Car Diagnosis: it won't start

After an online discussion elsewhere late last week, I felt a need to visit a the subject of car repair.

Or diagnosis.

Some time ago, a distant relative posted a question on FaceBook. The question was something like, "My car won't start. Any advice before I call to have it towed to the shop?"

This question ought to be easy to answer.

There are three very big domains that the problem could be in. And "the car won't start" could mean any of these.
  1. Turn the key, and nothing happens
  2. Turn the key, and hear the starter motor turn the engine over...but the engine doesn't 'catch' and start running on its own
  3. Turn the key, and the engine starts running, but dies almost immediately.
Each of these point to the problem, but don't give definite answers.
(I'm assuming, while going through this list, that you have a choice between doing something yourself, a friend whose done some car repair, or a tow-truck trip to a repair shop.)
  1. Problem: key turned to "START", but no response from starter motor.
    1. Can you test the headlights?
      Turn the key to "RUN", but not to "START".
      Find the switch for headlights, and try to turn them on.
      1. If the lights are dim, or not visible at all, then the problem is likely a low battery
        You might be able to start the car by the method of jump-starting.
        To do this, you need jumper cables and another vehicle. 
      2. A helpful guide for diagnosis, and attempting a jumpstart, is here.
        However, if you can't confidently identify the "+" and "-" terminals on both batteries, it's not a good idea to guess.
    2. If the lights are bright, the problem is likely the starter motor.
      1. Fixing a starter motor typically involves un-bolting it from the underside of the engine, and putting a new one in.
      2. If you don't feel confident in finding the motor, or successfully un-bolting it, you might be able to find someone to help you.
        Or call a tow truck to have the vehicle delivered to a repair shop.
  2. Problem: key turned to "START", engine turns over but doesn't catch.
    1. It could be that the ignition system isn't delivering a spark where it is needed, when it is needed.
      Do you feel confident you can identify a spark plug and its associated wire?
      If not, find someone who can help you...or you'll have to find a repair shop that can handle it.
      1. Assuming you have identified one of the spark plugs, and own (or are willing to spend money on) a spark-tester:
        Unplug the wire from the spark plug, plug the wire into the tester, and use the tester to discover if if you have spark. 
      2. Assuming you have the confidence and tools to un-screw the spark plug from the engine:
        Unplug the wire, un-screw the spark plug, and put the plug back into the wire.
        Hold the wire-boot, such that the side of the spark plug is touching the engine-block.
        Have someone else try to start the engine.
        1. If you see a spark jumping across the gap when the engine turns, then the ignition system is delivering a spark when appropriate.
          You should probably go to 2.B. below.
        2. If you don't see a spark, then you need to fix something in the ignition.
          You could end up replacing spark plugs, wires, ignition control modules, distributor caps, crank-position sensors...
          Or you could end up asking a talented friend, or a repair shop, to check/replace these things for you.
    2. It could be that the fuel system is not delivering fuel to the engine.
      1. If you had the spark plug out while testing in 2.A.ii, you ought to be able to tell whether any gasoline was delivered to the engine. You should be able to smell gasoline inside the cylinder.
      2. Another test: most modern cars use a fuel pump. This pump turns on automatically when the ignition goes from "OFF" to "RUN". 
        1. If you can hear a slight hum from the rear of the car after turning the ignition from "OFF" to "RUN", then the fuel pump is probably providing fuel.
          You might have a leaky fuel line, or a blockage somewhere between the fuel pump and the point where fuel is injected into the engine.
          This is probably something to take to a repair shop. 
        2. If you cannot hear the hum of the fuel pump starting...then you probably also want to take the vehicle to a repair shop.
          Replacing a fuel pump usually involves removing a fuel tank and opening it up. Easier than replacing all the fuel lines between the tank and the engine. 
    3. It could be that the engine has fuel and spark, but no air providing oxygen.
      This is somewhat rare. 
