Your Car Diagnostic Guide
A proper car diagnostic test is the first (crucial!) step towards working out a solution. Without properly working out what's wrong you can waste a lot of time and money chasing down possible causes without actually solving the root cause.
It's better to test & not guess
Don’t allow an untrained workshop play 'swap-a-tronics' with your car. This will cost you more in the end or not get the fault repaired correctly.
Car Diagnostic Cheat Sheet
Ask these 3 Questions to work out if your mechanic has the right equipment to diagnose your car.
Benefits of Getting it Right:
Problems are identified FAST - no waiting around while endless tests are done.
Save money on repairs by fixing faults before they spiral out of control and cause damage to other areas.
Lower fuel costs
Your car's emissions and electric systems are tested to ensure you aren't wasting money on unburned fuel.
When should you invest in a car diagnostic test? Does your car have any of the following symptoms?
- You have a problem with your car and it's not performing as it should be
- Check engine light is on, blinking or coming on intermittently
- Other lights like your battery light are on
- Car is underperforming and have gone into 'limp home' mode
- Your car is using a lot of fuel, you may see black smoke from the exhaust or notice a burnt fuel smell
- Your car is running rough or jerking
These are just a few faults that warrant a car diagnostic test being done. If you answered yes, to any of the above then:
- Book your car into a workshop that has the correct equipment. Use this cheatsheet so you know what questions to ask
- Complete a diagnosis sheet - the more information you can provide the better
The reason that diagnostics have become more important is that most vehicles now have computers that control the function of the operations from increasing fuel supply to allowing the air conditioning to operate. These computers operate within a preferred scale and if it goes out of range then problems will result.
Problems don't always trigger an alert...
The check engine light will come on and alert you that there is fault with your car only if its safety related or to do with fuel emissions. But there are codes that won't be apparent to the driver but will show up next time your car is scanned.
Correct servicing of your vehicle will catch these faults before they become an issue if your servicing centre has the right equipment and is actually doing the test as part of their service.
Sometimes these checks are done and a fault may occur with an electronic component. You may need to have your vehicle examined and a number diagnostic tests will be needed to pinpoint the defective part.
Testing Equipment - What to Look For in a Workshop
- EXHAUST TEST
- FUEL PUMP
- VACUUM LEAKS
- infrared camera
- SCAN TOOLS
- COIL TESTER
- BORE SCOPE
- DIESEL FLOW TEST
Exhaust Emission Test
This is done with a gas analyser and will show how the engine is breathing:
- Is it getting the correct amount of fuel
- Is the exhaust system in good condition?
- Is the engine misfiring?
- Are the valves sealing correctly?
Fuel Pump Pressure & Flow Test
Testing a fuel pump will indicate is the engine is operating at the correct pressure. This is critical because it determines whether the correct amount of fuel is being supplied to the engine. Most pumps are electric and do wear out. Signs of a fuel pump wearing out can be tested using an amp clamp and oscilloscope to check how the pump is work electrically.
Vacuum Leaks & Exhaust Leak
A smoke test machine is used to pinpoint small vacuum leaks or gaskets that are critical in allowing the correct amount of fuel to be match with the correct amount of air. Also small exhaust leak will cause check engine light to come on in late model vehicles.
This tool is used to measure the temperate of different components. If a part is failing it can heat before it fails completely. This is mostly used to
- find blocked radiators
- fuses that over heat
- belt pullies heating up and being noisy
- components that are supposed to run at a certain temperature and aren't, it can be used to rule out a potential cause.
This instrument is used to measure the operation of electrical components. Whilst working it measures the reaction of the current under load. Its useful in determining a faulty part they may not have completely failed. This is very handy when trying to find intermittent faults.
There are a lot of myths about scan tools, in particular, how they make things a lot easier as you just plug it in and the tool tells you what's wrong. If only it were that easy!
The scan tool shows a code that has been logged. It's then up to the operator to research, test and verify that related components are not causing the fault. Too many times money is wasted on replacing parts that were not at fault.
Scan tools do allow access to live data and with the correct information compare good data from a test car with the vehicle which has faults.
This is a great tool to find ignition coils that are breaking down under load. The tester simply loads the coil up and you can see a strong steady spark that is faultless. If it misses a beat then it should be replaced.
This tool is used to look inside a cylinder that may be leaking coolant or has been damaged.
It's also used to look up an air intake that is blocked and full of carbon. With some of the emission requirements on car we're seeing an increasing amount of air intakes blocked and full of soot.
When this happens it causes the diesel engine to emit high levels of black smoke. What’s actually happening is blocked air intake with soot deposits. The engine doesn’t receive enough air at the same a regular dose of fuel is being injected. This makes the mixture too rich and the engine simply can’t burn all the fuel as it hasn’t received enough air.
Diesel Flow Test
This is a great way to find out how each diesel injector is performing. As each injector is recycling a small amount of fuel back to the tank, this instrument means you can test the performance of each injector and measure the return flow. If its all over the lace we then know that a replacement or service of the injectors is required.
A Practical Example - Engine Misfire Diagnostic Process
To better explain how all this works, here is an example of how Auto Edge would deal with an engine misfire. The car owner reported that the car was running rough, the vehicle was vibrating and had black smoke coming out of the exhaust, so this is what we did:
- Collect Information - check diagnostic form, date of last service and information from car owner
- Check car - how does car look, sound, feel and smell?
- Test drive if possible
- Scan for faults using the correct scanner
- Research codes if any have come up on the scan
- Test battery and charging system
- Conduct emission test with a 5 gas analyser (this measures things like air/fuel ratio and the percentage of hydrocarbons)
- Test spark plugs, leads and coils electronically with a KV (kilovolt tester) at idle and load
- If coil was poor then remove it from the vehicle and load test it on bench tester
- Replace coil and recheck, exhaust emission and test drive
- Depending on readings obtained from KV test, all coils and spark plugs may need to be replaced as a preventative maintenance measure
- Car is then retested to ensure all faults have been eliminated and a final test drive to confirm results.