Hi all. This document is intended to be constantly evolving, if you have something to add or have spotted a mistake then please let me know and it'll be incorporated. It is not meant to be a complete problem solving guide, but it should help give you some understanding of the diagnostic process, and means you can post with more specific problems and we can work from there 
Basics
For an engine to work, it needs 4 main things;
- Fuel
- Spark
- Compression
- Timing
If one or more of them is missing, or wrong then the engine will not run properly; if at all. So if your engine is not right, then you need to check and test them to find the problem.
Let's start with fuel.
There are two slightly differing types of fuel systems found on modern petrol engines, Single Point Injection (SPI) and Multi Point Injection (MPI). SPI has now been phased out; any vehicle after 2001 is 99.9% going to be MPI. As the names suggest, an SPI has a single fuel injector and an MPI system has more than one, normally it has one per cylinder. A fuel injector has a pressurised fuel feed to it, and two wires connected to it. It is a type of electronic solenoid, a powerful spring holds the injector shut and when power is applied to it - the solenoid overcomes to the spring, opening the injector, allowing fuel to be injected.
On an SPI system, the injector is normally found in the throttle body - above the throttle plate. If you removed the cars airbox, you are likely to see it sat on the very top of the throttle body.
On an MPI system, the injectors are found in the inlet manifold - very near to the inlet ports on the cylinder head. They are linked by a hollow bar, this is the fuel rail - it contains pressurised fuel to supply the injectors. They will also have a two pin plug connected to each injector.
Before attempting to check the injectors, you need to check that there is fuel getting to the fuel rail. The fuel rail is fed by a pump in the petrol tank. This pump is normally under the rear seats. When you switch on the ignition, you will normally hear this pump buzz for around 2 seconds whilst it pressurises the fuel system.
If you hear this buzzing sound, and the tank has fuel in it, then you can assume that there is fuel getting to the fuel rail. You now need to check the power to the injectors.
The injectors have a fused, ignition switched 12v feed to them. If you disconnect one of the injectors, and probe the pins on the plugs with a multimeter, you should find 12v between one of them and ground. The other pin on the injector connects to the injection drivers inside the ECU. You cannot test this without an oscilloscope.
Assuming the injectors have fuel, and power then you now need to check they are all firing. The easiest way to check if the injectors are firing is turn the engine over a few times and then remove the spark plugs. If all of the spark plugs are wet and smell of fuel, and they are wetted to the same or similar degree then it can be assumed that the injectors are all firing.
Spark
To ignite the fuel, a spark is needed. This is done by pulsing high voltage through a spark plug. The voltage travels from an HT coil into the spark plug, and then is dissipated through the outer metal of the spark plug and into the head, to ground.
You are likely to find two types of spark systems on modern cars. Assuming a 4 cylinder engine, one has two Coil Packs and is called wasted spark. The other has one coil per cylinder, and is called Coil On Plug.
A wasted spark system has HT leads linking the coil to the spark plug; it fires the spark plugs in pairs. The firing order for the cylinders will be cylinder 1, then 3, then 4 and then 2. The coil packs will be setup so that one coil fires cylinder 1 and 4, and the other will fire 2 and 3. This means when the plug in cylinder number 1 is fired, the plug in number 4 also fires - hence the term wasted spark.
If the car is running on only 2 cylinders, it will be running very rough, vibrating and have restricted power then it is possible that one of the two coil packs has failed. With the engine running, using a pair of pliers remove one HT lead from the spark plug. If the engine stalls, then you can assume that that spark plug was firing and you should reconnect it. If the engine does not change, then it is fair to assume that that spark plug was not firing, follow the HT lead back to the coil and then remove the other HT lead that is fed from that coil. Again, if the engine note does not change then it is fair to assume that that coil pack is not working. Turn the engine off. Disconnect the LT wires from the coil packs and swap them over, then swap all the HT leads over. Start the engine and repeat the test. If the fault has moved to the other pair of cylinders, then you need a new coil. If it has not, then the fault lies elsewhere.
If the car is not running at all, then remove the HT and spark plug for cylinder number 1. Connect the HT lead to the spark plug and holding it by the HT lead, press the metal part of the plug onto the top of the engine and get some to turn the engine over. The plug should spark.
A COP setup has the coil sat directly on top of the plug and so has no visible HT leads. If you suspect a fault on a COP system, then you can disconnect each coil individually to locate the coil that is not firing.