      1. Many modern engines have an airflow sensor that is used to control fuel injection and ignition.
        Replacement parts can be purchased. In a pinch, it might be possible to clean the sensor.
        There is little risk in getting help from a knowledgeable friend in this. There is little risk in taking this to a repair shop, but you'll end up paying a somewhat-high hourly rate for replacing a part that is easy to replace.
      2. If the problem is a dirty air filter, it is possible to attempt to start the engine without the filter.
        However, it is very foolish to run the engine for more than a few seconds without the air filter. The filter is designed to keep dirt and grit out of the engine. If too much dirt or grit gets into the interior of the engine, it can lead to scratches on important surfaces (like cylinder walls) inside the engine. These surfaces are supposed to be smooth. Scratches can introduce all kinds of expensive-to-repair problems.
        Happily, air filters are usually easy to replace, even for automotive-repair novices.
        They are also usually very affordable, and usually on-the-shelf at the car-parts store. Make sure you get the correct filter. 
      3. Very few cars use carburetors these days. If your car does use one, it probably needs to be cleaned.
        If you're deciding between a knowledgeable friend and a repair shop, this one is a toss-up. Either method is good.
      4. If the car uses some sort of fuel-injector to mix fuel and air, then you likely need to diagnose/replace the part.
        In this one, a knowledgeable friend can help, but a repair shop might be better.
  3. Problem: the starter motor turns the engine, and the engine sputters and dies.
    This falls into the same diagnostic pattern as Problem 2...Spark, Fuel, Air.
    Except this time, you have enough to get the engine running for a moment, but not enough to keep it running.
    1. If you are able to test spark-plugs, test each of the spark plugs on the engine.
      It's generally a good idea to pull out only one plug and wire out at a time. The pattern for connecting wires to plugs may not be intuitive, easy-to-remember, or obvious. If you unplug them all, and plug them back in the wrong order, you're worse off than when you started.
      1. Once you figure out which spark plug(s) are not firing, you probably have a very simple repair. Usually, it is specific wires or specific plug(s) that are bad and need to be replaced. 
      2. However, as long as you're replacing some plugs (or wires), it might be good to replace them all.
      This kind of work is easy to do yourself, if you have the ability (and tools) to pull a spark plug. However, not all combinations of spark-plugs-and-wires will be available at the car-parts store.
      A talented friend or a repair shop can do this repair also.
    2. If the problem is with lack of fuel, then either 
      1. The fuel pump can't supply enough pressure, because the pump is wearing out.
      2. The fuel pump can't supply enough pressure, because a leak in the fuel-line between pump and engine
      3. The fuel pump can't supply enough pressure because a fuel line is partially clogged.
        All three of these generate the same advice as 2.B. above. You'll probably want to take the problem to a repair shop.
      4. It's also possible that a fuel injector has failed, or is clogged.
        If clogged: depending on how serious the clog is, and how soon the engine dies, you might be able to fix this with a fuel-injector-cleaner, purchased as the car-parts store.
        However, if the injector has failed, it needs to be replaced. This is probably best done at a repair shop.
    3. If the problem is with air-supply, then either
      1. An air sensor is malfunctioning. (same advice as in 2.C.i above)
      2. The air filter is clogged, and needs to be replaced. (same advice as in 2.C.ii. above)
      3. If a carburetor is present, it may have a partial clog that needs to be cleaned out. (same advice as in 2.C.iii above)
      4. Otherwise, an injector may need to be cleaned/replaced. (same advice as in 2.C.iv above.)
Most car-won't-start problems can be diagnosed with this process. However, some don't quite fit in that list.

I have, in my car-ownership career, met a problem that looked like a bad starter motor (1.B.) Except when I took out the starter motor and took it to the car-parts store for a test, the starter motor worked fine.

After several hours of study and puzzlement, I figured out that the electrical relays that were supposed to trigger the starter motor weren't working. The relay replacement was cheap and easy, once I knew what the problem was.

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