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Bosch K-Jetronic – Part 3, Tuning and Fixing

Part 2 focused on how to test the system, now using the information from testing, we can move onto tuning and fixing any issues with the KJet system.

K Jetronic is a simple system really, but it needs every part of it running correctly to run well.

My fuel flow test was low

If the flow test failed there are only a couple of things it could be.

First, the fuel filter could be blocked. This is particularly common in older cars that have steel tanks. If there is any rust in the system it will block the filter in no time. Replace Filter.

Next, it could be a failing fuel pump that cannot output the required amount of fuel. This will become obvious with a System Pressure test, as it is likely it won’t be able to supply enough pressure. Replace Pump.

This could also be caused by fuel bypassing the system via a leaking Fuel Accumulator. If the vent is leaking fuel, the flow/pressure may not be making it to the Fuel Distributor. Replace/Remove Accumulator.

The final, and rare, failure is that the Main Fuel Pressure Regulator is sticking and will not allow fuel to bypass. This will also show in a System Pressure test as a low system pressure as the regulator will be jammed shut. Strip regulator and clean/free up.

My System Pressure is low

If the accumulator, fuel filter and pump are OK, we need to look at the regulator. First, replace the regulator seals as a matter of course. If this hasn’t fixed the issue, we need to try shimming the regulator.

Take care when adding shims as a small shim can add a lot. A 0.1mm shim can add about 0.8-1PSI. I needed an extra 7.5psi/0.5BAR, so added about 0.7mm of shim to get it correct. I made a shim from a copper washer carefully sanded down to the correct thickness.

The above photo shows the original pair of shims, which I added my extra shim to.

My System Pressure is high

As expected, this is the opposite. If the rest of the fuel system before the regulator is OK, if the pressure is still outside of the spec you may need to try removing a shim, or substituting in a thinner shim to get the correct pressure.

Now your system pressure should be good. If not, go back and start again until it is.

My Cold Control Pressure is wrong

Let’s start with cleaning and resetting the WUR. After so many years they can get rather tired. Remove and disassemble the WUR my by removing the four screws in the bottom (more detailed information here)

Now strip the unit completely by removing the heater strip, and fuel unit to access the metal diaphragm (be careful, the diaphragm is very fragile). With the diaphragm removed, start with blasting the filter mesh on the inlet out with carb or brake cleaner, from the inside out. I also used an ultrasonic cleaner, which I believe helped. This mesh is made of 5 fine layers, and each layer will get blocked.

Now reset the adjustment pin with a hammer and punch by tapping it from the inside out, until it is flush with the housing. Preferably do this with without the heater strip installed, lest you cause damage to it.

Now is the perfect time to make the WUR adjustable. If you don’t, expect to have to disassemble the unit a few times to reset the pin. Follow the information and guide here.

Now let us test and configure the WUR.

Step 1. Check the diaphragm

Reinstall the diaphragm, O-ring, cup and its retaining plate into the WUR. Install nothing else, just this. Connect the pressure tester as it was before, as well as the return line on the WUR. It should now just be a floating WUR top, with fuel lines on it and no bottom half.

This should mean there is no pressure on the diaphragm. Now run the fuel pump and check the pressure. It should be around 0.5BAR. Now, using the pin and mexican hat, gently press on the center of the diaphragm, on the cup. If you can raise the pressure by hand to above 4.5BAR, you’re winning.

If it doesn’t raise that high, something isn’t right with the diaphragm. Strip and clean the diaphragm, replace the o-ring, and try again. Make sure the diaphragm can move freely and isn’t stuck to the housing. If the initial pressure without pressing on it is too high, there is a restriction in the inlet (likely the damn mesh). Clean the mesh more, and try again. DO NOT remove the mesh completely as it is needed for that base restriction.

Step 2. Test the heater gets hot.

Hold the heater unit (the green thing on the metal strip) in your hand, connect the power plug to it, and turn the ignition and pump to ON. It should get noticeably hot. Not enough to burn you, but once you notice it getting hot, you are good to go.

Step 3. Test the pseudo-warm pressure.

Reassemble the WUR with the springs on the bottom half, but without the heater/strip. This applies pressure to the Mexican hat and diaphragm, to emulate the warm pressure. This should be 3.5bar or HIGHER.

If it’s low, like mine was, you need to find a way to apply more pressure to the diaphragm. In my case on the underside of the unit, in the base, was a press-fit pin. Punching this further up into the body increased pressure on the springs, which increased pressure on the diaphragm. It didn’t take much to increase it. Some units have an adjustment screw here instead, or you could try substituting or stretching the springs. I went up to 4BAR from just under 3BAR.

This is crucial. If you cannot get this high enough, you will never be able to lean out the Warm Control Pressure enough.

Step 4. Reassemble and set Cold Control Pressure

Fully reassemble the unit, heater and all. Refit to the car, and connect the fuel lines. Leave off the vacuum and heater connections.

Now its go time. Fire up the fuel pump and see where the pressure is. It should be high. We want to get it down to the required Cold pressure from Part 2. In my case, I needed 0.7BAR. Using a punch, hammer down on that pin you pushed outwards (or on the screw if you made it adjustable) until the pressure drops to what you need.

