Mercedes-Benz Fuel Injection: K & KE-Jetronic

Mercedes-Benz vehicles boast a rich heritage of innovation, and their fuel injection systems are no exception. While early models experimented with unique direct injection methods, akin to diesel technology, the systems you're most likely to encounter in a classic Mercedes-Benz workshop today are the Bosch K-Jetronic and its evolution, the KE-Jetronic. Introduced in the 1970s and continuing well into the 1990s, these systems marked a significant leap from carburettors, offering enhanced precision and efficiency. Understanding their fundamental operation is paramount for any mechanic or enthusiast aiming to diagnose and maintain these iconic cars effectively. This article will unravel the intricacies of both systems, providing a comprehensive overview of their components and functional principles.

K-Jetronic: The Continuous Flow Innovator

The defining characteristic of K-Jetronic systems, a unique trait among widely distributed fuel injection systems, is their continuous fuel spray. As long as the engine is running, fuel is constantly sprayed through the nozzles. The injectors themselves are entirely mechanical, opening when fuel pressure reaches approximately 3.8 bar (though this can gradually decline to 3.0 bar with use). They deliver a volume of fuel that mechanically corresponds to the volume of intake air. There are no electrical components or connections to these injectors; nothing switches them on or off, nor does anything pulse or cycle them beyond the hydraulic force of the petrol. High-speed photography has occasionally revealed the nozzles 'chattering' at around 1500 Hz, a phenomenon solely attributable to the resonances of the needle, spring, and the fuel's viscosity and delivery speed. Certain K-Jet injectors were meticulously engineered to allow the needle to 'walk' around the opening, further enhancing fuel atomisation and vaporisation. To shield the injectors from the engine's heat and vibration, they are not threaded into the manifold but simply pressed in by hand, secured by non-conductive rubber collars. The hex head merely assists in steadying the injector during fuel line connection or disconnection. Many injectors also incorporate an air shroud around the tip, which, particularly at idle speeds, insulates the fuel spray cone from the manifold sides, preventing condensation. This focused, high-speed airstream also aids in vaporising the fuel even when the engine is turning slowly with no load.

The Fuel Pump: Heart of the System

Our journey through the K-Jet system begins at the fuel pump. In the earliest and simplest K-Jet configurations, this was the sole electrical component, controlled by the fuel pump relay on the firewall. Early systems were devoid of sensors, computers, or actuators. Conceptually, the fuel pump could have been mechanical, driven by a crankshaft pulley. However, for practical reasons like minimising vapour lock and managing heat, the pump is ideally located away from the engine, often near or inside the fuel tank. The basic K-Jet fuel pump has since become a de facto standard for almost all automotive fuel injection systems.

It comprises a small yet powerful electric motor with a positive displacement hydraulic pump on one end of the shaft. Fuel continuously flows through the pump as it operates, effectively cooling the armature windings. While the carbon brushes and copper commutator are submerged in petrol, the absence of air or oxygen means there's no risk of ignition. Sparking from an electric motor's carbon brushes could ignite fuel vapour in air, but not when submerged in liquid petrol.

Early versions of these pumps were simple 'on or off' devices. Any fuel not injected into the intake runners was recirculated back to the tank, helping to keep the under-bonnet fuel plumbing relatively cool. Later iterations introduced more sophisticated features, such as an additional, lower-current pump delivery rate for quieter low-speed driving, vehicle-speed limits controlled electronically, and even multiple pumps in series (one for pressure, the other for volume).

The initial pumps operated using 'roller cells', where cylindrical bearing rollers were held in channels around an eccentric drivewheel. As the rollers moved, they drew fuel from the tank and propelled it towards the engine by displacement. These were later superseded by 'Gerotor' pumps, internal/external gearsets that function much like engine or transmission oil pumps. Other designs include cycling-vane and peripheral-pump types, and on current Mercedes-Benz models, counter-rotating helical spindles, which force fuel through with a geometry similar to a scroll-supercharger. In every design, the objective remains the same: to move fuel and build pressure quickly and reliably. Many pump design changes have focused on reducing noise. Due to the highly effective cooling provided by the fuel, fuel pump electric motors can be surprisingly compact for their power output. Most K-Jet pumps draw between 6 and 10 amps. A lower current could indicate resistance in the circuit, while a higher current might suggest an internal short or a fluid blockage forcing the pump to work harder. A healthy pump for a six-cylinder engine typically delivers a litre of fuel in about 40 seconds, with larger displacement engines requiring more fuel.