Compression
It is pressure inside the cylinders that actually moves the crank via the pistons and con rods. The only places where gas can get in and out of the cylinders should be inlet and exhaust valves. If it is leaking out anywhere else, then the engine will not perform properly as the pressure that should be pushing the piston down is escaping. To test the compression of an engine, you can buy a tool called Compression Tester. It is a pressure gauge that screws into the spark plug holes; they normally cost around £20. To use it, you need to remove all the spark plugs and ideally disconnect the fuel injectors, then screw the tester into one of the spark plug holes, press the throttle to floor and turn the engine over for around 5 seconds. Record the value on the tester, move to the next cylinder and repeat the test for each. If there is a variance of 1bar (around 14.7psi) between any two of the cylinders then that shows a problem.
Timing
Fuel, spark and compression have to happen at the right time for the engine to work. On an engine there is valve timing to look at, as well as ignition/injection.
Valve timing is concerned with the opening and closing of the inlet and exhaust valves. It is controlled by a camshaft, which is driven by a belt or chain from the crankshaft.
Ignition / Injection timing is handled by the ECU. One of the ECU's critical sensors is the TDC / Crankshaft Sensor. This sensor looks at a toothed wheel on the crankshaft, and when a missing tooth passes the sensor it sends a pulse to the ECU. The ECU uses this pulse to determine when to spark and inject fuel. If the ECU does not receive this pulse then it will not even attempt to inject fuel or spark. If the sensor or its cabling is damaged, then the pulse may be scrambled and the ECU will get confused and inject / spark at the wrong time. If your car was randomly cutting out, and then restarting shortly after seeming perfectly fine but now will not start at all then it is likely that the TDC sensor is at fault.
And finally..
If the car has thrown its Engine Management / Check Engine / Injection System light on, then it means that the ECU knows that there is a problem and that it has logged a fault code describing the problem. If you plug a diagnostic machine into the ECU, it will tell you the code and should help in narrowing down a fault.
If you have no fuel being injected, and no spark then often the fault is the TDC sensor.
As I said at the beginning, this is not intended to be a complete fault finding and repair document - it is just enough to get you started with some basic testing, if you now post the results of your tests and any further questions in the relevant section of the forum then I’m sure someone will be able to help you further narrow the problem down.
Mark
PS If anyone has any model specific guides for doing any of the above, then please let me know and i'll get them added.
Basics
For an engine to work, it needs 4 main things;
- Fuel
- Spark
- Compression
- Timing
If one or more of them is missing, or wrong then the engine will not run properly; if at all. So if your engine is not right, then you need to check and test them to find the problem.
Let's start with fuel.
There are two slightly differing types of fuel systems found on modern petrol engines, Single Point Injection (SPI) and Multi Point Injection (MPI). SPI has now been phased out; any vehicle after 2001 is 99.9% going to be MPI. As the names suggest, an SPI has a single fuel injector and an MPI system has more than one, normally it has one per cylinder. A fuel injector has a pressurised fuel feed to it, and two wires connected to it. It is a type of electronic solenoid, a powerful spring holds the injector shut and when power is applied to it - the solenoid overcomes to the spring, opening the injector, allowing fuel to be injected.
On an SPI system, the injector is normally found in the throttle body - above the throttle plate. If you removed the cars airbox, you are likely to see it sat on the very top of the throttle body.
On an MPI system, the injectors are found in the inlet manifold - very near to the inlet ports on the cylinder head. They are linked by a hollow bar, this is the fuel rail - it contains pressurised fuel to supply the injectors. They will also have a two pin plug connected to each injector.
Before attempting to check the injectors, you need to check that there is fuel getting to the fuel rail. The fuel rail is fed by a pump in the petrol tank. This pump is normally under the rear seats. When you switch on the ignition, you will normally hear this pump buzz for around 2 seconds whilst it pressurises the fuel system.
If you hear this buzzing sound, and the tank has fuel in it, then you can assume that there is fuel getting to the fuel rail. You now need to check the power to the injectors.
The injectors have a fused, ignition switched 12v feed to them. If you disconnect one of the injectors, and probe the pins on the plugs with a multimeter, you should find 12v between one of them and ground. The other pin on the injector connects to the injection drivers inside the ECU. You cannot test this without an oscilloscope.
Assuming the injectors have fuel, and power then you now need to check they are all firing. The easiest way to check if the injectors are firing is turn the engine over a few times and then remove the spark plugs. If all of the spark plugs are wet and smell of fuel, and they are wetted to the same or similar degree then it can be assumed that the injectors are all firing.
