How to Diagnose an Engine Performance Problem

Diagnosing an engine performance problem is almost a thing of the past without the aid of a Diagnostic Readout Box. On newer cars almost everything is controlled or affected by a computer. For performance the computer looks at several sensor readings and adjusts for fuel mixture and spark timing to keep the engine running smoothly. There are many variables to consider when diagnosing a performance problem but here are a few things you can do to help narrow them down.

- First of all, is the "Check Engine" light on or flashing? If the check engine light is on then you most likely have a sensor related problem. If the light is flashing then you have a major emissions problem and need to get it to the garage quickly. When the check engine light comes on that means the parameters in a sensor have gone out of range. It doesn't necessarily mean the sensor is bad because something mechanical may be causing it to go out of range to turn the light on as well. If you have a diagnostic readout box then you can at least see which sensor is out of range and of course read the code that is displayed in the computer's memory.

- If you don't have a readout box (which most people wouldn't) then there is another way to help determine the area of the problem. Having a repair manual for your particular vehicle will be a major help, make sure it's an electronic engine control manual with diagnostic information in it. You can often retrieve diagnostic codes from the computer without the readout box. On some vehicles (Chrysler is this way) you can turn the key on and off 3 times and with newer vehicles it will actually display the diagnostic code in the odometer display. Older vehicles will just give you a flash code and you will need to know how to read this, thus the handy repair manual.

- Once you know the diagnostic code and if the manual is extensive enough, you will need some electronic testing equipment such as an ohmmeter and a 12-volt tester. You can now make an educated decision on whether you have a bad sensor that is causing your performance problem. The repair manual should give you a step-by-step testing procedure to check certain sensors using your test equipment. Obviously you will need to have some common knowledge on how to use and read your test equipment.

- If the check engine light isn't on and you have a performance problem then you need to lean more towards a component failure such as spark plugs, coil, distributor, spark plug wiresDiagnosing an engine performance problem is almost a thing of the past without the aid of a Diagnostic Readout Box. On newer cars almost everything is controlled or affected by a computer. For performance the computer looks at several sensor readings and adjusts for fuel mixture and spark timing to keep the engine running smoothly. There are many variables to consider when diagnosing a performance problem but here are a few things you can do to help narrow them down.

- First of all, is the "Check Engine" light on or flashing? If the check engine light is on then you most likely have a sensor related problem. If the light is flashing then you have a major emissions problem and need to get it to the garage quickly. When the check engine light comes on that means the parameters in a sensor have gone out of range. It doesn't necessarily mean the sensor is bad because something mechanical may be causing it to go out of range to turn the light on as well. If you have a diagnostic readout box then you can at least see which sensor is out of range and of course read the code that is displayed in the computer's memory.

- If you don't have a readout box (which most people wouldn't) then there is another way to help determine the area of the problem. Having a repair manual for your particular vehicle will be a major help, make sure it's an electronic engine control manual with diagnostic information in it. You can often retrieve diagnostic codes from the computer without the readout box. On some vehicles (Chrysler is this way) you can turn the key on and off 3 times and with newer vehicles it will actually display the diagnostic code in the odometer display. Older vehicles will just give you a flash code and you will need to know how to read this, thus the handy repair manual.

- Once you know the diagnostic code and if the manual is extensive enough, you will need some electronic testing equipment such as an ohmmeter and a 12-volt tester. You can now make an educated decision on whether you have a bad sensor that is causing your performance problem. The repair manual should give you a step-by-step testing procedure to check certain sensors using your test equipment. Obviously you will need to have some common knowledge on how to use and read your test equipment.

- If the check engine light isn't on and you have a performance problem then you need to lean more towards a component failure such as spark plugs, coil, distributor, spark plug wires and even the fuel pump depending on the type of performance problem. These are some of the things you can physically look at, put a pressure tester on or still use a meter, but sometimes these types of problems won't turn the check engine light on. Even fuel injectors can fail mechanically and not turn the light on so you may have to put a little more elbow grease into your work and study that manual a little more to check these items.

- With an engine miss you would want to look at the ignition system first (spark plugs, wires, ignition coil, distributor etc.). Then you may need to check the injectors if nothing shows up in the ignition system. If you have a lack of power problem then the fuel pump would be a place to look. Some vehicles have a handy connection on the fuel rail to quickly check fuel pressure, but take caution, as fuel pressures are very high on new cars. Wear eye protection when installing your tester.

We could go on and on about trying to diagnose a performance problem. There are some common variables between different brands of vehicles but for the most part each one has it's own unique control system. If you feel competent in doing some interrogation on your own then just make sure you understand the safety issues with certain areas of the car. If you don't want to deal with it then find an ASE certified mechanic who has the knowledge and the equipment to work on your vehicle.

Take care and stay safe!