Mercedes-Benz W202 ignition coils and a dead cylinder
My W202 runs on fewer cylinders. Is the fault in the ignition coil?
Often it is. You can check a coil by swapping the coils with each other: if the fault moves with the coil, the coil is broken. A multimeter can show a good reading even though the coil does not work at operating voltage, so a practical swap test is more reliable.
What other possible causes are there besides the coil?
The spark plug leads can be broken inside or perished even though they look intact from the outside. Also check the spark plugs, the suppression resistor between the coil and the plug (about 2 kOhm) and the insulating rubbers. Sometimes replacing the coils alone is not enough.
There is no spark on cylinders 1 and 4 even after swapping the coils. What is the cause?
The cause can be the ignition control unit, a black box under the washer fluid reservoir on the left fender. It has to be replaced, repaired or tested. Moisture can get into it along the vacuum hose and cause a fault in winter.
The fault only appears when the engine is warm. How do I find it?
A diagnostic tool may not show it, so trying a working coil is a practical way. Fault codes can be read, but a spark plug or ignition fault does not always leave anything in memory. The mass air flow sensor (MAF) can also cause misfiring.