The pump for any given model is designed to deliver more fuel than the engine could consume even under the most extreme fuel-demand conditions, such as wide-open throttle in freezing temperatures. This excess pump capacity ensures full engine function and consistent fuel pressure. The only factors likely to prevent a mechanically and electrically sound K-Jet pump from delivering sufficient fuel are a completely clogged fuel filter (a rare occurrence) or a crimped or blocked fuel line. These issues commonly result in a noisy pump, as it has to exert more effort, and you'll often hear the pressure pulses. If rust develops in the system after the fuel filter (perhaps due to moisture in the petrol), clogs can also form in the small internal filters within the fuel distributor and the injectors themselves.

Fuel Filter and Pressure Accumulator

Within the fuel outlet of each pump is a fluid check valve. Its purpose is to retain fuel within the injection lines, preventing vapour lock and ensuring the plumbing remains full of pressurised liquid fuel for the subsequent engine start. This check valve merely seals the fuel in the system; the pressure itself is maintained by the pressure accumulator. Prolonged cranking times are often the first symptom of a check valve or pressure accumulator failure. For diagnostic purposes, a 'long cranking time' is generally considered more than four seconds before the engine fires.

The fuel then proceeds directly from the pump to the fuel filter. While this is the standard layout today, it's worth noting that in the era of carburettors, fuel filters were frequently placed between the tank and the pump. Such an arrangement would often lead to vapour lock with modern fuels in a fuel-injected vehicle (and it often caused vapour lock even with older fuels in carburetted vehicles!). Placing the fuel pump ahead of the filter, however, means the cleanliness of the fuel from the filling station is more critical than ever, as any grit or rust particles pass through the fuel pump largely unfiltered, save for a coarse-mesh screen just ahead of the pump inlet, primarily designed to block particles large enough to jam the pump.

Historically, replacing a fuel filter was standard practice with every 'tune-up.' However, 'tune-ups' as they were known largely ceased with the disappearance of ignition contact points in the 1970s. Modern fuel filters feature a large surface area of super-fine, accordion-fold filter paper, capable of removing particles down to single-digit micron sizes and absorbing or neutralising most gums and tars. A filter actually becomes more effective after some use, as it accumulates a patina of dirt on its active surface. Unless there's a clear, demonstrable need to replace a fuel filter – such as a healthy fuel pump failing to deliver the volume and pressure specified by the carmaker – replacing an existing filter is often unnecessary.

Following the fuel pump and filter in the hydraulic circuit is the accumulator. These three components are often arranged in a row in early systems. The accumulator consists of a container with a diaphragm separating the fuel and air chambers, and a spring to maintain pressure from the air side of the diaphragm. Its function is similar to accumulators in automatic transmissions, absorbing and storing fuel pressure to smooth out pressure peaks from the pump (reducing noise and ensuring smooth fuel flow) and to retain pressure in the system when the pump switches off. After engine shutdown, the system pressure should ideally remain at least 2.7 bar after 10 minutes and 2.6 bar after twenty. Not all K-Jet systems require fuel pressure accumulators, as some vehicles have sufficient volume in the lines and components to achieve this.

As mentioned, K-Jet injectors release fuel whenever the pressure exceeds approximately 3.8 bar (varying slightly by model and year). This maintains sufficient residual pressure in the system to keep it primed for a quick restart and to prevent vapour lock across most ambient or under-bonnet temperatures. If a K-Jet system fails to hold pressure after shutdown, especially if it emits an audible 'electric-motor' noise corresponding to pump speed, inspect the pressure accumulator for a broken spring or ruptured diaphragm. A direct pressure check at the fuel distributor inlet is the primary diagnostic procedure. Also, inspect the pump's check valve by confirming the system holds pressure after engine shutdown. If these components are sound, look for a dribbling injector.

