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Bosch K-Jetronic – Part 1, The Basics

Bosch K-Jetronic seems to be a bit of a black art these days. Almost anyone you mention it to backs off and cringes. Hopefully this series of posts clears up some of that misinformation and scaremongering, and opens more people up to working with their KJet instead of binning it, or neglecting it until it inevitably fails.

First things first, what is K-Jetronic?

In simple terms, its a form of mechanical fuel injection.

In more elaborate terms, KJet is a Continuous Injection System (commonly called CIS), where unlike later L-Jetronic or “standard” Electronic Fuel Injection, the injectors do not pulse open and closed; once the system pressure is high enough they all inject a constant spray of fuel, in all cylinders at once, which is varied in volume by the system depending on various factors.

Kjet was used from the 70s, into the mid 90s, and was primarily used on European cars. Mercedes, VW, Audi, Porsche and Ford were the more prolific brands to use it, and when it worked, it worked well. It was used on many different engine configurations, displacements, and even on Forced Induction engines; it’s quite flexible.

The system consists of the following components,

And the basic Schematic works like this

Easy? Yup, cool. Air and fuel go in, spark makes it go bang. Engine goes vroom.

The basics of the system are the easy part, it’s the actual settings and internals of the system that is complicated, but even then, once you understand it, it’s really quite simple.

What do those things do?

Fuel Pump

Pushes fuel from the fuel tank, into the injection system. The pump must be capable of producing more than 6BAR as the system pressure of most KJet systems is approx 5 to 5.5BAR and if the pump can’t keep up, the system will starve. The pump should be producing more fuel than the engine needs.

Fuel Accumulator

The accumulator has two basic functions. First, it’s used to dampen out pulsation in the fuel from the fuel pump. This is apparently to ‘deaden” noise from the pump, but to what end, I’m not sure. The other, more important function, is to hold pressure in the fuel lines when the engine is shut off, to assist with hot starting and reduce the possibility of vapour lock. Inside this unit is basically a large diaphragm, which pushes against a spring.

Fuel Filter

The fuel filter in KJet systems is very important. Everything from the filter onwards runs at very fine tolerances, and any fine particles in the fuel will cause havoc. If the filter wasn’t there, everything from the fuel distributor, to the injectors and even the warmup regulator would become clogged and/or damaged.

Air Flow Sensor

This is one of the major components of the system. The sensor plate is used to regulate the fuel flow through the injectors depending on engine load and speed. The sensor plate sits within a specially shaped cone, which is tuned for correct fuelling at certain airflow. As the intake vacuum above the plate increases, the sensor plate is lifted further and increases the air and fuel flow (contrary to popular belief, it’s not just airflow that raises the sensor plate, its the engine sucking against it).

Warm-Up Regulator

The Warm-Up Regulator (WUR) is a rather misnamed and misunderstood unit. Sure, it does the function of an old school carby choke, to enrich the fuel mixture when the engine is cold, but it is also used to regulate the fuel Control Pressure when the engine is warm or warming up. This cold enrichment is controlled by a bimetallic strip acting on a diaphragm, which is warmed by both an internal element, and ambient heat. There is a fine multi-layer filter mesh on the inlet to the WUR. This can become clogged and must be cleaned thoroughly, but not removed completely.

Fuel Distributor

The next major component is the Fuel Distributor, which is also called a Metering Head. This also contains the Main Fuel Pressure Regulator. The regulator controls the main System Pressure and holds it at a steady pressure, whilst bleeding off excess back to the tank. The Metering Head distributes fuel to the injectors via the Control Plunger and a series of Differential Pressure Valves. The Control Plunger resides inside the Metering Barrel, which has a series of very small (0.2mm wide) slits in it, one for each injector. As the airflow changes, the Control Plunger moves up and down, changing the fuel flow through the slits. The WUR Control Pressure acts on the top of this plunger, which alters how far the plunger moves, thus altering the mixture. There may be filters in the outlets to the injectors, under the fuel pipe fittings.

