Adjusting the control pressure regulator on the K-Jetronic injection system2

On the photos 1 and 24 you can see how a control pressure regulator (CPR) looks like.

The CPR is a device used for the enrichment of mixture during the engine warm-up phase. The upper connection hose is attached to the fuel distributor pipe duct. The initial pressure is more or less equal to the number limited by the pressure regulator – 5 kg/cm². The lower connection hose is attached to the pipe that is connected to the K-Jetronic fuel distributor control input. There is also a 12V power socket on the top of the CPR. It is connected to a bimetal plate heater that is located inside and used to regulate the fuel pressure. On the photo 2 you can see the reverse side of the regulator.

The exposed surface borders on the engine body head joint face. The joint face is needed to transfer the engine heat to the CPR body. This is required to prevent the bimetal plate from cooling too much during short engine stoppages – otherwise this would lead to an overfueling after starting the warm engine. You can see the ventilation outlet in the middle as well as the air channels system (the perpendicular and the circular grooves). The injector engines of the W123 model with the 102 motor had the control pressure regulators with this precise Bosh ID. On the photo 3 the ID is 0 438 140 065.

Having unscrewed the four screw-bolts that fix the body you can see the internals of the control pressure regulator.

The bimetal plate with the heating element attached to it is fixed by an M6 screw-nut on the rod press-fitted to the body. Moreover, there is a spring and hard plate with a dowel – you can see them on the photos 6 and 7.

On the photo 6 you can see that the dowel is inserted into the conic hole of the plate. It’s shape resembles a sombrero hat. There is lubricant grease inside which prevents the dowel from falling out and lets it move around freely.

After unscrewing the M6 screw-nut it becomes possible to take out the bimetal plate with the heating element attached to it. On the photo 8 you can see the element resistance level that is equal to 26 ohm.

To remove the heating element you will have to unbutton the holding plate. After that it will look the way it is on the photo 10.

After removing the bimetal plate you can see the adjustment unit inside of the CPR body that is basically being controlled by the CPR. On the photo 11 you can see a disc that is fixed by four screw-bolts. Inside of the big disc there is a fill-piece with a hole. The dowel abuts on it with one end while the other end is inserted into the hard plate.

Under the disc there is a thin plate that is used to regulate the control pressure. The plate is made out of carbonaceous material and it’s pretty fragile, that’s why you should be especially careful when handling it. There is a fill-piece inside of the disc. The dowel that abuts on it blocks the fuel pipe located directly under it. There is a rubber spacer around the pressure regulator room – it prevents any possible leaks. The regulator room and the plate can be seen on the photos 17 and 19. On the photo 19 you can also see the big disc with the fill-piece. It should be able to freely move inside of the disc for the system to function properly.

The fuel moves through the central hole out of the CPR and gets into the fuel distributor.

The thin plate should be flat when you are assembling the whole unit to prevent sudden change of settings. After some time the plate will bend a little bit towards one side.

The connection hoses that connect the CPR to the fuel line can be seen on the photo 12. And on the photo 13 you can see the small filter inserted into the big connection hose.

If you carefully push the filter out of the connection hose, you’ll get the following (photos 14 and 15).

There’s no screen on the body of the filter – it is totally dissolved in the petrol.

After turning off the connection hoses you can see the holes inside of the CPR. There is a dense grid under the big connection hose (photo 16). When it’s clogged the regulator won’t function properly as the pressure can’t very within the required range. After clearing the regulator you can try to blow into these holes before installing the plate. As long as the holes are clean the difference between the small and the big hole is insignificant due to the fill-pieces. If you have to make a great effort to blow into the big connection hose then it’s pretty clogged.

After removing the heating element from the bimetal plate it needs to be put on properly. In order to do it you will need to place the plate as shown on the photo 22. First put on the heater and then place the locking plate. To fix the heater move it towards the locking plate until it locks.

On figure shown on the photo 31 you can see how the control pressure depends on the temperature. On the photo 32 the control pressure regulator mode of functioning is displayed.