The Fuel Distributor: Precision Delivery

As petrol continues through the line, it encounters one of the most intriguing devices in automotive history: the K-Jet fuel distributor. The name might suggest it functions like an ignition distributor, sorting high-voltage sparks to the correct plug in firing order. However, its operation is different: all cylinders receive fuel continuously and simultaneously. The 'distribution' in question refers to delivering precisely the same quantity of fuel to each cylinder. A K-Jet bank of injectors can maintain cylinder-to-cylinder mixture variation at six percent or lower, a significant improvement over carburettors which typically varied by 40 to 60 percent.

The fuel distributor features a separate delivery tube for each cylinder's injector. Inside, each fuel line terminates in a differential-pressure regulator, metering fuel into its line based on the fuel pressure and the deflection of the spring-steel diaphragm beneath it. While all pressure regulators share the same hydraulic force in their conjoined lower chambers, they have separate upper chambers, each filled through its own laser-cut metering slot. They deliver a precisely equal quantity of fuel to each cylinder due to the exceptionally precise machining of the components, with the control plunger and its sleeve, a matched set, machined to an accuracy of 0.00025 inches.

This level of precision is critically dependent on factory assembly conditions and controls, making it highly unlikely to successfully reassemble a fuel distributor on a workbench and have it function correctly – or even simply not leak. While it's interesting to open a defective one to see its internal workings, don't count on reusing it. If you find one internally clogged, acquire a replacement and return the faulty one for remanufacture. You can remove the inlet filter (if present) or the control plunger and its surrounding filter, along with the metering slots, for cleaning, but these are not standard maintenance procedures. If particles are found in these filters, replace the main fuel filter, as it indicates grit is passing through.

Each fuel injection line ends in the centre of a small, fuel-filled chamber with a steel diaphragm at the bottom. Below this diaphragm is another chamber of the same dimensions but without an injection line. When the pressure above the diaphragm is higher than below, the force slightly bends the diaphragm downwards, driving fuel through the fuel line and (assuming it exceeds the opening-pressure threshold) out the fuel injector tip. When the pressure difference between the two matching chambers is lower, the diaphragm moves back up, reducing the amount of fuel injected. If the pressure in the lower chambers is greater, the steel diaphragm rises, completely shutting off fuel flow. The pressure of the fuel and the vacuum in the intake manifold also influence the fuel volume. It's important to remember that the diaphragm position is determined by the pressure differential between the chambers, not the pressure itself. However, for a given diaphragm position or injection-line opening, the upper chamber pressure (combined with the intake manifold vacuum) dictates the momentary volume of fuel flow.

The Airflow Sensor: Mechanical Air Measurement

The K-Jet airflow sensor is distinct from any other intake-air measurement device, even vaguely resembling the swinging-door sensor on Bosch L-Jet systems. The fuel distributor and the airflow sensor are integrated into a single assembly, jointly known as the mixture-control unit. The K-Jet airflow sensor physically and mechanically determines the fuel flow by altering the pressure in the upper chambers of the fuel distributor.

The airflow side of this unit consists of a finely machined air cone with an air flap positioned centrally. This air flap is located in the intake airflow between the air filter and the throttle plate. It moves in response to air flowing past it, drawn into the engine when the throttle opens. The movement of the air plate directly corresponds to the volume of air passing it, thanks to the very precise tapering of the air funnel. The open cross-sectional area around the cone accurately reflects the air volume, ensuring the mixture control unit meters a larger quantity of fuel for a correspondingly larger amount of air, and vice-versa. As the air plate and lever move from their rest position, the lever moves the control plunger to uncover more of the fuel control slots, increasing pressure in the fuel distributor and thus the amount of fuel delivered.

