How to track down a misfire
QMy Volkswagen Passat MKIII has developed an intermittent misfire, which can be extremely severe at times. Can you offer any clues on tracking it down?
Mike Stephenson, Bath
AThe most common cause is a fault somewhere in the ignition system. A misfire occurs when an insufficient voltage is delivered to the spark plug, either because not enough is being generated by the system or because it’s escaping or being blocked en route to the plug.
Does a single cylinder drop out each time? In that case, find out which one and substitute that spark plug, its HT lead and the distributor cap. If the misfire is quite random and happens under any and all driving conditions, look for a loose connection in the 12V ignition
circuit, especially if the ignition seems to die completely once in a while. Check the earth braid inside the distributor and the wires from the sensor or pickup in an electronic distributor.
Many misfires occur under load, usually when labouring the engine at low rpm. This is because a greater voltage is needed to create a spark under these conditions. Make sure all HT components are clean and their connections and contacts are corrosion-free. If a car starts to cut out under load then dies, try swapping the rotor arm. A failing condenser will cause misfires and breakdowns whose exact nature is hard to pinpoint, except that the misbehaviour will happen when the engine is hot and go away once cooled.
Misfires caused by the fuel system are rarer and happen when the fuel supply is for some reason inadequate. This may be caused by a blocked jet in the carburettor, a failing fuel pump or a blocked fuel line, filter or tank pickup. The misfire starts as a stutter, becoming a heavy hesitation under load. The final stage of conking-out may be a valedictory backfire.
Try pulling the choke out when the symptoms occur. If the choke stabilises the problem, a blocked jet is likely to blame. If it has a temporary effect, with the symptoms returning after a matter of seconds, the problem is probably fuel supply.
The final misfire type is caused by burned, damaged or maladjusted valves. You’ll feel a rhythmic pulse if you place the back of your hand near the exhaust tailpipe. Confirm it’s a valve by removing the plug lead of the offending cylinder. With an ignition fault, it would
make no difference – the cylinder would be completely dead anyway. With a faulty valve, however, the miss will become more pronounced with that lead removed.
Finally, what to do on the road if the car starts to misfire? Pull the choke out. If the miss goes away look at the carburettor and fuel system. If it doesn’t, push the choke in and alter the engine load. If it responds better to high rpm at light load, suspect a breakdown of an HT part. If it’s a shambles at high rpm but less so at low rpm, look at the condenser and the spring and pivot of the points. If the miss is rhythmic and across all rpm and loads, suspect something relating to one cylinder only, be it a valve or an HT component.