No start

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Engine does not turn over

If you get in, turn the key, and nothing happens- here's an explanation for all the various things that can go wrong.

  1. Battery: does the battery have enough power? Turn the ignition on and switch on a relatively high-current item like your headlights. Sometimes batteries are charged but have high internal resistance, or there is corrosion or a poor connection somewhere. At a low current, a small resistance causes only a slight voltage drop. At 50-100A required to turn over your engine, even a slight resistance causes a large voltage drop.
  2. Ignition switch: there are two ways to tell if your ignition switch is working:
    • Load reduction: your Audi may switch off many electrical items in the car when the key is turned to "start". Think back: do your headlights, radio, or what have you normally switch off when you start the engine? Does the same thing happen now?
    • Starter solenoid: the starter solenoid is an electrically-actuated switch located on the starter, activated by current from the ignition switch. If both the ignition switch and solenoid are working, you will hear a loud click from the engine compartment. This test is not conclusive when the solenoid does not click. It could be a failed solenoid or a failed ignition switch
  3. Starter solenoid: The starter solenoid switches current from a heavy battery cable to the starter (the starter is grounded through the engine block, usually.) With age, the moving parts of the solenoid may gum or rust up.'
  4. Braided cable: On many cars, there is a thick braided copper cable that runs from the solenoid to the starter. It is known to corrode with age (when installing a new or rebuild starter, it is strongly recommended to heavily paint this braid with a tough paint, preferably one that is flexible. Many rebuilders do this.)
  5. Starter:
    • Rotor winding or commutator (contacts which mate with the brushes)
    • Brush wear
    • Bendix Gear: The Bendix gear is the gear on the head of the starter which engages the engine flywheel. The angular momentum of the gear and the sudden motion of the starter spinning causes the gear to pop out via curved grooves on the starter shaft, and engage the flywheel. A spring keeps the gear away from the flywheel otherwise. With age, particularly disuse- rust may build up, the lubricant could dry up, or dirt could accumulate the gear may not pop out.
  6. Flywheel teeth broken: it is possible for teeth to break off the flywheel, and things may line up such that the bendix gear ends up positioned such that it has not teeth to engage. This type of failure is extremely rare.

Starter damage from impact

Do not use a hammer to "test" if the starter is good or not. Bosch places warning labels on their starters specifically warning against this. Why?

  • You could cause the permanent magnets to break; they're brittle.
  • You could cause the case of the starter to be deformed and push the magnets against the rotor, causing quick death to the starter
  • You could cause the permanent magnets to demagnetize; magnets demagnetize when struck or dropped.
  • There are about a dozen things that could be wrong that are more likely than "it needs smacking with a hammer"
  • In that dozen-thing list are a lot of non-destructive, easy-to-check items.

Engine turnover but no catch

If you turn the key and the starter turns the engine, but the engine does not start, then this tutorial should help you pinpoint the cause.

To run, your engine needs three things:

  • Fuel
  • Air
  • Spark

When you turn the key to start the car, a few things happen.

  1. First, turning the key activates the fuel pump in (or near) the fuel tank. Fuel is pumped from the tank to the fuel rail, which feeds the fuel injectors. The ECU tells the injectors when to open and spray fuel into the cylinders.
  2. Second, the starter motor turns the engine over, forcing the pistons up and down. On the four cycle engine in your Audi, the cycles are: intake, compression, ignition (power), and exhaust. In the vernacular: suck, squish, bang, blow. On intake, the downstroke of the piston pulls air, mixed with fuel from the injector, into the cylinder.
  3. Third is spark. There is a coil or coil pack(s) that delivers high voltage to the spark plug at the right moment, as directed by the ECU.

When all three things happen in the right order at the right moment, your car should start. If any one of those three things is missing, your car will not start.

Problems

The following sections will help pinpoint the problem.

No Fuel

Lack of fuel to the engine can be caused by a few things. Starting with most likely and easiest to diagnose causes:

  1. There is no fuel in the tank (don't always trust your fuel gauge).
  2. A fuel line is clogged
    • A fuel filter is clogged, keeping fuel from flowing.
    • A metal fuel line is clogged due to being crushed or
    • A rubber fuel line has collapsed, probably due to age.
  3. The fuel pump is not working.
    • The pump is good, but is not getting power (12V).
    • The pump is getting power, but it is broken.
  4. Injectors are not working.
    • An injector is not working; the car may start, but it runs poorly. It is rare for all (4, 5, 6, or 8) injectors to go bad at once.
    • Broken wiring or a corroded connection keeps the signal from the injector.
    • A bad ECU cannot trigger the injectors.

More details can be found at Testing_for_fuel.

No Air

Starting with the most likely causes:

  1. The air cleaner is clogged.
    • Dirt has built up over time, clogging the filter.
    • An animal has built a nest in the air cleaner.
  2. The timing belt is broken.
    • If you have the misfortune of a broken timing belt, the valves won't open, which prevents the air/fuel mix from being drawn into the cylinder.
  3. The exhaust system is clogged. If the air can't exit, new air can't enter.
    • Clogged catalytic converter.
    • Clogged muffler.
    • Crushed exhaust pipe.
    • Someone stuck a potato up the tail pipe. Seriously, it will prevent your car from starting.

No Spark

Lack of spark is probably the most common of the three problem areas that might keep your car from starting. The old adage, "most carburettor problems are electrical" still applies. Most likely causes first:

  1. Bad spark plug wire(s), most likely due to old age.
  2. Bad spark plug(s), again due to age.
  3. Bad wiring between the Engine Control Unit (ECU) and Power Output Stage (POS), or the POS and coil pack.
  4. Bad coil(s).
  5. Bad POS.
  6. Bad ECU. The ECU is the brains of the engine. If it is dead, so is your engine.

See Testing_for_spark for more information.

Conclusion

Your first step should be to pull the diagnostic codes for your car. Most of the problems listed above will throw a DTC code. The DTC will tell you where the problem lies. On my car, DTC 4332 pointed to the ignition. If you get no code, look to the causes that don't have sensors, things like an empty gas tank or clogged catalytic converter.

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