Sure you could throw a set of spark plugs at it and see what happens. Might as well throw a set of coils in there at the same time. What if that does not fix it. In reality the only approach should be to test and inspect. Let’s take a look at how we go about this. This is a fairly generic approach that can be applied to many spark ignition engines.
Absolute first thing to do is confirm the fault. Is a misfire present? What does it feel like? Experience is a big help at this point. Code scanning is next on the list. Clearly if we are dealing with a non EFI / engine management vehicle this is not possible. On a side note, EFI is electronic fuel injection. This is a fuel only system with spark controlled by a distributor and advance mechanism. Engine management is the fuel and spark controlled by the ECU.
If we have a misfire code present that will give us a clear indicator of where to go next. A misfire code will be in the form of P03**. If the code is P0304 that is telling us that the ECU has detected an issue on cylinder number 4. Code P0300 is more generic and is only an indicator that the ECU has detected a misfire but has not isolated it to a specific cylinder.
Let’s say we have a P0304 coded detected by our Haltech ECU connected to a SR20det. The misfire can be felt when the engine is running. Fault is confirmed. We need to visually inspect items like wiring, any fuel, oil or vacuum leaks and any abnormal engine noises? Any fault found will need to be repaired prior to moving forward. It is simply silly to move on with other testing when a visible fault has been found.
With any visible faults rectified and the misfire confirmed to still be present we need to take the next step. Number 4 cylinder has been identified by the ECU as having an issue. We now need to physically inspect the spark plug and coil from this cylinder. Where visible inspection shows no fault a simple little trick is to swap the spark plug from number 4 to number 3 and the coil from number 4 to number 2. When the misfire moves to another cylinder you have confirmed the component that is causing the issue.
No matter if the spark plug or coil was the issue it is best practice to replace the plugs or coils as a set. Even if the coil was the issue were visible wear is present on the spark plugs and the history is not known replace those as well.
But hang on. All this and number 4 cylinder is misfiring. Our time so far has been well spent. We now know that the misfire is due to an issue specific to cylinder number 4. We need to confirm that the coil for number 4 is firing. This is easily done with a HEI spark tester. No spark at this point indicates a Power or Signal issue to the number 4 coil. We know the coil is not faulty as we tested it on a different cylinder.
Were spark is present often the injector is the next easy test to perform. Check for power to the injector and pulse using a noid light. No power to the injector gives a clear direction. Same as if we have power but no pulse. Where power and pulse are good I like to listen to the injector tick, this confirms it is opening. No tick at this point tells us the injector is faulty and we can confirm this with a resistance test.
With all this testing as good. We need to move onto a compression test. Testing compression gives us information about the mechanical condition of the engine. The list of possible causes of low compression are many and varied, too many to list in this. A good topic for another blog perhaps.
It is completely possible for all of the above to pass testing and the issue be unresolved. Modern engine and the issues that occur are many and varied and this is only the start of testing that is required for some problem cars.