There is rarely a need to loosen or remove the K-Jet airflow plate from its lever, as its precise centering is critical; if it touches, it can snag and stick in the funnel bore. If access is required to any part of the airflow sensor below the plate, the sensor must be removed from the intake tract anyway. Without a special tool, a good method to re-centre the plate after removal is to fit paper around its perimeter while tightening the positioning bolt. Using paper of uniform, correct thickness will create a uniform, correct gap around the sensor plate. In the absence of such perfect paper, toilet paper can be used; it will be too thin alone, but enough layers can be built up to achieve the correct gap. Mercedes-Benz K-Jet airflow plates typically feature curved or shaped top surfaces, a design not common to all Bosch K-Jet systems. This curved surface aerodynamically allows greater airflow compared to a flat sensor plate of the same diameter. Each airflow sensor plate also has a stop bar above it, combined with a rubber bump-stop. These elements allow the lever and plate to bounce above the funnel's centre and release backpressure should there be a backfire through the intake manifold, usually preventing damage to the airflow plate and lever. The funnel is tapered above, partly to orient the incoming air around the plate and partly to provide pressure relief in case of a backfire.

While Bosch K-Jet systems on many cars draw intake air upwards through the airflow sensor and through a tube to the throttle body, on most Mercedes-Benz vehicles, the air flows downwards through the air funnel and into the engine. Among other advantages, this eliminates 'false air' entering through a cracked, porous, or loose air hose. Functionally, the system operates identically, with only a change in the geometry of the air sensor lever and fulcrum. Although the air flap presents an obstruction to the intake airflow, its aerodynamic resistance is quite low, generating no significant vacuum between it and the throttle plate. All intake air, except for what comes through the PCV (Positive Crankcase Ventilation) and EGR (Exhaust Gas Recirculation) systems, passes the airflow sensor. Idle air, in particular, transits the sensor.

At engine shut-off, with no air passing through the sensor, spring tension and the weight on the arm bring the lever to a position that moves the fuel distributor's control plunger to shut off all fuel flow. While early and late fuel distributors have different internal plumbing, most have fuel system pressure above the control plunger, meaning the return force of the airflow arm must be sufficient to overcome this. Depending on the application, this return force originates either from a spring or a counterweight on the opposite end of the lever.

As emissions requirements became stricter, a later version of the K-Jet system, sometimes referred to as K-Lambda, emerged. This system incorporated an oxygen sensor for mixture feedback and a separate frequency valve on the fuel return circuit. By duty-cycling the return flow, the system could fine-tune the intake mixture by precisely adjusting the fuel pressure within the fuel distributor. However, to achieve the level of combustion control necessary for the next set of emissions standards, the system had to evolve further into the KE-Jet.

KE-Jetronic: Embracing Electronics

The primary distinctions between the basic K-Jet injection system and the KE-Jet are the Electrohydraulic Actuator (EHA) and the associated subsystems and components that enable its operation. Due to a pressure regulator within the fuel distributor, the pressure difference between the upper and lower chambers remains constant at approximately 0.4 bar, similar to the earlier K-Jet. As the airflow lever moves, the control plunger directs fuel into the fuel distributor. The EHA's role is to modify the lower chamber pressure in response to the oxygen sensor feedback signal, independently of the rest of the fuel distributor. Modifying the lower chamber pressure alters the pressure differential between the chambers, changes the position of the spring-steel diaphragm, and consequently adjusts the volume of fuel delivered for the amount of air passing the airflow sensor.

The EHA contains a variable electromagnet that moves a metering plate, which covers or uncovers a fuel inlet to the lower chambers. This electromagnet, like all others, is a current-responsive element, operating in direct response to amps – or, in this case, milliamps – rather than voltage. The windings on most KE-Jet EHAs should measure about 19.5 ohms at room temperature.

The KE-Jet system employs a control unit to manage intake mixtures and maintain combustion as close to stoichiometry as possible (a feature also present in the final K-Jet versions). The continuous feedback information for mixture readjustment comes from the oxygen sensor, similar to most other vehicles. What sets the KE-Jet apart is how the mixture command operates. In essence, it partially controls the fuel flow to the lower chambers, thereby regulating the pressure differential. The EHA functions as a type of electrical solenoid, but instead of abruptly moving an element to one extreme, like a starter solenoid, the EHA applies a measured electromagnetic force to the metering plate, subtly bending it in one direction or the other. By varying not only the current but also its polarity, the computer can adjust the mixture from full rich to complete fuel shut-off (for example, during extended deceleration). It's crucial to remember that electromagnetism here, as in all solenoids, functions purely by current, by amperage. Voltage is irrelevant, except that polarity cycles back and forth during closed-loop driving, and if polarity changes, voltage does too.