If you go too low, if you haven’t made it adjustable, strip the WUR and hammer the pin back out and start again. If you made it adjustable, use the nut to pull the pin out a bit and get it within range.

Once you’re happy with the Cold Control Pressure, move on to Warm.

Step 5. Set Warm Control Pressure

There are two ways that WUR gets heated. First by the heating element inside it, and secondly, via heat from the engine conducting through the body of the unit. Both end up with the same result though, flexing the bimetallic strip to change the pressure on the diaphragm. In this instance, we will use just the internal heater to warm it up. Connect the heater, and turn the ignition and pump on. Watch as the pressure rises as the strip gets warm.

After a few minutes (about 5) the pressure should stop rising. At this point you want it to be within your spec. Mine was close to the required 2.9BAR, but slightly low.

If it rises too much, you may still have a blockage in the inlet filter. You may be able to work around this by tapping the cold pin down a little more, but this will also cause the cold pressure to change. I believe you can also get this lower by lowering the System Pressure too, by changing shims. If it’s too low, you need to adjust the WUR to bring it up.

In my case it was low, but only by a few PSI. This can be adjusted by tapping the main body of the WUR down. I used a punch and alternated it on each side (orange arrows) until the pressure came up. This takes very little to make changes, you can’t even see it move, but the pressure goes up as you increase the pressure on the back of the diaphragm.

With the Warm Control Pressure set, now you should have a car that runs.

My Fuel Distributor Plunger is sticking

Remove the distributor from the airbox. Be careful lifting it not to let the plunger drop out the underside. If the plunger comes freely out, clean the plunger and bore with carb or brake cleaner until the plunger (lightly oiled) moves freely in the bore. It shouldn’t stick or catch. BE VERY CAREFUL WITH THE PLUNGER! It’s finely machined and cannot be replaced as its matched to the bore. Any damage/marks/scratches will render it useless as it uses its tight fit to be fuel tight.

If the plunger doesn’t drop out, you can sometimes use a little compressed air in the top fuel port of the unit to force the plunger out.

This is also a good time to clean out the unit, and the outlet filters if it has them. I filled the unit with brake clean and left it to soak overnight before rinsing it out with more brake clean.

My Accumulator is leaking from the vent

Bin it, or replace it. They cant be fixed. In my case I know my car runs happily without it, so I removed it and altered my injection system to run the pump with the key turned to ON. This allows me to prime the fuel system before starting, which helps with the warm start issue the accumulator is meant to stop. So far it hasn’t really been an issue, other than a couple of longer than usual (but still very reasonable) cranking times when warm.

My Cold Start Injector is leaking or doesn’t trigger

If it’s leaking, replace it. If it’s not triggering, it’s up to you if you replace it or not. I’m not convinced that unless it’s freezing outside that you need it, but I guess it doesn’t hurt to have if it’s working properly. You could try soaking it in carb or injector cleaner, or using an ultrasonic cleaner to clean it.

My Thermotime Switch is dead

Replace it (unless you don’t use the Cold Start Injector).

My Auxiliary Air Device is jammed open or closed

Sometimes you can clean and free these up. Mine was jammed, so I soaked it in brake cleaner and then used a heat gun to warm it up and cool it off to force it to open and close. Eventually, after a lot of cleaning and heating it started to work correctly.

My Injectors are leaking, the pattern is bad, or flow is uneven

Replace them. You could waste your money trying to have them serviced, but they are cheap enough to just pick up a new set and be done with it.

My engine runs but idles bad

This is where the idle mixture adjustment on the airbox comes into it. This adjustment ONLY sets the idle mixture and you need everything else to be ticking along nicely before you mess with this. Ideally, you would want a CO meter to adjust this to the correct spec, but without one, you can do it by ear and feel.


If you rev the engine and let the RPM drop, it should drop straight to a nice idle. If it drops and rises back up again, or hunts at idle, the mixture is wrong; this is the system trying to compensate. You also need to get the idle speed right at this point too, as altering the idle speed will alter the mixture demand.

By slightly raising or lowering the sensor plate you can tell if it needs to be richer or leaner based on how the engine responds. A small turn of this screw, with a 3mm hex key, will do a big change. Left is Lean, Right is Rich, so turn the hex key counterclockwise to lean the mixture out. With a little playing, you should be able to note the point where between rich and lean the engine runs best. The idle speed will usually come up slightly, and it will run smooth. Tweak the idle speed down if needed and adjust again until its happy. Once you have it in the happy place, you should now note that if you bring the revs up and let them drop to idle it should settle straight into a nice idle.

Sometimes if the fuelling changes have been dramatic you may need to tweak this a lot to even start the car. Try starting it a few turns rich and see if it starts. If not, try a few turns lean from the start point, and see what happens. At this point, it doesn’t matter where the starting point was as you will need to tweak the mixture once it’s running anyway.

My engine runs and drives bad

If all the above tests and settings are right, you may need to look at something other than fuelling. It could be ignition, compression, or like me, you could just be low on fuel.

My engine runs and drives great

Congratulations, you have now mastered getting K-Jetronic working properly. Now go out and enjoy it. Look after it, and it will look after you.



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Edward Marsh
Edward Marsh
1 month ago

amazing write up! thank you! just what i needed to add fuel for a supercharged setup

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