Is it just me who thinks that when most mechanics are presented with a misfire on a petrol engine, they go straight for the ignition system as it’s the ‘most common’ system to cause the problem? Then, depending on the arrangement, they usually replace the ignition coils, spark plugs and either strike it lucky and get a fix or are still met with a misfire fault. Only then do they start doing some digging. I recently had to deal with three different vehicles, all petrol, and all with misfires. I want to show that there is more to it than just spark plugs, coils and ignition leads. Running issues My place of work includes car sales. With stock running low, my boss decided to buy some more cars. This included three vehicles that were described as having “warning lamps illuminated and running issues.” Being a blind sale online you have to take a risk as you cannot view before you buy. You then have to hope the damage is not catastrophic or that it has not been around the houses with no joy and sent to auction to become someone else’s problem. The vehicles purchased with running faults were a Citroen DS3, Dacia Duster and a Nissan Qashqai, all of which had petrol engines. After a week or two, the vehicles arrived and I set to work on diagnosing each of them. I chose the Citroen first which had the 1.6 EP6C engine fitted, simply because it was the first off the lorry. The car started and ran but had a misfire accompanied by warning lights and messages for a fault. Scanning the car for faults showed a cylinder 4 misfire code. This code doesn’t tell you why it is misfiring, only which cylinder is misfiring. We now a had direction in which to head. For a petrol engine, the key elements required for correct running are the fuel system, ignition system and mechanical integrity, i.e. good compression. This also must all be happening at the correct time. If we have a misfire one or more of these must be off. A good habit I have gotten into over the years with misfires is to check the mechanical condition of the engine first in order to ensure it can create compression. If all is ok, only then do I move onto the ignition/fuel side of things. To do this I first prevented the engine from starting, which can be done in several different ways, and cranked the engine, listening for any irregularities as it was turning over. A trained ear can pick up an issue before any tools need to be used. In this instance, all sounded well but to confirm this I carried out a relative compression test using an oscilloscope. While this can’t physically measure the compression, it can compare each cylinder to the others which confirms if they are even or relative to each other. This is done by placing a current clamp around a battery cable and measuring the current draw from the starter motor in amps. In this way we can visually see the work done by the starter to crank the engine over. A key thing to remember though is that the scope trace may look good and have even peaks throughout, but if all cylinders are low on compression the waveform will also look good. As I said, we are testing them relative to each other. A good thing to do while carrying out this test is to check the amperage at the top of the peaks and compare it to the battery AH (amp hour). A rough guide is to expect three times the AH of the battery. So, if we have a 60AH battery I would 30 AFTERMARKET APRIL 2023 TECHNICAL Below: Dacia Duster www.aftermarketonline.net DEMYSTIFYING MISFIRES Offering three misfire problems for the price of one, Neil shows how experience and good process will help you get the correct diagnosis BY Neil Currie
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