Tire pressure, octane level and injectors aside..
What are the key components that affect the fuel economy or fuel consumption of the W124.
Starting with the older KE-Jetronic system present on the pre-1991 W124s.
The KE-Jetronic is an electro-mechanical fuel injection without any form of closed loop feedback [ as compared to newer EFI systems with O2 sensors as feedback control ].
The fuel delivery of the KE system is largely dependent on the fuel pump, fuel injectors, fuel pressure regulator, fuel distributor, the EHA valve and the air-flow flap that detects the amount of air flowing into the air intake.
Firstly, the fuel pump provides a constant 6 bar of pressure for fuel delivery.
A symptom of a failing fuel pump may be diagnosed by an audible whirring during a hot weather or when the engine has been running for more than 1 hour.
Next, the Fuel Pressure Regulator [FPR] is a control valve that opens or close the fuel return line in order to provide the fuel pressure at 3.5 bar as when this pressure is reached, the mechanical injection valves of the fuel injectors will begin to open and close rapidly, spraying the fuel into the combustion chamber.
The control of the opening and closing of the fuel pressure regulator is by means of a vacuum hose which moves the inner diaphragm to open and close and the variance of the opening and closing is fully dependent on the engine vacuum. [ See this post for the picture of the vacuum hose on the FPR ].
The fuel is also routed through the fuel distributor that distributes the incoming fuel line from the FPR to 4 outgoing pipes evenly to the 4 injectors [ for 4 cylinder W124s ]. The fuel distributor has an internal metering system whereby a spring control plunger opens and closes the metering slits that distributes the fuel to the individual pipes. The spring control plunger in turn is actuated by the air flow flap as seen in this diagram here.
|Air flow flap affecting fuel distributor plunger|
The air flow flap is also directly affecting another fuel intake valve [ aka EHA valve ].
The EHA valve is a electro-solenoid that increments the opening of the main fuel line that enters the fuel distributor. It is mounted at the rear of the fuel distributor as shown below.
|EHA Valve mounted at rear of the FD|
The EHA valve takes its input from the potentiometer that is located in the front of the air flow metering unit. This potentiometer is merely a variable resistor that measures the sweeping distance of the air flow flap. By how much the air flow flap moves down from its closed position, the potentiometer is able to detect this distance and send this to the EHA valve which will then open up to a certain degree based on the input it receives.
|Air flow potentiometer|
Thus, by the air flow potentiometer affecting the EHA valve, fuel distributor plunger and the fuel pressure regulator being controlled by the engine vacuum, the KE Jetronic fuel delivery system is somewhat rudimentary but yet capable of providing a reasonably capable system of delivery the "almost" correct level of fuel required.
However, the tolerances of its upper and lower limits of fuel required may be very large and though it was sufficient back in the days when fuel were cheaper, it may not be up to the expectation of the budget conscious W124 owners of today.
And as age catches up on the old components, the variance become larger and this affects fuel consumption almost definitely.
To spruce up the system, we recommend replacing a few components.
1) The Air Flow Potentiometer
As this component depends on the resistance of carbon tracks, they may be worn and providing inaccurate resistance feeding the input of the EHA valve.
2) The Fuel Pressure Regulator
The FPR works correctly if its diaphragm is still soft and supple, but with rubber parts, they tend to harden and crack over time. With a faulty diaphragm, it may not be opening or closing as expected.
See here on how to test if the FPR is working by means of testing the idle microswitch.
3) Spring within the air flow metering unit [ This could be the most difficult job ]
The spring provides the tension of the air flow flap to return to the topmost position.
As the air flow flap returns to the topmost position, this is the position whereby the system detects that there is almost zero air coming in [ ie throttle fully closed ]. It should return to the top as fast as possible in respond of the closed throttle. If the spring is not as taut as it used to be, it might take a longer time to move all the way to the top or it might even not fully retract causing a somewhat almost closed position where it'd cause the KE-Jetronic system to think that there is still some open throttle and fuel will be catered for this situation.
Last but not least, the KE-Jetronic system requires the regular manual tuning of idling CO%/AFR by means of an external lambda(O2) sensor in order to ensure the system is calibrated.
For the post 91 makes of W124 [ eg 16v E200 or any other cars with catalytic converters for that matter ], as there is a closed loop feedback of the fuel delivery system, there are only 2 critical components to replace that will directly affect the air fuel ratio or fuel consumption of the engine.
1) O2 exhaust sensors
The O2 sensors along the exhaust will tell the ECU how well the fuel is being burnt and the ECU will react by injecting more or less fuel to achieve stoichometric ratio of 14.7 for perfect combustion and maximum power.
|Common O2 Exhaust Sensor|
2) Mass Air Pressure(MAP) or Mass Air Flow(MAF) sensors
Modern engine's ECU decides the amount of fuel to inject by detecting the amount of air entering the combustion chambers, the MAP and/or MAF sensors will let the engine know the amount of air flowing in. Depending on your ECU design and algorithm, it may have either the MAP or MAF or combination of both MAP and MAF for better accuracy of measuring the air entering the combustion chambers.
|Typical Mass Air Flow (MAF) sensor|
|Typical Mass Air Pressure (MAP) sensor|
For any cars of more than 3 years of age, a replacement of the 2 key components above will almost definitely improve the fuel consumption significantly.
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