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3.1 Basic Troubleshooting Principles

The basic function of any fuel injection system is to meter fuel in the correct proportion to the amount of air being drawn into the engine, thus achieving the correct air/fuel mixture. Since proper fuel system operation depends on accurate measurement of the intake air, any unmetered air entering the system via vacuum leaks will cause poor running.

The entire system should be inspected for disconnected or damaged hoses, loose connections, or other air leaks. The continuous injection systems (CIS, CIS-E, and CIS-E Motronic) are particularly sensitive to fuel pressure. Check the fuel lines and connections for signs of leaks. Especially inspect the electrical connections to the various components. Aside from loose connections, many problems can be caused by contamination at the connectors which affect the electrical signals. Make sure all connections are clean, tight, and dry.

Generally, fuel system problems fall into one of four categories: cold start, cold running, warm running, and hot start. Warm running is the most basic condition. Before troubleshooting a problem in any other category, the system should be working well and properly adjusted for normal warm running. If the car will not start, even the simplest potential problem cannot be ruled out. Make certain that the battery is adequately charged and that there is fuel in the tank.

Various adjustments to the basic fuel metering are required to adapt to other than warm running conditions, and certain components are responsible for adapting the fuel metering in certain ways. By concentrating on the sensors and components that adapt or correct the fuel metering for the particular condition, troubleshooting can be greatly simplified. For example, if the engine will not start when cold, the components responsible for cold start enrichment are most likely to be at fault, and should be tested first.

Regardless of the type of fuel system, the cars covered by this manual may experience the gradual onset of driveability problems such as rough idle, stalling, hesitation, or poor acceleration, mainly during warm-up. These problems are frequently related to the use of gasoline with insufficient cleaning additives which results in clogged fuel injectors and intake valve carbon deposits. To prevent such problems, Volkswagen recommends the exclusive use of high quality gasoline and the periodic use of a fuel system cleaner added to the gas tank. See LUBRICATION AND MAINTENANCE for more information on the fuel additive. In extreme cases, more comprehensive work may be required. For complete information on approved methods of fuel system cleaning and intake valve decarbonizing, consult an authorized Volkswagen dealer's service department.

The following cautions and warnings apply whenever servicing the fuel system.


During many fuel system test procedures, fuel will be discharged. Do not smoke or work near heaters or other fire hazards. Have a fire extinguisher handy.


Before making any electrical tests with the ignition turned on, disconnect the high-voltage cable from the center tower of the distributor and ground it to some clean metal surface to prevent sparks.

To prevent damage to the ignition system or the electronic fuel system components, connect and disconnect wires and test equipment only with the ignition off.

Thoroughly clean connections and surrounding areas before loosening.

Cleanliness is essential when working with parts of the fuel system open. Avoid the use of compressed air, and avoid moving the car. Only install clean parts.

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