Press "Enter" to skip to content

Bosch K-Jetronic – Part 2, Testing

In Part 1 we figured out the basics of what the components are and what they do. Now it’s time to test the system and see what we are starting with.

How do I test it?

All measurements quoted are based off the Ford Cologne V6 and TVR Tasmin 2.8i manuals. Please check your relevant manuals for your correct measurements. These are examples.

First things first, if you don’t have a known good KJet specific pressure tester, don’t bother going further. If you cannot test the pressures, you cannot diagnose or set the system correctly. Throwing parts at a system that has incorrect pressures will cure nothing. A good pressure tester can tell you almost everything you need to know with why the system is acting like it is. Either obtain a good quality tester or take it to someone that has one.

This is the exact kit I purchased, and it’s proven to be reliable, high quality and well made. I originally purchased a full kit in a fancy red case from America, which turned out to be drop-shipped from China, and was a complete fail. Inaccurate, and low quality. It set me back quite a bit by not giving me the correct readings. This is the sort of work where quality and accuracy is important. Buy right, buy once.

Get ready to take some notes of your readings.

Now, the first thing you need to test is the fuel flow from the pump. Disconnect the fuel return line off the main regulator at the fuel distributor, extend it about 700mm and direct it into a suitable container of 1L or larger. I used a 5l fuel can for this.

Now run the fuel pump for exactly 30 seconds and measure how much fuel is in the container. You should have at least 750ml. Anything more, good (within reason). Anything less, and there is either a restriction in the system such as a blocked fuel filter, or the fuel pump could be failing. This is a test of the fuel flow from the tank, to the regulator, so a low reading should be something before the fuel distributor.

If the fuel flow is OK, reconnect the return line and proceed with testing the System and Control pressures.

Connect the pressure tester between the fuel distributor and the WUR.

The shut off valve must be on the side of the gauge that connects to the WUR.

You don’t need to run the engine to test the pressures. Disconnect the electrical connector from the WUR for now (to stop it heating up and changing pressures) and run the fuel pump.

With the shut off valve open, you should be reading the Cold Control Pressure from the WUR. As mentioned, the Control Pressure is what controls the enrichment.

The Cold Control Pressures are on a scale depending on the ambient temperature.

As you can see from that graph, at 20c control pressure should be 0.6-0.9BAR. 30c would be 1-1.3BAR.

If the control pressure is within the correct range, you’re good to go. If not, there is a problem, which we will look into later on.

Next, close the shut off valve, and you’re now reading the System Pressure. System pressure is the main overall pressure the injection system is running at. If it’s not set correctly it will throw the Control Pressure out too.

This should read about 5.1BAR (0.4BAR +/-). If not, as long as the fuel flow test was OK, you may need to shim the regulator or you could have a failing fuel pump that cannot produce the pressure required. I set mine on the higher side, but within the acceptable range.

To test the Warm Control Pressure you can either warm the engine up if it runs, or leave the ignition and pump running with the electrical connector connected to the WUR. The heater should warm the WUR up and allow us to test the warm pressure. It should start from the Cold Control Pressure (if it’s low enough. If it’s already 3BAR or higher there is no point testing for Warm pressure) and steadily rise until it hits a plateau. The Warm Control Pressure should be about 3BAR depending on spec.

The last test for the gauge is to test the accumulator and for leaking injectors. With the fuel pump running, have the test gauge shut off valve open. Now turn off the fuel pump and the gauge should drop to about 2.5BAR. This is the accumulator pressure. After 10 minutes, it should still be 2.5BAR. After 20 minutes, it should drop to 2.0BAR. If its lower than this, either the accumulator is leaking internally, the injectors are leaking, or the cold start injector is leaking.

If the pressure drops more than it should, you can run the pump again to build pressure, shut it off and shut the test gauge shut off valve. If the pressure still drops, this indicates the leak is before the gauge, so likely to be the accumulator.

If the pressures are 100% bang on, there is only a couple of other tests to run to check the system. The next tests require disassembly and are a last resort.

First is to test the accumulator. These are a common failure point and can cause a bunch of weird issues, including low system pressure. The accumulator usually lives down by the fuel pump, and should be in the fuel system post pump.

Left to right are fuel pump (without feed hose fitted), fuel filter, and fuel accumulator. The hose coming from the accumulator is the vent that returns to tank.

The accumulator will have three fittings. Two on the front, for fuel in and out, and one on the rear, which is a vent (and failure overflow). The vent is on the dry side of the diaphragm, so if any fuel comes out of it, the diaphragm has failed. The easiest way to test is to clamp the hose from the outlet of the accumulator (usually the one that goes to the fuel filter), take the vent hose off and direct a hose from the vent into a container, and run the fuel pump. If the accumulator is OK, not a single drop of fuel should come out of the vent. If anything at all comes from the vent, the accumulator has failed.

