K-Jetronic fuel control unit on a 102 engine at a glance

I wanted to fill in the gaps in knowledge and get and idea of how the K-Jetronic system looks like from inside for a long time already. As the KE system has been described and pictured for many times I decided to show you how the K-Jetronic system on a 102 engine of the W123 model operates. A sharp desire to see what’s there inside has inspired me to make the pictures and write this article. I hope that everyone can satisfy his or her curiosity now. We are going to disassemble the device mentioned above completely. So, let’s begin!

Let’s explore the K-Jetronic fuel control unit by Bosh. The device consists of two major parts.

1. The air regulator (photo 11).

2. And the fuel distributor (photo 12).

On the photos 8 and 15 you can see the fuel distributor pressure disc, the fuel inlet connector that supplies fuel from the controlling pressure regulator, four branch pipes to connect with the fuel-injection nozzles, the mixture quality regulator (a column in the middle) and the fuel pipe unions (photo 15).


From left to right (photo 8):
a. fuel supply from the fuel pump;
b. fuel supply to the controlling pressure regulator;
c. reverse line supplying the fuel back to the gasoline tank;
d. fuel supply to the electrical starting nozzle.

The fuel distributor is shown on the photo 3 together with the mixture quality regulator.

It’s worth mentioning that both parts are assembled in such a way that their ID numbers are located on the same side.

On the upper part a connection hose can be seen that connects to the fuel line of the controlling pressure regulator (photo 3).

On the lower part of the fuel distributor a rubber cover is located. The big hole connects to the throttle gate. The small one connects to the junction pipe of the air-delivery system that is needed for idle running and the additional air shutter (photo 9).

Let’s have a closer look at the fuel distributor. On the photo 22 you can see the fuel line leading to the nozzle. Inside of it a secondary fuel filter is located – its blockage often leads to malfunction or failure (photo 22).

Between the outlet ducts a hexagon is placed. By unscrewing it you can get access to the line that is connected to the lower inner tube of the fuel distributor (photos 62 and 63).

Inside of the line a spring-mounted piston can be seen together with the line that connects the lower inner tube of the fuel distributor and the upper one (photo 60).

A removable fuel inlet connector is located on the fuel distributor body (photo 26). It is equipped with a secondary fuel filter. Its blockage leads to system pressure decrease.

On the photo 49 an inlet connection is shown in close up.

On the photo 26 on the right you can see the system pressure regulator that limits the pressure by 5.2 bar.

The pressure regulator is basically a spring-mounted valve (photo 51) that brings the excesses of fuel back to the gasoline tank via the reverse line.

It consists of the following parts. On the photo 54 you can see the holes that are needed to limit the fuel delivery. The disc shown on the photo 54 to the left side of the spring is the adjusting plate. The fatter one allows to increase the system pressure and the thinner one – to decrease it.

The mixture quality regulator (a column on the body of the device) contains a spring-mounted guide bar with a 3-mm hex key groove on its outer side. The guide bar is hexagon-shaped from inside. The hex key is used to force in the inner hexagon that connects to the hex guide bar. The latter gets into the hexagon-headed bolt located on the air regulator carrier. If you push the pressure disc you can see the adjusting screw (photo 25).

On the photo 41 both the regulating stem and the mixture quality adjusting screw can be seen very well.

Next to each of the fuel line outlets a fuel supply adjusting screw is located. It is covered by a hexagon closing screw. A copper spanner used together with a blind plug prevents any possible fuel leaks out of the control unit (photo 18).

If you unscrew the blind plug you will be able to see the fuel supply adjusting screw. On the photo 28 it is the small hole with the hexagon inside.

The adjusting screw itself can be seen on the photo 56.

In the hole on the photo 59 you can see a plate that leans against the adjusting screw assisted by the spring.

To detach the fuel distributor from the body you will need to unscrew the three screw-bolts (a common screwdriver can be used). To unscrew one of them first disconnect the fuel line connection hose from the controlling pressure regulator (photo 17).

After carefully detaching the fuel distributor from the body take care not to loose the rubber spacer. Using a damaged spacer or not using one at all will result in additional air suction that will break down the normal carburetion process. In the middle of the lower part of the fuel distributor on the photo 29 you can see the external part of a plunger. The plunger is spring-mounted. To prevent it from falling out of the distributor a plate is installed. The plate should lean against the conical surface in the lower part of the plunger, but shouldn’t touch its thin part.

You can take out the plunger by slightly bending the plate. Don’t loose the spring. You should perform all these actions on a clean and comfortable workplace (photo 30).