To regulate the pressure two manometers should be used. The first one should be connected to the start valve and the second one – via a T-connector to the CPR port and to the fuel distributor as shown on the photo 28. For safety reasons the connection hoses have been made similar to the original ones, but allowing a gas hose with a collar to be used. Experience has shown that great pressure inside of the gas hose (up to 5 atm) requires additional attention to fire safety engineering. After the gas hose attached to the original connection hose broke and the fuel started leaking out of it directly onto the discharge manifold, there were no more doubts that other connection hoses are needed (photo 30).

Unfortunately, there’s no simple way of setting the service pressure in the original regulator design. A guide bar with the bimetal plate attached to it is regulating the pressure in the CPR. The guide bar has been inserted into the hole tight. To set the required pressure you need to move it within the hole by slightly tapping it. The deeper the guide bar is in the hole – the lower the maximum control pressure will be (you need to disassemble the CPR to do this, so good reliable connection hoses are a good practice).

To perform these actions you will need a piece of some thick cable that will be used to turn on the fuel pump while the engine is off. It should be inserted between the 7-th and the 8-th contacts of the fuel pump relay.

After attaching the manometers let’s start with the meterage.

Let’s insert the piece of cable and turn on the fuel pump. The first manometer that is attached to the start valve will show us the system pressure (around 5 atm). As soon as you connect the contacts to the cable the second manometer that is connected to the CPR will display the pressure of approximately 1.2 atm. As the engine is cool the arrow should indicate the pressure that corresponds to the performance schedule. At 68°F (20°C) it should be 1 atm. If you remove the cable now turning off the fuel pump in such a way the second manometer will display the pressure of about 3.2 atm – this is lower than the pressure needed for the nozzles to open.

From the CPR manual we know that the control pressure is 3.4 to 3.8 kg/cm², so after starting the engine we let it warm up to 176°F (80°C). The heating element is warming up permanently together with the engine and the pressure of the second manometer will be growing up smoothly, decreasing the mixture enrichment. After achieving the 176°F temperature the pressure should be no more than 4 atm. Now you can press the accelerator pedal for several times to see how the engine reacts. Both manometers will display the pressure increased by 0.2 atm for a while and then return to the original values.

If the control pressure grows up to 5 atm, you should slightly knock the guide bar and let it get deeper into the CPR hole. Use a beam caliper and measure how deep the guide bar is getting inside in comparison with the regulator body. When the engine is on, knock the guide bar and let it get deeper inside for less then a millimeter. In the same time the control pressure will be decreasing – you should stop when it achieves 3.8 atm. Quickly press the accelerator pedal for several times and let the engine run for a while. As it continues to warm up the control pressure may rise a little bit (0.2-0.4 atm) – in such case you need to lower it once again. You need to achieve stable pressure that shouldn’t grow up when you press the accelerator pedal. After you’re done with that – you may kill the engine.

Warning: if you make the guide bar get too deep inside into the hole, the pressure will get out of the required range and you will have to re-do the whole procedure once again. In such case you will have to detach the CPR from the lines, disassemble it and slightly knock the guide bar for it to get out of the hole for about 1 mm. After that you will have to repeat the procedure from the very beginning.

At this point I’d recommend letting the car cool until the next day leaving the manometers connected. On the next day the following procedure should be done to check the lower value of the control pressure. This is important to be able to start the engine when it’s cold outside. Insert a piece of cable instead of the fuel pump relay. The CPR heating contact should be disconnected. The pressure should fall to 0.8-1.0 atm at 64°F (18°C). If it does, we succeeded and the control pressure range is correct. Start the engine and let it warm up till the working temperature. Check the highest possible pressure. If the lowest possible pressure was too high, wait until the engine is warm and knock the guide bar slightly to decrease the pressure from 3.8 to 3.6 atm. This will let you get even lower pressure when then engine is cool and will help you to start the engine more quickly during winter time.

