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K-Series Fuel Injection SystemsPosted 6/11/1997By Jim Newkirk
1993-95 Chrysler Concord, New Yorker, LHS, Dodge Intrepid and Eagle Vision.
The K-Series fuel injection systems are continuous mechanical fuel injection systems used on a wide variety of European vehicles, including such makes as Volkswagon, Audi, BMW, Mercedes, Porsche, Volvo and Saab. The system is one of the most common fuel injection systems on the market today, but also is one of the least understood. The K-Jetronic System constantly injects fuel into the engine as long as the car is running and air flow is present to move the sensor plate in the airflow sensor. The sensor plate is connected to an arm that pushes up on a plunger located in the fuel distributor. As airflow changes, the movement of the sensor plate and the plunger increase and decrease the volume of fuel injected into the engine. Since fuel is being injected constantly, fuel pressure will have a direct affect on driveability. As a matter of fact, fuel pressure is the single most critical element when diagnosing driveability problems in the K-Series fuel injection systems. An accurate fuel pressure gauge must be used when testing these systems, with a range of 0 to more than 100 psi. You will also need a digital volt ohm meter (DVOM) that can read milliamps. You will work with three types of pressures when diagnosing these systems: system pressure, control pressure (also known as counter pressure) and rest pressure. System pressure is the total fuel pressure produced by the fuel pump on a constant basis. The fuel pump must be able to maintain this pressure during all driving conditions from idle to wide open throttle. As a rule, system pressure will run about 5 to 5.5 bar pressure, or 75 to 85 psi (1 bar = about 15 psi) and the pump should be able to produce a minimum volume of 1 pint in 15 seconds. When deadheaded, the K-Series fuel pump will produce about 1.5 times the system pressure or about 110 to 120 psi. System pressure is a function of volume of fluid moved against a restriction, so to maintain system pressure at the desired level, there must be some type of restriction built into the fuel system. This restriction is more commonly known as the fuel pressure regulator. The fuel pressure regulator restricts the return of fuel to the tank by a calibrated amount, maintaining system pressure at the desired level. On early K-Jet systems, this regulator was a slide valve (also known as a push valve) internal to the fuel distributor. Fuel pressure could be adjusted by adding or removing shims from the valve. On later K-Jet systems, the regulator is the conventional diaphragm type. Control pressure (or counter pressure) is the pressure that is metered to the top of the fuel plunger on a K-Jet system. By changing the counter pressure, the resistance to plunger movement is changed, allowing enrichment and enleanment of the fuel mixture to the engine. On a car equipped with K-Jet, this pressure is controlled by the warm-up regulator. The warm-up regulator only compensates for engine temperature and is therefore a rather coarse control of fuel mixture. (Some K-Jet warm-up regulators also have a vacuum port to help with the acceleration enrichment and deceleration enleanment function.) Typical control pressures on a K-Jet warm-up regulator are 55 psi with the engine at full operating temperature and 20 to 30 psi on a cold engine. (The colder the engine, the lower the pressure.) A car equipped with K-Jet Lambda also changes control pressure with a warm-up regulator (with pressures similar to a plain K-Jet system), but also controls lower chamber pressure in the fuel distributor by bleeding pressure through a frequency valve. By modifying lower chamber pressure, a change in volume of injected fuel is made, enriching or enleaning the mixture. The frequency valve is nothing more than an electrically duty-cycled fuel pressure regulator controlled by an on-board computer in response to an oxygen sensor signal. This system provides a more precise and rapid control of fuel mixture. Typical duty cycle on a properly running engine is 45 percent to 55 percent duty and fluctuating. A quick test of this system is to start the engine and test the frequency valve for vibration or noise -- it should vibrate. Also, unplugging the oxygen sensor will put the system in open loop and fix the frequency valve at a 50 percent duty cycle. The KE-Jet system provides quicker response and more precise control of fuel mixture than the K-Jet Lambda system and is the current K-Jet system in use. This system uses a device called a differential pressure regulator to control fuel mixture in response to both engine temperature and oxygen sensor signals. In the KE-Jet system, counter pressure is broken down into primary counter pressure and control counter pressure. Primary counter pressure is the pressure applied to the top of the fuel plunger. This pressure stays constant and is the same as system pressure. Control counter pressure is modified by the differential pressure regulator and is actually the lower chamber pressure in the fuel distributor. By modifying lower chamber pressure, the fuel mixture can be enriched or enleaned in response to temperature and oxygen sensor signals. Typical control counter pressures are 4 to 7 psi less than system pressure on a fully warmed engine and 17 to 20 psi less than system pressure on a cold engine (typical system pressures are 5.0 to 5.5 bar or 75 to 85 psi). The signal to the differential pressure regulator from the computer is measured in milliamps of current. To test this signal, a DVOM must be placed in series with the differential pressure regulator. Typical current values are 80 milliamps cold engine (15k ohm resistor in place of coolant temp sensor to simulate a cold engine condition); 120 milliamps during cranking (this is a crank enrich function to aid starting); and 8 to 12 milliamps warm idle. (Note: always check service manual for values.) These values correspond to the fuel pressures listed. In other words, at 80 milliamps current you should show 17 to 20 psi less than system pressure. Rest pressure is the fuel pressure maintained in the system by the fuel accumulator after engine shutdown. The fuel accumulator is a large spring-loaded diaphragm that maintains a pressure of about 1.5 to 2.0 bar for 30 minutes or more after engine shutdown. This rest pressure provides for fast restart and prevents fuel percolation or boiling (vapor lock). Always check the service manual for the car line you are working on for proper rest pressures and times. Typical symptoms caused by accumulator problems are extended crank time and hard start hot. With an understanding of the system and the proper tools, K-Jetronic fuel system service is a straightforward procedure that can keep your service bays full all year long. Give me a call if you have any questions!
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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