A mileage over 250,000 km, configure the M120 engine and automatic transmission, chassis model W140 Mercedes 600SEL sedan. The car engine idle speed is normal, but accelerated to 1500 ~ 3000r/min the exhaust black smoke.
M120 engine is V12 engine, have two sets of ignition components. Confirm the failure symptoms, when the engine speed from 1500r/min increased to 3000r/min, the vehicle acceleration weakness, hard acceleration failure more apparent. But the engine speed exceeds 3000r/min or more, accelerate performance back to normal. Connection mb star c3 to self-diagnosis, no fault code. Measure the fuel pressure, the idle condition of the fuel pressure is 330kPa. Unplug the fuel pressure regulator vacuum tube, the fuel pressure becomes 390kpa. That means the fuel pressure is normal. Measure two ignition coil resistance are normal. Measure high-voltage resistors, are all within the standard values. Cleaning fuel injectors, replace fuel filter and spark plug, testing, fault still.
Check the valve timing, no problem. Start the engine, in idle condition unplug the camshaft adjust solenoid valve harness connector, the engine operating conditions have changes, means variable timing camshaft adjustment is normal. Measure all cylinders pressure are within the standard value. Check the exhaust condition, the left exhaust uniformly, the right exhaust have intermittent pause phenomenon, and the right exhaust pipe temperature is significantly higher than the left, means the right column of the cylinder working poor.
To do disconnect cylinder test, keep the engine speed in 2000r/min, Unplug the high-voltage of the right column cylinder one by one, each plug one, the engine speed decrease is not obvious. But when check the left column cylinder, each plug one high-voltage, the engine speed decrease are obviously. In order to verify whether the fault is related with the ignition energy, respectively unplug one high-voltage of right column and left column cylinder, plugged spark plug, observe flash over situation.
The result is when idling condition two spark plug flash over situation as same. When the engine speed reaches 1500r/min, the right column cylinder spark plug flash over energy began to weaken; when the engine speed to 3000r/min, the right column cylinder spark plug ignition back to normal. Replace the right column cylinder ignition control module, the fault completely ruled out.
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