Mercedes-Benz W124 K-Jetronic: hard starting and fuel pressure
Why does a K-Jetronic W124 start poorly after standing?
The cause is usually fuel pressure leaking away while the car stands. The injector nipples can loosen, the cold start injector usually the most, or the inside of the accumulator can be rusty. If the pressure drops too quickly after shutdown, the fault may be in the pressure regulator, the injectors or the check valve.
How is fuel pressure measured and what is the correct pressure?
The reference value is about 5–5.5 bar; 8 bar is too high. The gauge is connected to the fuel hose, and the pump can be made to run by bridging the fuel relay with a wire. With too low a pressure, check the filter and lines first, then the pressure regulator. With too high a pressure, check the return line. Bridge the terminals only for the test, remove the wire immediately afterwards and watch out for fuel leaks and sparks.
How are the fuel pump and its relay checked?
The pump should buzz every time the ignition is switched on. If no sound is heard, the fault is in the pump or its relay; the relay is near the battery next to the overvoltage protection relay. The relay's solder joints can be cracked, and the relay can also stick closed, in which case the pump runs continuously.
How are the K-Jetronic injectors checked?
A good injector sprays an even mist and does not drip when the throttle is closed and the system is under pressure. The difference in delivery between injectors may be at most about 10%, otherwise the injectors are replaced. The opening pressure of a used M103 injector must be at least 3.2 bar.