The plunger itself is shown on the photo 32. The plunger and the hole inside of which it is located are highly sensitive. Any dirt or mud is inadmissible here! It is not recommended touching the work surfaces of the plunger as well as placing it on surfaces covered with nap. When assembling the plunger you need to be careful not to let any dust or hair get inside. When withdrawing or mounting the plunger it’s recommended to drop some pure gasoline on it. This will help to avoid scratches on the surface.

On the photo 34 you can see the fuel distributor mounting seat. The rubber spacer is inserted into the round groove – this is done to prevent additional air suction. There is an air regulator carrier inside of the hole. The regulator plunger leans against the roller bushing. The bushing itself should be able to rotate freely.

After unscrewing the eight screw-bolts that connect the two parts of the fuel distributor you can start disassembling it using the torx screw. There is a membrane between the two parts of the fuel distributor. After years, being compressed all the time, the membrane sticks to the parts. That’s why in order not to damage the thin membrane you should hold the fuel distributor with your left hand and use a wide and thin screwdriver to disjoin the upper and lower parts a little bit. The screwdriver should be inserted in the spot where the screws are located. In the very first time you need to carefully check each of the four sides with the screwdriver to determine the side that can be disjoined with less effort.

More than that, you need to make up your mind on where exactly you are going to insert the screwdriver: between the top and the membrane or between the membrane and the bottom of the fuel distributor. If you managed to make at least a tiny gap you should carefully push the screwdriver inside. To increase the gap you should use your screwdriver as a lever, but leaning against the metal part and not against the membrane.

To make the process easier you should pour some gasoline into the gaps. It will prevent the membrane from breaking apart. After that take a small thin screwdriver and delaminate the membrane circle-wise.

After that hold the top of the fuel distributor with one hand and the bottom with the other one and twist them in different directions for just one millimeter or so. The lower part is inserted into the upper one through a sealer so you should carefully move them apart to unstuck. Help yourself with gasoline to delaminate the fuel distributor membrane completely.

Caution! The next procedure should be done extremely carefully to be sure not to loose the ceramic plates, the springs and the adjusting plates. This all may fly apart, so as the journal box is being pulled out make sure your movements are getting more and more careful. The best way to do it is to place your hands holding the fuel distributor into a plastic bag and then proceed with disjoining the parts. To disjoin twist them in the opposite directions by 5-10 degrees pulling the lower part out of the upper one in the same time. The result is shown on the photo 84.

Now you can hold the lower part of the fuel distributor with the journal box.

There is a big rubber spacer on the journal box as well as the four oval rubbers on the pipes. Avoid damaging them.

On the photo 85 you can see the fuel distributor journal box. You can see the metering ports inside of the ellipse. Do NOT to clean them mechanically!

The upper part separately can be seen on the photo 78.

The small holes in the middle of each pipe are the nozzle outlets. These holes are really tiny. Only a thin needle is able to get into these holes. You should insert the needle without any effort to check whether the pipe is choked. If you feel any resistance you should stop inserting the needle to avoid piercing the filter. The needle should be inserted with the blunt point. On the side of each pipe in a groove a reverse side of the adjusting screw can be seen (the screw itself is shown on the photo 56).

The fuel distributor membrane is shown on the photo 81 (this side up). You can see how it is bended as affected by the ceramic plates.

Because of the springs each of the ceramic plates pushes the membrane that leans against the saddle in the lower part of the fuel distributor. The wide cross-arms can be seen on the photo 76.

On the photo 87 the all adjusting plates, the springs, the ceramic plates, the journal box screw-head and the plate that holds the plunger can be seen.

The metering ports are extremely small. On the photo 90 you can compare its size with the size of the hole in the lower inner tube.

On the photo 92 you can see the plunger groove. Short marks are the metering ports and the large ellipses are the holes in the lower inner tube.

Not let’s explore the air metering unit that is called the Venturi tube.

This device serves as an air measuring instrument needed to reach the required proportions in the gasoline-air mixture. Inside of it a pressure disc is located that can move together with the live lever. After removing the holding rod of the rubber in the lower part of the device you can see the mechanical system of the air regulator (photo 35).

The lower part of the air regulator is shown on the photo 46.

The flexible part of the air regulator is basically a double lever that hangs on the main axle and has fast response. On the one side of the lever a pressure disc is located and a screw can adjust its position. On the other side a counterbalance is located that balances the whole system. On the photo 45 a damping mechanism is shown – it protects the mechanical part from damage during the snap-backs in the intake manifold.

The damping mechanism is basically a steel plate that receives the lever return motion. The plate leans on the pressure disc position limiter. This can be seen very well on the photo 40.

On the photo 41 you can see a projection (to the right of the adjusting screw) that leans against the plate at rest. During the snap-backs in the intake manifold this projection pushes the plate damping in such a way the air regulator lever return motion.
Also a stem and a mixture quality regulator screw can be seen on the photo.

