How to set the MAF screw accurately on Motronic 2.5...

This also works on Motronic 2.7 as it is the same setup, but I only tested it on a Motronic 2.5 car.

Install new or use known good O2 sensors.

Note your original MAF numbers and if possible do a gas analysis looking particularly at the HC and CO numbers

Warm the engine up properly (until Oil Temp is in normal operating range)

Leave engine at IDLE for whole procedure from here... Disconnect the O2 sensors for 2 minutes before carrying out the procedure to ensure the ECU goes to open loop and all gases settle down. Connect a multimeter with the scale set to read milliVolts. Connect the positive probe to the black (if bosch) signal wire, and the negative probe to body earth if 3 wire or to the earth wire if a 4 wire O2 is used - ignore the heater wires as the system would already be at operating temperature. The O2 sensors GENERATE a voltage without a supply.

S-L-O-W-L-Y turn the screw until the multimeter reads 465mV +- 5% - this is a slow and delicate process. On the Bosch O2 sensor .465V or 465 mV represents Lambda 1 (AFR 14.7:1) as n average between the oscillations of 100 mV (Lean) and 900 mV (Rich).

Rev the throttle a few times, let it all settle down and repeat if the resulting numbers vary more than 5% and repeat a few times. You can do a GAS analysis looking for the HC count as well as the CO.

When done, shut down, note the new MAF numbers and reconnect the O2, disconnect the ECU plugs/battery for 30 secs and then reconnect and fire the car up.

FYI... when the car runs in closed loop (O2 connected) you can measure the O2 sensor's output to ensure it stays within the same RANGE on each bank, AND that it oscillates the same number of times per minute on both bank, this will tell you the O2 sensors are equally worn/accurate.

I had one MAF at 343 and th other at 370 to hit the magic number on the O2 sensor output. This generated an identical CO and HC (within 3%) gas analysis from each bank when in open loop. The previously set "factory default" 383 ohm, had a difference of over 20% in the HC number and 15% on the CO.

My $0.02 and a lot more balanced engine when in open loop.

Marco

 

Bank 5-8 520 ohm, 1-4 430 ohm to balance the O2 output on new set of plugs.

The engine runs a lot smoother than when set to 383,, especially in the 2-3000 RPMs. The smell has also reduced dramatically.

The "tune point" is when you see a quick drop from 800mV to 200mV for a VERY small variation in the CO screw setting, then going rich again (towards the 6-700 mV area) by 5-10 ohms. When you plug the O2 sensors back in, the output voltage oscillates between 900-100 mV about once every 2 seconds.

This made a HUGE difference to the engine's smoothness at idle and low RPM's (partial throttle) - from a rumble punctuated by pops, to a deep regular hum.

Marco

 

465mV is the voltage generated by the sensor when the exhaust is at stoichiometry... An arbitrary voltage, characteristic of all thimble zirconia sensors...

Marco