So you go out one morning and your engine will turn over but it won't start...what could be wrong?
Now that you know how an engine works, you can understand the basic things that can keep an engine from running.
Three fundamental things can happen - a bad fuel mix, lack of compression, or lack of spark. Beyond that, thousands of minor things can create problems, but those are the 'big three.' Based on the simple engine outlined, here is a quick run-down on how these problems affect your engine.
Bad Fuel Mix
A bad fuel mix can occur in several ways:
You are out of petrol, so the engine is getting air but no fuel
The air intake might be clogged, so there is fuel but not enough air
The fuel system might be supplying too much or too little fuel to the mix, meaning that combustion does not occur properly
There might be an impurity in the fuel (like water in your tank) that prevents the fuel burning
Compression
If the charge of air and fuel cannot be compressed properly, the combustion process will not work as it should.
Lack of compression might occur for these reasons:
Your piston rings are worn (allowing air/fuel to leak past the piston during compression)
The intake or exhaust valves are not sealing properly, again allowing a leak during compression
There is a hole in the cylinder
The most common 'hole' in a cylinder occurs where the top of the cylinder (holding the valves and spark plug - also known as the cylinder head) attaches to the cylinder itself
Generally the cylinder and the cylinder head bolt together with a thin gasket pressed between them to ensure a good seal
If the gasket breaks, small holes develop between the cylinder and the cylinder head and these holes cause leaks
Spark:
The spark might be non-existent or weak for a number of reasons:
If your spark plug or the wire leading to it is worn out, the spark will be weak
If the wire is cut or missing, or if the system that sends a spark down the wire is not working properly, there will be no spark
If the spark occurs either too early or too late in the cycle (i.e. if the ignition timing is off) the fuel will not ignite at the right time, and this can cause all sorts of problems
Many other things can go wrong!
If the battery is dead, you cannot turn the engine over to start it
If the bearings that allow the crankshaft to turn freely are worn, the crankshaft cannot turn so the engine cannot run
If the valves do not open and close at the right time, or at all, air cannot get in and exhaust cannot get out, so the engine cannot run
If someone sticks a potato up your exhaust, gases cannot exit the cylinder so the engine will not run
If you run out of oil, the piston cannot move up and down freely in the cylinder, and the engine will seize
In a properly running engine all of these factors are within tolerance.
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