Hi! I have misfire on cylinder 2.
Audi A2, Same engine AUA 1.4. Same problem.
Usually it appears as said here: “The car starts fine and then after a few minutes starts to misfire on No.2 cylinder. The EML light will come on, car runs on three cylinders. If the car is started and driven immediately, it drives fine, until you need to wait any length of time at a set of lights and then the misfire/signal cut starts again. Resetting the ECU with the engine running immediately returns the car to four cylinders, until a lengthy tickover causes the problem again.”
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Case, which is described above is typical driver's problem description with that mystery misfire on cylinder 2. Some Audi A2 owners have changed camshafts, head, belt, etc. even pistons. Usually without success.
I have to point out, that in most cases “Resetting the ECU with the engine running immediately returns the car to four cylinders, until a lengthy tickover causes the problem again.” is not actually what it seems.
Misfire detecting is done in ECU. Algorithm is about as: if misfire is detected, it should occur repeatedly (how many times is not documented) in 3 minutes. Only then ECU will produce P0302 - cylinder 2 misfire fault code and engine running will go worse. ECU resetting (clearing fault codes) will indeed affect engine running, but it actually reset misfire count at a same time.
It is not clear, how it affect registers in different microchips drivers, but that can affect in the end on ignition driver signal.
But, as I have notice, misfires are not gone after fault codes reset, they are still present. Check misfire counter in VAG-COM diagnostic.
And, that is also true, that some times during testing, if misfires are present, they can disappear after engine restart, and engine will run as smooth as new one. Misfire counter in VAG-COM diagnostic will show 0 or under 10. But after some idling time misfire will start to occur.
So what you should do:
First, test compression for all four cylinders. Should be at least 10. May be up to 14 bar.
No more as 1 (max 1.5 on old one) bar different between any cylinder.
Second, change spark plugs, ignition coil. Change ignition wires.
Test income manifold for leaks, same for exhaust manifold.
Test EGR, test Lambda.
Read fault codes from ECU. Reset, check after test ride again.
Should maintain only one, misfire in cylinder 2.
Problem, which is described above, is quite well known, and few people have came with same symptoms.
I myself, have pointed out actual problem to the ECU.
To do that, you have to use oscilloscope with at least two inputs.
Ignition here use waste spark system, which is not as reliable, as some may think.
Ignition coil on that motor have two actual coils inside. As I remember one for 1-2 cylinder, second for 3—4 cylinder. Ignition signal to that coils came directly from ECU.
That are two thin wires (0.35mm2). First wire is green/white, which came from ECU pin number 102 and on ignition coil goes to pin number 1. It drives cylinder 1 and 2.
Second wire is green/red, which came from ECU pin number 103 and on ignition coil goes to pin number 3. It drives cylinder 3 and 4. Two wires-> two signals.
Signal, which ignition coil produce is square wave, as I remember is about 4.8V. Frequency depends on motor rpm.
You have to connect oscilloscope first input to first ignition coil input signal, and second input to second signal. Then adjust oscilloscope, that both signals received from ECU can be read clearly on oscilloscope screen.
Then you can observe, depending on you oscilloscope type, both signals on the same time, or one signal on a time. Start motor.
Then, you will notice, that signal to ignition coil pin number 3 have always same square waveform, and the length of that weave is always the same (motor idling).
Then observe signal, which goes to ignition coil pin number 1. It drives 1 and 2 cylinder.
You will notice, that when misfire will happen on cylinder 2, on the same time waveform will alter. Signal will fall from it's top 4.8V before it will normally reach end of waveform. It will fall:
1) Directly from rising edge, abruptly towards 0V (bottom), but it will not touch bottom until it will reach end of wavelength. That kind of misfire is a worst one, idling very unstable at the same time.
2) From about half (or more towards the end) of wavelength, abruptly towards 0V (bottom), but it will not touch bottom until it will reach end of wavelength. That kind of misfire is a less impact, idling unstable, but not so badly.
You can observe on the same time misfire counter in VAG-COM diagnostic ( 01 Engine - 08 Measuring Blocks- Group 014 (last block, total misfire count), Group 015 (misfire count cylinder 1, 2, 3), and Group 016, first block (misfire count cylinder 4).
So, main problem is probably with ECU ignition coil driver. There is no freely available datasheet, but possible cause is microchip L9119D – which (probably) control STI bus, k-line, ignition control.
Reference: Audi A2 current flow diagram No 2/1, 1.4 litre fuel injection engine (55kw – Magneti Marelli – 4-cylinder), engine code AUA. From model year 2001.