Injection Valves

The Injectors are the final part of the fuel injection system. The injectors are quite simple in their design, consisting of a metal cylinder housing a small valve, a spring and a filter. During operation a correctly functioning injector “sings” by making a squealing noise. The valve is set to open at a certain pressure, which in the case of the Cologne V6 is 3.3BAR. Over that pressure, the injectors are open and constantly injecting fuel. Below that pressure, the injectors should be sealed and must not leak. The internal filters are not serviceable. The injectors are sealed into the manifold with an O-Ring.

Cold Start Injector and Thermotime Switch

The Cold Start Injector and Thermotime switch go hand in hand. The Cold Start Injector is a primitive electric injector inserted into the plenum chamber, before the main injectors, which when triggered, fires a mist of atomised fuel into the intake system. As it’s injected into the plenum, it more or less gets sucked into all cylinders to enrich the mixture across the board. The Cold Start Injector is only triggered when the Thermotime switch meets the required conditions and makes or breaks the circuit. The Thermotime switch is both heated by the coolant temperature and an internal heating element. This allows the injector to operate in cold temps, but also stops the injector from triggering multiple times (in the event of a failed start), or triggering too long and flooding the engine.

Auxiliary Air Device

This device controls the flow of additional air into the engine when cold, which increases the cold idle speed. This is done by a shutter, which slowly closes off a passage which bypasses the throttle. This should only be open when the engine is cold.

So that’s the parts, how does it all work?

The fuel pump pushes fuel through the accumulator and filter to the metering head. This fuel acts against the main regulator, which pushes the System Pressure to 5.5BAR. Most of the fuel continues on into the metering head, but the excess is returned to the tank. The fuel, now at high pressure, continues into the differential pressure valves, and to the WUR. When the engine is cold, the control pressure set by the WUR will be low, at about 0.5BAR. As the engine warms up, and the WUR warms up, the fuel pressure should increase steadily to its warm pressure of about 3BAR.

So remember this, LOW Control Pressure = RICH. HIGH Control Pressure = LEAN.

Of course, this Control Pressure isn’t what the injectors see, that is the System Pressure (5.5BAR). The Control Pressure is only used to offset the height at which the Control Plunger in the metering head can increase to. A high control pressure places more force on the top of the plunger, reducing fuel flow to the injectors, thus allowing a lean mixture.

This GIF shows how the WUR regulates the Control Pressure

There is a vacuum fitting on the top of the unit, but no one can really confirm if its actually for full load enrichment or not. The manual indicates there is a diaphragm in the base of the unit that changes the mixture with vacuum, but I couldn’t see how it worked when I had my WUR apart. More research needed I feel.

For the basic system, you can almost just ignore the cold start injector and thermotime switch. As long as it isn’t leaking, it won’t be a problem.

The one other part that can be important is the idle mixture adjustment screw. This little screw resides down a tube between the sensor plate and fuel distributor, and uses a long 3mm hex key to turn it. This screw acts directly on the pivot for the sensor plate and raises or lowers the height of the plate. Less is more if you do tweak this, as a small change can make a big difference to the mixture. This screw does not alter warm mixtures at all, only idle.

If all of that is working as it should, the car should run and respond fairly well. The biggest problem is when the fuel pressures are wrong. It only takes one wrong figure for the whole lot to fall over, and then its a case of tracking back and finding out where that pressure has gone. This is why people dislike KJet.

Moving on to Part 2, We begin testing the system.


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3 months ago

Good description!Congratulations!!

3 months ago

Before I retired from the motor trade many jobs have gone through the w/shop with the K-Jetronic system, quite a few XR3i’s, some Capris and a few Mercedes. One of the Mercs proved to be a simple fault, but not easy to discover. The accumulator would depressurise overnight with the fuel running through the injectors and ending up in the engine’s sump. The owner had already made several trips to a local Merc specialist to no avail including having the metering unit/distributor replaced. The fault turned out be a seized pivot where the idle mixture screw lever when backed off… Read more »

4 days ago

So what if your idle adjustment key broke off how do you fix that?

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