To diagnose an EHA, first check for continuity as described above, then for dynamic function. When the engine first starts cold, a steady current of several milliamps will be sent to enrich the mixture. Once the system enters closed loop (typically within two minutes of startup), the current should be seen cycling, reflecting the oxygen sensor's signal. Many testers display this as an on-off signal, but it is actually current in milliamps reversing polarity. If the EHA current cycles but not within the expected milliamp range, first check for air leaks. If these are eliminated, the 'tamper-proof' cap can be opened to access the mixture adjustment screw. This should not be a regular service; if the engine has not been disassembled or the fuel distributor replaced, prioritise checking for a vacuum leak. When using a sensitive amp-clamp to test the EHA, ensure the two wires are separated, as catching both in the clamp will result in cancelling currents and an inaccurate measurement.

KE-Jet ECU Parameters and Operation

The KE-Jetronic ECU considers several parameters when calculating the EHA control current. These include:

• Cranking signal: An electrical signal parallel to the starter solenoid current.
• Engine temperature: A reference signal dropped to a voltage corresponding to the engine coolant's temperature.
• Intake air temperature: From a sensor in the air intake duct.
• Engine speed (TD or crankshaft position signal): From an inductive signal generator at the flywheel.
• Engine load: From a MAP (manifold absolute pressure) sensor.
• Airflow sensor position signal and movement: From a sensor at the end of the airflow sensor lever pivot.
• Oxygen sensor or Lambda signal: From a conventional heated oxygen sensor in the exhaust stream.

During warm-up, the computer sends a command current that slightly restricts fuel flow by moving the EHA valve plate. This causes pressure to drop in the lower chambers, and the pressure differential bends the metal diaphragm downwards, allowing relatively more fuel to flow. Once the engine reaches normal operating temperature and operates in closed loop, the ECU sends commands of varying polarity and current. This causes the valve plate to cycle back and forth across its travel, enriching or leaning the intake mixture as required by the oxygen sensor signal. A scope, set to simultaneously monitor the EHA current and the oxygen sensor signal, graphically illustrates this interdependence.

When the computer's sensors indicate deceleration (closed throttle, high engine RPM, strong intake manifold vacuum, or low MAP), the ECU sends current in a polarity that pulls the metering valve plate all the way back, allowing pressure to equalise between each of the chambers. The additional pressure from the small spring in the lower chamber is then sufficient to stop fuel flow.

The system possesses an inherent and transparent limp-home mode. Should one or more circuit failures prevent the computer from sending a signal to the EHA, spring tension on the valve plate brings it to a centre position. The engine will then continue to run as if it were an ordinary K-Jet system, rather than a feedback-modulated KE-Jet. On most models, this condition will illuminate the Check Engine Light (CEL) and, if the car has self-diagnostics, set a fault code for the failed circuit. Because the KE-Jet uses an electrically triggered cold-start injector spraying into the intake manifold log, the engine can typically still start when cold.

The computer control system on KE-Jet is almost exclusively dedicated to fuel mixture; its primary purpose is to calculate and send the correct current command to the EHA. While it receives input signals from the crankshaft position sensor and, in some models, from the speedometer, and some models include a transmission upshift delay function (to accelerate engine warm-up), it is not yet a complete engine management system that coordinates fuel, spark, A/C compressor engagement, cruise control, and other functions into one integrated unit (that development came with the subsequent Motronic system). Control of the ignition system remains with the EZL/AKR control unit, with only minor interconnections with the fuel system. KE-Jet is purely a fuel injection system. Nonetheless, it is crucial never to connect or disconnect the control unit with the ignition key on, as this can result in damaging voltage spikes.

Mixture Control: Rich and Lean Scenarios

Fuel enrichment is crucial during cranking and the initial second or two of engine operation. This enrichment can continue for up to two minutes, depending on the coolant temperature. It also occurs during acceleration to compensate for the fact that air accelerates into the intake manifold more quickly than fuel. Finally, at high speeds and high loads, at or near wide-open throttle (WOT), the mixture also goes rich. In this latter mode, there is no mixture feedback function and no EHA control current; the engine operates in 'open loop'.