Inlet and outlet on the left, vent on the right. Mine was so bad the “dry” side of the unit was full of fuel and would drain out of the vent if you tipped it up.

Another major item that can have issues is the fuel distributor. The plunger can stick, which leads to erratic fuelling. With the engine off, fuel pump on, you can do a quick test to see how free the plunger is. If you raise the sensor plate by hand, drop it and then quickly bring it back up again, can you feel it hit the plunger high up the bore, or has the plunger mostly, or completely dropped with the sensor plate? If your plunger is still high in the bore, it is sticking. It should more or less follow the sensor plate as it drops. Be aware that raising the sensor plate will inject fuel into the cylinders, so don’t play with this too much.

The next test is to check the auxiliary components; the Cold Start Injector, Thermotime Switch, Auxiliary Air Device and WUR Warmup Circuit.

First is to test the Cold Start Injector. This is done by removing the injector from the manifold, and directing its nozzle into a container. Now, connect the power connector from the Aux Air Device to the Cold Start Injector and turn the ignition ON and run the fuel pump. This should cause power to open the injector and spray fuel. Now disconnect the power from the injector and wipe the nozzle dry. Hold the injector, with the ignition and pump ON, in the container for 60 seconds and observe it for any leaking. Shut off ignition and pump, and if it is still dry, it has passed the test.

Next is to test the Thermotime Switch. This triggers the Cold Start Injector based on temperature (and time). Testing this is done by testing for resistance and continuity between the circuits.

When Cold, the resistance of Terminal A seems to be irrelevant, as long as it is lower than the resistance of the same terminal when the coolant temperature is hot. When Warm Terminal A should have a higher resistance.

When Cold terminal D should have continuity with the body of the sensor, with no resistance. When Warm, the same terminal should show as open circuit when tested against the body.

The Auxiliary Air Device (AAD) is another item that is common for causing issues. The sliding plate inside the unit can stick, meaning the unit sticks either open or closed when it shouldn’t. This is fairly easy to test.

When cold, you should be able to look through the unit and see the shape of the cutout inside

When Warm, that should be a solid plate blocking it off. You can also do a quick test by pinching off the hose on either side of the unit when the engine is at operating temp. If the idle speed drops when the hose is pinched, air is still flowing through the AAD. One quick test if the plate isn’t closing, is to see if there is power at the connector when the ignition and fuel pump is on. Use a test light to check the terminals on the connector, one of them will have power.

The last item in the list is the electrical side of the Warm Up Regulator. If this isn’t functioning correctly, the WUR will not lean out the pressures correctly, and the mixture will be out when warm.

First test there is power at the plug. Check this like the AAD; turn the ignition to ON with the pump on (or run the engine) and use a test light to see if one of the terminals has power. If so, continue.

Now shut off the ignition and test for resistance of the heater coil in the WUR. All heater coils will have a different value depending on spec of the unit. In my case its 26Ohm, but generally as long as you don’t have an open circuit you should be OK. Test by checking resistance between the two terminals on the WUR.

Now, the final, and hardest to test item is the injectors. These are known to leak and have erratic spray patterns when old. The only way to test these is to remove them from the manifold and test them. In this case it requires removing the plenum to access them.

Remove all the injectors from the manifold, and using a series of jars or containers of similar size, place the injectors into the jars.

Now, be safe because there is about to be raw fuel all over the show.

Turn the fuel pump on. Without lifting the sensor plate, check to see if any fuel is leaking from any injectors. Any leaking is bad. Now lift the Air Sensor Plate to trigger the injectors. Inspect the flow pattern of each. After a few seconds, shut off the pump and measure how much fuel came from each injector. It should be very similar between injectors.

If they are leaking, the flow pattern isn’t a nice cone, or there is a variation in flow, scrap the injectors and replace them. They aren’t really serviceable, although some people do claim to “clean” them, there is no way to remove and replace the filter inside the injectors.

With all the testing complete, refer to your notes and see how you are doing. If all tested well, all you may need to do is tweak the idle mixture if it isn’t idling well, or the issue may reside somewhere else in the engine (ignition, compression etc).

If you have issues with the injection, the next part will show you how to adjust everything. Part 3.

Was this post helpful? How about dropping in a donation to help with the running costs of this site

4.1 15 votes
Article Rating
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
1 year ago

Warming up the WUR: In my car, 1974 911, the WUR gets power from the alternator, (ie a spinning alternator), not direct from the battery. Makes sense, you want the WUR to get warm as the engine gets warm, not simply when someone turns on the key, and then begins 10 minutes of texting before remembering they were getting ready to go somewhere.

8 months ago

Can you please let me know your WUR and Fuel distributor numbers?

7 months ago

Thank you for this very complete documentation! 52,000 kms… when i kick the gas, and then take my foot off the pedal , the idle goes down very low, around 5500 rpm, then goes back up. DAA to be verified. But it is not easy to access. romain from France,mercedes 190 E 2.0