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K-Series Fuel Injection Systems

Copyright Jim Newkirk, Autoinc Magazine, 1997 - All rights reserved

[Jim's original article contained details of a system not used in the UK. It also used limited American English terminology and SAE units. Minor editorial amendments have been made to cater for these issues. The 'K' in K-Jetronic is from the German word 'kontinuierlich' - continuous.]

The K-Series fuel injection systems are continuous mechanical fuel injection systems used on a wide variety of European vehicles. The system is one of the most common fuel injection systems on the market, 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 6 bar.

You will work with three types of pressures when diagnosing these systems: system pressure, control pressure and residual 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 5.5 to 6.5 bar pressure and the pump should be able to produce a minimum volume of 900ml in 30 seconds. When deadheaded, the K-Series fuel pump will produce about 1.5 times the system pressure. 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 - the kit is Audi part 035 198 685. On later K-Jet systems, the regulator is the conventional diaphragm type - Audi part 034 133 534L for the ur-quattro's MB engine.

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. Typical control pressures on a K-Jet warm-up regulator are 3.4 bar with the engine at full operating temperature and 1.2 bar on a cold engine. (The colder the engine, the lower the pressure.) There is an electric heater in the warm-up regulator, driven by the fuel pump supply circuit. The WR and GV engines should have a warm (oil over 50°C) control pressure of 3.45 to 3.75 bar at idle with the hose to the control pressure regulator in place, and 2.75 bar to 3.05 bar with it disconnected.

Residual 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 2.8 bar for 30 minutes or more after engine shutdown. This residual pressure provides for fast restart and prevents fuel percolation or boiling (vapour lock). Always check the service manual for the car line you are working on for proper residual pressures and times. Typical symptoms caused by accumulator problems are extended crank time and hard hot starting.

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!

Jim Newkirk is ASE master and L1 certified. He is an AutoLine Telediagnosis import specialist with 19 years of experience. At least, that was his experience when he wrote this article eleven years ago.

AutoInc. Magazine ®, Vol. XLV No. 6, June 1997

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