Tuesday, January 15, 2008

Last weekend, for fun, and because it was necessary, I changed the fuel injectors in the white Jetta. This is the car - if the reader recalls (Hi mom!) - that lost two manifold studs last summer. Take heart, replacing fuel injectors is far easier than replacing exhaust studs. I’ll go into this procedure in just a moment, but first, a word on the Digifant fuel injection system.

The Digifant Engine Management system is a computer-controlled fuel injection system designed by Volkswagen and first introduced in 1986 on its aptly named van: the Vanagan. A year later the system became standard on Volkswagen Golfs and Jettas equipped with 8-Valve, gasoline-powered four-cylinder engines. The sporty Golf GTi and Jetta GLi, both powered by 16-valve four cylinders, retained a mechanical/electrical hybrid fuel injection system known as CIS-Motronic. It is also important to note that the Digifant system employed on the Golf/Jetta platform – with the exception of Golfs and Jettas sold in California after 1991 - was a variant of the original Digifant system and was, therefore, known as Digifant II.

The Digifant II system is often criticized by Volkswagen enthusiasts. The main complaint being that the finicky system is prone to frequent component failure and allied performance and drivability issues. These criticisms are not without merit, however, it is equally important to recognize the attributes of this injection system. In the Jetta and Golf models, Digifant II directly replaced the antiquated - if not abnormally reliable - family of Bosch mechanical, fuel injection systems known simply as CIS - which stands for Continuous Injection System. All CIS systems relied upon an air-vein to measure intake airflow and to control fuel flow through a mechanical fuel distributor and injectors. Though rugged and reliable, the CIS system was not designed to account for factors such as intake air temperature and specific throttle position and is, therefore, limited in its ability to tailor fuel delivery for every situation. It is true that later iterations of the CIS system incorporated electronic controls that worked in tandem with the CIS mechanics. These controls included – among other things - an oxygen sensor tied into a frequency valve to monitor and control fuel/air mixture as well as a full-throttle enrichment switch to prevent lean-running during full-throttle acceleration. However, the Digifant system integrated all of these features into a fully digital system.

Starting in 1993, with the introduction of the third generation of the Golf/Jetta platform, Digifant II was replaced by the Motronic fuel delivery system. Variations of the Digifant system, however, continued to be employed by other Volkswagen models including the Fox, Corrado and the Mexican produced classic Beetle.

This is not as boring as you think it is.

At the heart of the Digifant II system is a computer brain better known as an electronic control unit (ECU). The ECU regulates fuel injection and engine timing based on inputs received by a suite of sensors that measure – among other things – throttle position, coolant temperature, intake air temperature and oxygen content in the exhaust.

Fuel injection is controlled via electronically actuated fuel injectors that are housed in a common fuel rail. Each cylinder is supplied by a single, solenoid-operated injector which sprays a fine mist of fuel toward the backside of each intake valve. Each injector is energized through the ignition coil and grounded through the ECU to complete the circuit. The amount of fuel injected into the cylinder is metered by the duration of time that the injectors are open. The ECU determines this duration by measuring engine load (this data is provided by the Air Flow Sensor) and engine speed (this data is provided by the hall sender in the distributor). The data is then compared to a map, or table of values, stored in the ECU’s memory. The map consists of 16 points for load and 16 points for speed. These 256 primary values are then modified to account for data concerning coolant temperature, intake air temperature, oxygen content of the exhaust and throttle position, allowing the ECU to select from 65,000 possible injector duration points. (To think, my dad used to complain about how tricky carburetors were.)

From reading all that crap above, one could get the impression that fuel injectors are a precision part, and, that if an engine’s injectors are leaky, dirty and not spraying properly the so equipped car will have drivability issues. That was my conclusion. After inspecting and testing the injectors, I realized that the old fuel-slingers were no longer making the grade. The first signs of trouble were a decrease in power and a persistent smell of gas in the motor oil. These symptoms alone could point to any condition that would allow excess fuel to wash past the piston rings and into the oil pan. However, performing a simple injector leak test uncovered the root of the problem to be – duh – a leaky injector.

To do this at home, you will need the Bentley manual! Follow the procedures for removing the fuel rail. It’s a straight forward project that requires you to remove the air box and intake plenum, idle stabilizer valve, after-run fan switch and a few vacuum hoses to get at the fuel rail. Once there, disconnect the fuel feed, fuel return line and signal input connection from the fuel rail. By the way, this is not a good time to light a blunt or burn incense. Next, remove the two Allen bolts that hold the fuel rail in place and pry the fuel rail and injectors free by wiggling the assembly side to side while pulling away from the engine head. If this doesn’t work, stop being a sally and pull harder.

Once the rail is removed, reattach the two fuel lines. For this test, do not reattach the fuel rail’s signal connector. Now have your lackey brother, despondent girlfriend or favorite pet goat turn the ignition to the on position. Do no engage the starter. You’ll hear the fuel pump turn on for an interval of time as fuel pressure builds in the system. Hold the rail so that the injector tips are pointed down and watch to see if the injectors leak. The standard is two drops per injector per minute. Anymore than this and the offending injector needs to be replaced. I found that only one injector was leaky. But, since the injectors were eighteen-years-old and – more than likely – near the end of their operational life, I opted to replace all four.

The injectors are held on the fuel rail via small clips that can be pried off with a flat-head screw driver (one of my all-time favorite tools). Also, each injector is wired into a small wiring harness that is part of the fuel rail assembly. When disconnecting the injectors from their individual electrical connections, take note of how the connector clip is fitted as it is a one-way deal. The injectors, once freed from the wire loom and retaining clips, slip easily out of the fuel rail ports.

If your injectors pass the leak test, refer to the Bentley manual for injector spray testing and continuity testing.

Otherwise, installation is the reverse of removal.

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