Q: car won't start but turns over

When a car won’t start, but the motor turns over, the first thing you need to figure out is what is missing. All gas powered vehicles need a few things to run. Fuel pressure, spark and injector pulse.

The fuel pressure is easily tested with an appropriate fuel gauge kit that you can rent from most local auto parts stores. The are a number of ways to test for proper fuel pressure, but the main specification for your car is 68-78 PSI with the fuel test gauge connected and with the key turned on. If the fuel pump is working, it should turn on for a few seconds, build pressure then turn off. Sometimes you will need to cycle the key twice in order for the fuel pump to reach maximum pressure. Usually with a crank no start this method is useful, but there other specifications that I don’t believe apply in your case.

The next thing is does the ignition system provide spark? Ideally, you should use a spark tester to test for spark. You will do this by removing a spark plug wire from a spark plug, plugging in the spark tester and cranking the motor. You are looking for spark at the spark tester. A spark tester doesn’t have to be used, but you will need to be creative and be sure you don’t shock yourself.

It is possible you will have spark and fuel, but if the PCM (Powertrain Control Module) is not opening the injectors, fuel will not be supplied to the cylinders. To test this you will need what is called a noid light. It is a very simple light bulb configured to plug into the injector connector of your car. You can usually find noid lights at your local auto parts store for fairly cheap. The use of a noid light requires you to disconnect a connector at one of the injectors and plugging the noid light into the connector. Trust me, this will all make more sense once you see what a noid light is. Now that you have a noid light plugged in, you will need to once again crank the motor. When the motor is cranking, the noid light should flash. If it doesn’t, I suggest trying another injector just to make sure it isn’t a poor connection somewhere at the first connector you tested.

After you have gathered this information it is time to draw some conclusions. This is where it gets complicated, if you didn’t think it was already. Interpreting the data you just gathered can lead to many conclusions, so I will have to narrow them down to the most common. But they are not at all the only possibilities. Sorry to say, but your vehicle is over 20 years old and it’s vehicles such as this that will often exhibit out of the ordinary malfunctions.

If your car doesn’t have fuel pressure, use a soft mallet type hammer or your foot to bang on the bottom of the fuel tank while someone cranks on motor. If the car starts, the fuel pump may be bad. If nothing happens, check for power and ground at the fuel pump or as close to the fuel pump as you can get. If it doesn’t have power or ground, this is where you will need to study a wiring diagram to figure out where the power supply has failed.

If the car doesn’t have spark and injector pulse, this points to something like a crank sensor or camshaft sensor. Possibly the engine temperature sensor as well. These types of sensor affect the PCM’s ability to initiate spark and fuel pulse. If it lacks only spark, you will need to focus on the ignition system only. Begin with the distributor cap and rotor. Are they in good condition. If you have spark at the spark plug wires, how long has it been since you replaced your spark plugs. If there isn’t spark and your ignition system components look good, I would suspect the ignition control module. In your case that is the PCM, which normally don’t fail.

If you don’t have injector pulse but you have spark and fuel pressure, you are looking for what the PCM uses to supply injector pulse. On most vehicle this is usually a crank sensor.

I could go on and on about various methods of diagnosing such a problem. Unfortunately your 1990 Audi lacks many advantages that newer OBD II systems have available that aid in diagnosis. This means you will likely have to perform pinpoint tests at the suspected failed components. You will definitely need quality information on your car. I recommend the one year subscription with All Data. Keep in mind, this is a professional information system and it is a lot for many people to take in. So if after reading this you feel frustrated and confused, I definitely recommend scheduling an appointment with a certified mechanic who can assess your starting problem firsthand because I have only just touched the beginning of such a diagnosis.

Good luck, I hope I have offered you information you can use.