Symptom
Strange engine behaviour between 1200 rpm and 3000 rpm. There was a little bit of engine knocking and some misfire — with black smoke coming out of the exhaust pipe. The engine had a fluctuating idle rpm — sometimes worse than others.
Investigation
My approach to this problem was to take the car for a spin and look out for driveability problems concerning the engine. I soon found out that the car was almost impossible to drive when engine speed was between 1200 rpm and 3000 rpm. Within this range the engine would cut out, but when passing 3000 rpm it ran almost like normal.
The next thing I did was to connect my serial tester VCDS (VAG COM) and look to see if DTCs (Diagnostic Trouble Codes) were present. To my big surprise — there were none stored. This engine ran really bad and yet no DTC!
When looking at the measuring block 003 on VCDS (this measure the MAF airflow) I could see that the specified vs. actual value was wrong, by around 130 mg/stroke. It showed 130 mg/stroke more than it should. My focus then shifted towards the EGR (Exhaust Gas Recirculation) valve and the operation of this as it determines how much air there is drawn into the engine.
I then switch to basic settings in VCDS to perform an actuator test on the EGR valve and its components. I did the test and could see that when the EGR valve was not active it draws 980 mg/stroke of air through the air mass sensor — it’s almost the double of what is in expected in limp mode (very basic mode in which the car runs in order to get the occupants home) which is 550 mg/stroke.
I then checked all the vacuum hoses for leaks with a vacuum pump and the function of the exhaust recirculation valve (N1), I found that nothing was outside the specified parameters.
With these preliminary checks in mind — I concentrated on the air mass sensor and its function. My conclusion was — if the EGR valve is okay — the only thing back that controls the amount the air through the engine is the air mass sensor.
With my PicoScope I connected to pin 5 on the air mass sensor. Pin 5 is the output pin and with engine idling it should output around 1.9 volts DC. Then I got confused — I thought I may have picked the wrong pin number. I checked my wiring harness diagram again and sure enough — it was the right pin number… “but it can’t be true?!” …the air mass meter was sending out a static value of 6.95 volts DC! Additionally there was no change in values when I revved the engine.
I now fully understand why it ran so poorly with such a bad air mass sensor.
Conclusion
A new air mass sensor was fitted and the engine got its normal idle again and could now take full throttle without misfire and cut outs.
What I don’t understand is why it did not set a DTC when the air mass sensor was that bad!
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