Spark
To ignite the fuel, a spark is needed. This is done by pulsing high voltage through a spark plug. The voltage travels from an HT coil into the spark plug, and then is dissipated through the outer metal of the spark plug and into the head, to ground.
You are likely to find two types of spark systems on modern cars. Assuming a 4 cylinder engine, one has two Coil Packs and is called wasted spark. The other has one coil per cylinder, and is called Coil On Plug.
A wasted spark system has HT leads linking the coil to the spark plug; it fires the spark plugs in pairs. The firing order for the cylinders will be cylinder 1, then 3, then 4 and then 2. The coil packs will be setup so that one coil fires cylinder 1 and 4, and the other will fire 2 and 3. This means when the plug in cylinder number 1 is fired, the plug in number 4 also fires - hence the term wasted spark.
If the car is running on only 2 cylinders, it will be running very rough, vibrating and have restricted power then it is possible that one of the two coil packs has failed. With the engine running, using a pair of pliers remove one HT lead from the spark plug. If the engine stalls, then you can assume that that spark plug was firing and you should reconnect it. If the engine does not change, then it is fair to assume that that spark plug was not firing, follow the HT lead back to the coil and then remove the other HT lead that is fed from that coil. Again, if the engine note does not change then it is fair to assume that that coil pack is not working. Turn the engine off. Disconnect the LT wires from the coil packs and swap them over, then swap all the HT leads over. Start the engine and repeat the test. If the fault has moved to the other pair of cylinders, then you need a new coil. If it has not, then the fault lies elsewhere.
If the car is not running at all, then remove the HT and spark plug for cylinder number 1. Connect the HT lead to the spark plug and holding it by the HT lead, press the metal part of the plug onto the top of the engine and get some to turn the engine over. The plug should spark.
A COP setup has the coil sat directly on top of the plug and so has no visible HT leads. If you suspect a fault on a COP system, then you can disconnect each coil individually to locate the coil that is not firing.
Compression
It is pressure inside the cylinders that actually moves the crank via the pistons and con rods. The only places where gas can get in and out of the cylinders should be inlet and exhaust valves. If it is leaking out anywhere else, then the engine will not perform properly as the pressure that should be pushing the piston down is escaping. To test the compression of an engine, you can buy a tool called Compression Tester. It is a pressure gauge that screws into the spark plug holes; they normally cost around £20. To use it, you need to remove all the spark plugs and ideally disconnect the fuel injectors, then screw the tester into one of the spark plug holes, press the throttle to floor and turn the engine over for around 5 seconds. Record the value on the tester, move to the next cylinder and repeat the test for each. If there is a variance of 1bar (around 14.7psi) between any two of the cylinders then that shows a problem.
Timing
Fuel, spark and compression have to happen at the right time for the engine to work. On an engine there is valve timing to look at, as well as ignition/injection.
Valve timing is concerned with the opening and closing of the inlet and exhaust valves. It is controlled by a camshaft, which is driven by a belt or chain from the crankshaft.
Ignition / Injection timing is handled by the ECU. One of the ECU's critical sensors is the TDC / Crankshaft Sensor. This sensor looks at a toothed wheel on the crankshaft, and when a missing tooth passes the sensor it sends a pulse to the ECU. The ECU uses this pulse to determine when to spark and inject fuel. If the ECU does not receive this pulse then it will not even attempt to inject fuel or spark. If the sensor or its cabling is damaged, then the pulse may be scrambled and the ECU will get confused and inject / spark at the wrong time. If your car was randomly cutting out, and then restarting shortly after seeming perfectly fine but now will not start at all then it is likely that the TDC sensor is at fault.
And finally..
If the car has thrown its Engine Management / Check Engine / Injection System light on, then it means that the ECU knows that there is a problem and that it has logged a fault code describing the problem. If you plug a diagnostic machine into the ECU, it will tell you the code and should help in narrowing down a fault.
If you have no fuel being injected, and no spark then often the fault is the TDC sensor.
As I said at the beginning, this is not intended to be a complete fault finding and repair document - it is just enough to get you started with some basic testing, if you now post the results of your tests and any further questions in the relevant section of the forum then I’m sure someone will be able to help you further narrow the problem down.
Mark
PS If anyone has any model specific guides for doing any of the above, then please let me know and i'll get them added.