The mixture goes lean not only during deceleration fuel shut-off (enabled by a throttle-closed microswitch) but, on many models, to prevent the engine from over-revving into the redline territory. It's also the initial strategy for the vehicle's maximum speed limit, followed by fuel pump shut-off. Since the vehicle maximum speed is a voluntary limit agreed upon by Mercedes-Benz and several other German carmakers for vehicles designed for the unlimited-speed German autobahns, this limit can be as high as 250 km/h (approximately 155 mph). On British roads, this should rarely inconvenience a driver. Deceleration fuel shut-off is disabled whenever the car is under cruise control.

On Mercedes-Benz cars equipped with KE-Jet, you might find one of three different coolant temperature sensor configurations: an internally grounded one-pin version connected solely to the KE control unit; a similar two-pin unit with pins for both the KE and ignition control units (or one for KE and one for the fuel pump relay); or a four-pin version with two pins for KE and two for the ignition system. All these sensors utilise NTC (Negative Temperature Coefficient) resistors, meaning their resistance decreases as the temperature rises. The four-pin versions are designed to be 'idiot-proof,' functioning identically regardless of their clock position when plugged in. Each second pin on the four-pin versions helps to mitigate any issues of excessive resistance on the ground-return circuit. For each sensor type, it's essential to check the continuity of the harness back to the computer, as well as the variable resistance through the sensor itself.

While there isn't a traditional throttle position sensor, there is a throttle switch with two outputs, indicating either the idle position, the WOT position, or (if neither of these) any position in between. Due to the ingenious design of the KE-Jet airflow sensor and fuel distributor, a finely discriminating throttle-position potentiometer is simply not required. These simple on-off switches should be tested for continuity in one position and an open circuit in the other.

Why Carburettors Faded Out

Carburettors were integral components of automobiles for almost as long as tyres, and like many other automotive technologies, they became increasingly complex as demands for fuel economy, power, and emissions friendliness grew. Even on the most intricate carburettors, the underlying vacuum and evaporation technology was well understood, and most vehicles performed adequately with them, despite overhaul and readjustment procedures becoming progressively more difficult.

The insurmountable challenges for carburettors lay in two critical areas: mixture control and fuel distribution problem. The ideal fuel/air ratio for an engine fluctuates considerably with changes in load, speed, and temperature. It also varies with the 'recent history' of the throttle; for instance, a richer mixture is needed at a given throttle position if the driver is rapidly opening it, whereas a much leaner mixture is required at corresponding throttle positions as the throttle rapidly closes. Most carburettors employed an acceleration pump or similar mechanism to briefly enrich the intake mixture when the driver pressed the pedal. However, the enrichment provided by these pumps was imprecise and prone to changing over time due to wear.

While carburettors could be adapted to vary the mixture in response to oxygen sensor feedback signals, this came at the cost of increased complexity and vulnerability to dirt or contaminants. Carmakers used duty-cycled solenoids to modulate either the fuel in the main circuits or the air in the emulsion tubes. Nevertheless, a feedback carburettor could not adjust the mixture rapidly enough to accurately respond to quick throttle openings or closings with the required precision. Furthermore, since the carburettor sits 'on top' of the intake manifold, there was no way to prevent the freshly atomised and vaporised fuel from condensing back into liquid on the manifold walls when pressure and temperature conditions favoured it. Even the most sophisticated mixture-adjusting carburettor could only maintain the overall mixture within the desired range at steady speeds, failing to cope with the rapid changes encountered in typical driving traffic.

Even if the mixture problem for carburettors had been perfectly resolved, the fuel distribution problem persisted. This issue is particularly pronounced in inline engines with single carburettors and 'log' manifolds. Imagine a six-cylinder engine with a single-barrel carburettor located at the midpoint of an intake manifold running parallel to the camshaft. Because heavier fuel droplets resist turning when the airflow changes direction, most tend to accumulate at the extreme ends of the manifold. Only the smallest droplets and properly vaporised fuel navigate the turns easily. Consequently, in such engines, cylinders one and six would typically receive the richest mixtures, while cylinders three and four would receive the leanest.

