Audi Cars 1999-05

Diagnosis & Testing


Before beginning any diagnosis and testing procedures, visually inspect the components of the ignition system and engine control systems. Check for the following:

Discharged battery
Damaged or loose connections
Damaged electrical insulation
Poor coil and/or spark plug connections
ECM connections
Blown fuses
Damaged spark plugs

Once an quick, overall inspection has been performed, if available, use a suitable Scan Tool, such as the VAG 1551 or VAG 1552, or their equivalent. These tools are required to interrogate the fault memory of the Engine Control Module (ECM).

When attempting to search for ignition troubles, also keep in mind that various sensor inputs that the ECM uses to calculate ignition timing may also affect engine performance.

Spark plugs are a good indicator for determining the operational condition of a cylinder. A misfire could be caused by a fuel related problem that acts as though it is spark related. If the plugs are removed, keep them in order to match the cylinder from which they were removed. For additional information, please refer to the following topic(s): General Information and Maintenance, Routine Maintenance and Tune-up, Driveability and Emission Controls.

Cylinder Drop Test

The cylinder drop test is performed when an engine misfire is evident. This test helps determine which cylinder is not contributing the proper power. There are only two safe methods to perform this test on engines using coil packs or individual ignition coils. These two methods are:

Grounding the high voltage plug wire for the particular cylinder
Disconnecting the fuel injector connector for the particular cylinder

Because of the location of the spark plugs on V6 and VR6 models, and because of the location of the individual coils on the 1.8L engines, grounding the ignition signal for a particular spark plug is difficult and potentially harmful.

The best and safest method to perform a cylinder drop test is to disconnect the fuel injector connector for the cylinder being checked.

Perform the test on all cylinders to verify which cylinder is suspect.

  1. Place the transaxle in P if an automatic, or in Neutral if a manual. Engage the emergency brake, and start the engine and let it idle.
  3. Disconnect the fuel injector connector for the cylinder being checked.
  5. The engine will sputter, run worse, and possibly nearly stall. If this happens reinstall the connector and move to the next cylinder. If the engine runs no differently, or the difference is minimal, shut the engine off and inspect the spark plug wire, and spark plug. If no problems are found with the plug wire or spark plug and the ignition system components are determined to be in proper working order, it may be necessary to check the fuel system. Check the following:

    Fuel injector operation
    Fuel injector signal from the ECM
    Fuel injector wiring
    Fuel injector electrical connector

  7. If the ignition and ignition system components are determined to be in proper working order, it may be necessary to check the mechanical condition of the particular cylinder by:
    1. Performing a compression test: if compressed air and a leakdown tester are available, perform a leakdown test, following the tool manufacturer's recommendations.
    3. Check for a vacuum leak, such as a leaking intake gasket or injector seal.
    5. Check for an excessive build-up of carbon on the intake valve. Excessive carbon buildup can absorb the fuel sprayed by the injector, much like a sponge.

  9. Make sure to perform the cylinder drop test on all cylinders.
  11. If the engine runs no differently on all cylinders, but the engine still lacks power, or the difference is minimal on all cylinders:
    1. Check the cam timing.
    3. If the cam timing is OK, verify the overall mechanical condition of the engine.
    5. Check the fuel system pressure, pressure regulator and fuel pump output.

Secondary Spark Test

The best way to perform this procedure is to use a spark tester (available at most automotive parts stores). Three types of spark testers are commonly available. The Neon Bulb type is connected to the spark plug wire and flashes with each ignition pulse. The Air Gap type must be adjusted to the individual spark plug gap specified for the engine. The last type of spark plug tester looks like a spark plug with a grounding clip on the side, but there is no side electrode for the spark to jump to. The last two types of testers allows the user to not only detect the presence of spark, but also the intensity (orange/yellow is weak, blue is strong).

  1. Locate and remove the fuel pump fuse.
  3. Start the engine and allow it to run until it stalls. I should stall in less than a minute. If it doesn't, make sure the correct fuse has been removed. For additional information, please refer to the following topic(s): Fuel System, Gasoline Fuel Injection System, and Chassis Electrical.
  5. On V6 and VR6 engines:
    1. Disconnect a spark plug wire at the spark plug end.

  7. On 1.8L engines, remove the coil pack.
  9. Connect the plug wire to the spark tester and ground the tester to an appropriate location on the engine.
  11. Crank the engine and check for spark at the tester.
  13. If spark exists at the tester, the ignition system is functioning properly.
  15. If the spark is weak or does not exist at the spark plug wire, perform diagnosis of the ignition system using individual component diagnosis procedures.