On the photo 70 a projection located to the left of the quality regulator screw can be seen very well.

To protect the mechanical part even more there is a crossbar with a rubber stop, strengthened from inside and outside, located directly over the pressure disc. Without this crossbar the whole device may become inoperative during major failures without any chance to be repaired.

In order for the air regulator to function properly the pressure disc should be at rest in a certain position. In such a position it should be located exactly on the opposite of the widest part of the Venturi tube. A slight deviation is acceptable. On the photo 24 the correct position of the disc is shown.

And on the photo 23 the position is above the line. Yet such position is also acceptable. I had two operational air regulators with the same disc positions.

To adjust the vertical position of the pressure disc a stem is used. The stem is located directly under the fuel connection to the fuel pump. The stem can be seen on the photo 48.

If the position of the disc is above the line you should lower it by slightly hitting the stem. If the pressure disc position is correct the fuel distributor should not let a single drop of the gasoline get out of the nozzle outlets while the engine is off because the fuel distributor plunger shuts off the metering ports completely.

To get out the mechanical part out of the air regulator body you need to remove the bushing lever pin spacing collars that are shown on the photo 27.

After performing this operation we will get the following details (photo 75):

a. two distance washers;
b. two thrust washers;
c. two rubber spacers;
d. two ball-bearings;
e. one spring.

It is worth mentioning that the spring is being inserted from the side specified. The spring is being inserted into the plug that can be seen on the regulator base. This plug is shown on the photo 4, it is located on the opposite side where the connection hoses are located.

After releasing the plugs you need to loosen the screw-bolts that fix the lever on the axle. They are shown on the photos 43 and 44.

The screw-bolts should be loosened just a little bit – make no more then 2-3 twists. After loosening them you need to kick out the axle by slightly hitting it. You need to be especially careful when doing this as the surface of the axle is polished and it rotates inside of the plugs thanks to the friction.

Now you can take out the whole mechanism out of the body. On the photos 67 and 68 the lever assembly can be seen – from the top and from the side.

The inner part of the regulator body can be seen on the photo 64.

Let’s have a closer look at it. On the photo 65 a plate can be seen – it limits the reverse lever travel and regulates the pressure disc position at rest.

The screw-bolt in the upper part is needed only to fix the plate inside of the body. The stem with the brass bushing is used to regulate the position of the pressure disc; it has been described on the photo 48 already.

From the inner side of the stem there is a rubber shock absorber. To adjust the pressure disc reverse position you should move it towards the inlet connection by slightly hitting it. It is not recommended moving it back and forth a number of times in order for the whole unit not to get loose.

Besides the adjustment rod there is also the reverse side of the thrust stem on the inner side of the lever. It can be seen on the photo 70. By pulling it out and pushing it in you can also adjust the pressure disc position. On the photo 42 you can see how it looks like on the lever assembly (between the parts in the bottom).

Now you can take the inner part out of the main lever where the mixture quality adjusting screw is located. This detail can be seen on the photo 74, there is a steel plate inside. It is needed to keep the adjusting screw in the required position preventing it from displacement.

The plunger leans against the roller located on the reverse side.

After taking out the inner lever you can see its mounting seat on the external lever body. It is shown on the photo 72. In the middle the lower part of the adjusting screw is located and to the right you can find the thrust stem that we have talked about on the photo 42.

The adjusting screw is needed to move the external lever in relation to the internal one – this makes the plunger change its position in the fuel distributor. In such a way the air-to-fuel ratio can be increased or decreased affecting thus the mixture enrichment level.

Finally, let’s consider some of the aspects when assembling the fuel distributor.

First of all you need to insert the lever on the photo 74 into the big lever. Slightly unbend the inner lever plate and insert the lever under the adjusting screw head.

Next you will need to put the levers into the air regulator body. Carefully push the axle on which the plug lever is rotating. Insert the ball-bearing from the side where no spring is required. Insert the rubber spacer and fix it with the distance washer. After that you need to move the axle towards the ball by pushing it.

Before fixing the lever on the axle you need to tighten the screw-bolts a little bit. They should be loose enough for you to be able to move the lever along the axle with some effort. To adjust the lever position on the axle you should insert the screwdriver into the plug and push the axle with it until it leans against the ball. Adjust the lever position on the axle by inserting the screwdriver into the grooves between the lever and the plugs.

The lever should be installed in such a way that there are gaps between it and the plugs. This can be seen on the photos 43 and 44. The accurate position can be reached by moving the pressure disc towards the “at rest” position. There should be equal gap around the disc.

Insert the second ball-bearing and fix everything with the distance washer.

Check the accuracy of the pressure disc position inside of the Venturi tube. Also you need to check whether the lever can move around smoothly. To do this you should turn the regulator to its normal position and gently draw the pressure disc all the way down. You should feel that it moves smoothly from top downward.