This is more than a minor or theoretical issue. According to Bosch experiments, a typical carburetted four-cylinder engine could exhibit mixture variations of approximately 60 percent. Clearly, to ensure reliable firing, the cylinders receiving the leanest mixture must be supplied with sufficient fuel. This inevitably means that the richest cylinders will always run excessively rich. This condition leads to increased wear in those rich cylinders due to cylinder wall washdown from the excess fuel. It's common in inline engines to observe greater wear in the first and last cylinders, although other factors, such as variations in coolant temperature further from or closer to the water pump, also contribute. The richer mixture naturally results in higher hydrocarbon emissions in the exhaust. Multipoint fuel injection systems like the K-Jets significantly reduced both mixture and distribution problems by a factor of about 90 percent, paving the way for cleaner, more efficient, and more reliable engine performance.

Comparative Overview: K-Jetronic vs. KE-Jetronic

To summarise the key differences and advancements between these two pivotal Mercedes-Benz fuel injection systems, refer to the table below:

Frequently Asked Questions (FAQs)

Q: What does K-Jetronic stand for?

A: 'K' stands for 'Kontinuierlich,' the German word for continuous, referring to the system's continuous fuel spray. 'Jetronic' is Bosch's proprietary name for their fuel injection systems.

Q: Are K-Jetronic injectors electrical?

A: No, K-Jetronic injectors are entirely mechanical. They open and close based purely on fuel pressure and are not electrically actuated or pulsed.

Q: What are common symptoms of a failing K-Jet fuel pump?

A: Common symptoms include a noisy pump (working harder), difficulty starting (especially after sitting), reduced engine power, and inconsistent fuel delivery. A current draw test can also indicate issues.

Q: Why is the fuel filter placed after the pump in K-Jet systems?

A: Placing the fuel filter after the pump helps prevent vapour lock, especially with modern fuels. The pump needs to be primed with liquid fuel, and a filter before it could impede this, particularly if partially clogged or if fuel vapour forms.

Q: Can I repair a K-Jet fuel distributor myself?

A: Due to the extreme precision required in its manufacturing (components machined to 0.00025 inches), it is generally not recommended to attempt to repair or disassemble a K-Jet fuel distributor on a workbench. If it's faulty, replacement or professional remanufacture is the advised course of action.

Q: What is the main difference between K-Jet and KE-Jet?

A: The main difference is the addition of electronic control in KE-Jetronic via the Electrohydraulic Actuator (EHA) and an ECU. This allows for closed-loop mixture control using an oxygen sensor, making KE-Jet far more precise and adaptable to emissions regulations than the purely mechanical K-Jet.

Q: What is the EHA and what does it do?

A: The EHA (Electrohydraulic Actuator) is a crucial component in KE-Jetronic systems. It's a current-responsive electromagnet that subtly varies fuel pressure in the lower chambers of the fuel distributor. This adjustment, based on feedback from the oxygen sensor, allows the ECU to precisely control the fuel mixture to maintain stoichiometry.

Q: Does KE-Jetronic have a 'limp-home' mode?

A: Yes, KE-Jetronic features an inherent 'limp-home' mode. If the ECU or EHA circuit fails, the EHA valve plate returns to a central position due to spring tension, allowing the engine to continue running, albeit without the electronic mixture control, similar to a basic K-Jet system.

Q: Why did Mercedes-Benz switch from carburetors to fuel injection?

A: Mercedes-Benz, along with other manufacturers, transitioned from carburettors to fuel injection primarily to overcome the inherent limitations of carburettors regarding precise mixture control and even fuel distribution across cylinders. Fuel injection systems, especially multipoint designs like K-Jet, offered significantly better fuel economy, increased power, and drastically reduced harmful exhaust emissions, which became critical with tightening environmental regulations.

Understanding the K-Jetronic and KE-Jetronic systems is more than just learning about old technology; it's about appreciating the ingenuity that paved the way for modern engine management. These systems, while complex, are remarkably robust and, with proper care and diagnostic understanding, can continue to deliver reliable performance in classic Mercedes-Benz vehicles for many years to come. Their legacy underscores Mercedes-Benz's commitment to engineering excellence and continuous improvement in automotive technology.

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