

We do know, that audi loves to run things at stoich as much as possible. Secondly we do know that they program to run very close to stoich even if the 02 sensors are missing.

So to simply make sure you are tuned correctly, allow the car to warm up fully and the rad fan to kick on atleast once.

Connect an ANALOG voltmeter, or a very very good digital voltmeter (must include a digital form of analog display), or o2 sensor gauge to the o2 sensor. But disconnect the o2 sensor from the computer. (you do not want the computer to be correcting the mixture during this proceedure, you only want the mapped fuel setting.)

While reading the voltmeter's display or the air fuel ratio display, you want to have .45 to .47 volts AVERAGE voltage on the meter over a 10-20 second period. Basically the signal will bounce high and low, but you want the average to be about .45-.47 volts. this will set you as close to stoich you can get. To adjust it, simply turn the adjustment screw on the MAF.

LASTLY once you acheive .45-.47 slightly richen it, just a touch will do. Since the stock computer uses a narrowband o2 sensor, and thats what we are reading from it should go indicate "rich" now and will not be easily averageable to a lower setting from about .6-.7 volts. this is fine. as soon as you connect the o2 sensor back to the computer, it will take over and run it back down to stoich.

There you go."

nothing

The O2 sensor requires power to function correctly. It won't get power if you disconnect it.

Pre95

thus making it impossible to tune the MAF.

Norty

but the design of the O2 sensor is that it creates its own voltage on the sensor wire. The only wire to disconnect is the black signal wire. Keep the two white wires (heater) connected so that you get an accurate reading on the signal wire.

You will notice when your car is cold, you will not get a voltage reading from the signal wire to ground. Once the car is at operating temp and running, you can pull off the signal wire and measure voltage against ground. You will see the voltage move on your meter back and forth.

4Driver4

With the sensor disconnected from the ECU, the ECU substitutes a constant value which is predictable. It effectively goes into limp mode. Fuel mixture is adjusted based primarily on MAF reading.

If the O2 is connected, fuel mixture is adjusted by the ECU using inputs from both the MAF and the O2, so changes to MAF may be cancelled by an ajustment at the ECU based on readings at the O2 sensor.

That's the right way to do it. You isolate the MAF as the main input influencing mixture, then use the O2 sensor to measure changes based on exhaust gas content.

4Driver4

The power is only the heater. The procedure sounds correct to me.

nothing

The CO potentiometer only shares a housing with the MAF, it is not an adjustment to the MAF signal at all. What it is adjusting is the way the ECM interprets the O2 sensor signal. Without that signal in the loop you are only getting close and will never nail the perfect mixture. Later ECMs eliminate the CO potentiometer by adding an adaptation channel that automatically sets the fuel trim - which it adjusts with the O2 sensor in the loop.

This procedure is covered in the Bentley, and the O2 sensor remains connected.

20VT90q

Post that there.<ul><li><a href="http://www.motorgeek.com/phpBB2/viewtopic.php?t=24872&amp;sid=accd0ded9b9d5e2812b1 1401d01f5360">MAF Adjustment</a></li></ul>

4Driver4

I'll have to take a look. I know of no moving parts in the MAF housing.

Most VAG CO adjustments are done with the sensor disconnected or the coolant temp sensor unplugged.