There is another way of checking whether the lever can move smoothly. Draw the lever all the way down and let it go; watch the way it is turning back to the original position. If the lever can rotate smoothly it will push off the plate one more time because of the counterbalance. If you put your ear to the cross-arm over the pressure disc you will be able to hear how the lever hits the plate twice.

If the lever is installed properly and moves smoothly tighten the screws on the lever pin.

Now you need to assemble the fuel distributor. Take its upper part and place it the way it is shown on the photo 79. Then insert the small springs into each of the big holes.

The plunger spring does not need to be inserted. Put the ceramic plates on the springs the way it is shown on the photo 80.

The membrane should be placed to its original position. Although the membrane is symmetrical it won’t be difficult to determine the correct way to place it. There is a hole in its middle part through which it was attached to the body by a screw. This hole is somewhat deformed and looks different from the holes on three other sides (photo 82).

The hole into which the journal box will be inserted should be moistened with some gasoline to reduce the friction against the rubbers. Take this part with your left hand and hold it strictly horizontally. Take the bottom part with your right hand.

Caution! Both parts should be taken in such a way so that the ID numbers located on their sides are on the same side. You can see them on the photo 3.

Insert the journal box into the passage till the ellipses and check once again whether the rubbers are tight enough. After that insert the journal box to the depth of these rubbers. Then carefully insert it all the way in.

In the very end you will feel some resistance. This happens because of the tight fit of the big rubber on the journal box inside of the groove hole.

After that you can take the screw-bolt that connects both parts and make sure the membrane is installed properly by inserting it into each of the holes. Tighten all the screws a little but and then tighten them even more in a criss-cross manner until the parts are drawn together equally.

Do not make it too tight as there will be too much pressure on the membrane.

If it happened so that the ellipse rubbers are broken or damaged, you can find similar ones in a plumbing shop. Buy rubber rings only after comparing them with the original ones. The diameter may be somewhat smaller and the thickness should better be measured with a trammel.

I had to choose among a wide variety of similar looking (but only looking that way) rubbers.

Here is how you can test the assembled fuel distributor.

Insert the spin, the plunger and tighten the plate that prevents the plunger from falling out. Hold the fuel distributor in its normal position so that the spring would push the plunger out to the bottom. In such a position the metering holes are closed.

By pouring the gasoline into each of the nozzle outlets you can fill the entire upper inner tube. If all the rubbers inside are hermetic the gasoline level will be equal. If at least one of the rubber spacers is not tight enough the gasoline will be getting inside quickly in a non-hermetic hole.

If everything is ok push the bottom of the plunger by slightly opening the holes. The gasoline should get inside practically simultaneously in each of the four holes.

Also, during the process of disassembling you can test the upper part of the device on correct fuel delivery by pouring the gasoline into the outlets. The small holes on the bottom should be closed with your fingers. If the pipes are clear the gasoline will get down in 4-5 seconds simultaneously and you will see four similar jets in the bottom. If not, you will have to use chemicals to clean the outlets as well as the filters inside of them.

Provided by Schegolev Vladimir aka  kit123

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7 Responses

  1. Virgas says:

    Hello.
    Very informative, thank you.
    Just one question about secondary fuel microfilters inside fuel line connection place: are they removable? How to remove and clean them and space between them and noozle?
    Thank you again.

  2. admin says:

    Hello, Virgas.
    I never did that, actually I dont know…

  3. Melvin Johnson says:

    My 1992 190e 2.6 Mercedes would just crank, not start. I removed the fuel distributer and found that the plunger needle was stuck and would not come out without me pulling on it. I pulled the needle out {was fairly easy} then pushed it back in after wiping it off and adding wd40. It then moved in and out easily with just a bit of resistance. Put the fuel distributor back on and it still didn’t crank. Did I mess the plunger up by removing the needle? It seemed to work as intended. Fuel pumps are good, new ignition parts. Cold start valve started leaking after reassembly. Any ideas on what the problems could be? Thank you Mel

  4. Olaf Larsen says:

    Excellent work, kit123! I have 230TE engines that need to be overhauled, and your photos and descriptions will be a great help to check the K-jet parts.
    Yours, Olaf

  5. tom says:

    Just came across your site, surprising as I thought I had seen most Merc sites out there, great tech articles, Many thanks 🙂

  6. Mickey says:

    Hi, Great Information!! my problem is more on hard starting..sometimes after it would start and when I try to adjust for idling some slight black smoke shows out ..and there after the engine begins to run rough ton even course the engine to shut off..after that again the hard start begins. another thing the fuel regulator seems to be good but, I would like to know where the regulating suction pipe comes from so I could locate it and reconnect for its normal operation? kindly help

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