Merc W203 200K misfire forever

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Merc W203 200K misfire forever
Car come in with "misfire" at idle. It can misfire ligthly for minutes then run ok.

Spark plug ok, decided to buy 4 coils. Test car ok. Give the car to customer.

Car come in with "misfire" at idle. It can misfire ligthly for minutes then run ok.

Decided to buy 4 injectors. Test car ok. Give the car to customer.

Car come in with "misfire" at idle. It can misfire ligthly for minutes then run ok.

Decided to test the timing of the engine, result the engine has correct sync.
Tested the wiring from ecu to injectors: found one wire of cyl 2 that has an high resistance
value. Put a new wire from ecu to injector 2.
Tested compression: cyl 2 has 2 bar less than the other cylinders.
Send the head to the rework center.
Head arrived with valve seat rework, valve guide, etc., also 1 new camshaft variator
New head gasket of course.

Test the car on street...guess what...between idle and 1500 RPM the engine collect
now and then "misfire" on cyl 2.

The engine run fine over 1500 RPM, the number of misfire on cyl 2 does not increase.
Leave the engine at idle, the number of misfire will increase.

Suspect of ECU but no one near there want to open this ECU.

We are litteraly tired of this car.
The owner is silly, could have bought a modern used car.

For your information:
+ swapped spark plugs countless time.
+ swapped coils a lot of time (new, used, used with new)
+ Compression is now the same for all the cylinder. (after head rework)
+ Ohm value for wiring of the injector is now the same for all the cylinder. (after repair wiring)
+ Catalytic converter found very clean

Kompressor kicks correctly.
MAF read ok.
Tried at idle without air filter.
Cleaned throttle body.

Countless swear because of the poor design of airbox, hard to remove, breaks easily, had to repair once
because we get DTC "air/fuel mixture adaptation" due to a crack in the airbox.

If you leave airbox pipe out you also get DTC "air/fuel mixture adaptation".

Sporadic DTC: RPM signal
Forever (especially at idle) DTC: misfire cyl2.

Before the head rework: Crank the car, leave it at idle, after 40 seconds engine begin to misfire.
Before the head rework: Crank the car, push throttle and you can stay at 2500 RPM for minutes
and engine could run regularly.

Unfortunately, we didn't repeat this kind of test after head rework but be patient, car will return,
and guess what DTC it will have.