A couple of issues regarding the adjustment process
1. For initial launch the spring height in the CPR should be 22.0-22.5 mm. You can either stretch or compress it to achieve this value. Measure it with a beam caliper but be sure not to squeeze the spring with it.
2. The guide bar can be pushed into the body for 3 mm. This lets you stay within the range that allows the engine to start.
3. In wintertime you may connect the CPR to the power through a 5-7 ohm/0.25 W resistor. Using the resistor lets the bimetal plate warm up more smoothly, as the heater manages to warm the plate quicker than the engine does. As a result, the increased engine rpm won’t decrease quickly – they will drop smoothly till normal values as the temperature of the engine reaches 140°F (60°C). In the summertime the resistor won’t affect the time needed for the engine to warm up. If you remove the heater in the wintertime it is going to warm up only thanks to the engine heat and the rpm will increase even slowly. They will reach their normal values only when the engine warms up till 176°F (80°C).
By the way, I didn’t find it useful to insert the adjusting plates under the regulator spring.

More than that, after having the guide bar got deeper than it should and having disassembled the CPR due to this, I have found the way to make this unit adjustable. Using a common screwdriver made it possible to adjust the pressure without having to disassemble the whole system.

The next part will be interesting for those who would like to make the CPR adjustable.

When I was thinking on how to implement the ability to adjust the pressure, I made myself a condition that all the changes in CPR should be reversible, i.e. after removing all the elements that I added to the system it should perform as it used to and should be adjusted using the old method (by slightly knocking the guide bar).

Basically, to adjust the pressure we move the guide bar inside of the hole. All we need to do is to make it possible to get the guide bar out of the hole without having to disassemble the CPR.

To implement this feature I removed the original guide bar from the body. I ordered the same guide bar to a turner, but having a smaller diameter letting it move freely inside of the hole after greasing it with a lubricant. This lets us pull the guide bar even if it’s deep inside. I have cut an M3.5 screw all along of the guide bar and drilled a small whole in its upper part.

To make the adjustment smoother and to prevent the guide bar from jumping off I placed a spring over it. The spring makes the guide bar get down moving the bimetal plate at the same time. To lift the guide bar a screw is inserted into it. The screw leans against the upper part of the regulator. In such a way the bimetal plate is lifted and there is an option of increasing the pressure as well as of decreasing it. To make the whole unit more secure I had to add one more holder that would fix the guide bar in the position needed. I drilled a hole in the CPR body just against the guide bar, cut a screw inside of the hole and inserted a secret screw-bolt into it. Thanks to this screw-bolt (see photo 25) the guide bar is tightly fixed after adjusting the pressure.

The disc, through which the adjusting screw-bolt is fixing the CPR, is complex: there is one more smaller disc attached to its bottom (the size is equal to the size of the hole). Such an implementation protects the screw-bolt from moving from side to side affecting the adjustment accuracy. You can see it on the photo 26.

As the guide bar is able to move freely inside the hole, a problem of fixing the bimetal plate appeared. When turning the screw-bolt the plate moved aside as well. To fix the plate I added an U-shaped incut the ends of which are attached to the grooves in the CPR body. They can be seen on the photo 27.

All the details and the spare parts needed to upgrade the regulator are shown on the photo 29. The small spring used in this device is from a set of brake block attachments. The width, the diameter and the spring rate of the spring appeared to be the most reasonable ones to provide a smooth adjustment.

Provided by Schegolev Vladimir aka kit123


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1 Response

  1. Pete says:

    Hi, Thank you for your site.
    I am busy with my Kjet from my 1982 500SE Mercedes.
    Bij adjusting the guide bar you only adjust the cold start pressure. When warm the Bimetal is loose from the spring anyway so this adjusttment does nothing to the control pressure when the engine is warm.

    My car is very fast when cold, but reacts slow to the gas pedal when warm. I compressed the spring somewhat and lowered the control pressure from 3,8 bar to 3,5 bar.
    Also I tried to get the vacuum system on the regulator to operate more efficient by adding a second restriction in the vacuum lines.
    I like to believe that things are better now but Im not sure. My former 450SE was way faster and responsive (it problably had a broken warm up regulator and used mach gas, but it was really responsive….)

    Best regards, Pete (The Netherlands)

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