Misfire Causes And Cures

Press, Volume CVII, Issue 31439, 4 August 1967, Page 7

Misfire Causes And Cures

TX TTENTION to low engine temperature, lack of firing voltage, and a sensible choice of projected-core spark plugs will help to overcome misfiring problems. This is said by Mr T. L. Thompson, of Christchurch, in a paper on misfire problems in high-compression engines which has won the Harry Turner Memorial Award of the Institute of the New Zealand Motor Industry. The awards—three are presented each year for original papers submitted by persons of different qualifications—are named after the late Mr F. H. Turner, who was a foundation member of the institute, a member of the committee, and a president.

FOULING A common cause of misfiring is chemical fouling of spark plugs caused by fuel additives, the paper says. This is most likely in city-used cars, and can be overcome by a combination of attention to engine temperature, firing voltage, and choice of spark plug. Another cause is carbon or cold-running fouling, which causes misfires because of the build-up of deposits caused by lack of engine temperature. The remedy for this is fitting a higher-opening thermostat and a higher-loaded radiator pressure cap.

“It is an accepted fact that most manufacturers fit thermostats which are too cold for winter temperatures. As the boiling point of a liquid is raised by increasing the vapour pressure over the liquid, there are no over-heat-ing risks in fitting a really hot thermostat if the system is safeguarded with a higherloading pressure cap.”

WIDE GAPS A third cause of misfiring is too wide a spark plug gap, which causes misfiring in the medium and high-speed ranges when firing voltage is falling off. Wide gaps are the result of too hot a plug, or ignition over-advance, which causes overheating and burning of the electrode. Wide gaps can also be caused by excessive mileage without a tune-up.

Low firing voltages can also bring misfiring, and three causes of this can be faulty suppressor high - tension cables, incorrect distributor dwell angle, or wrong coil polarity. Rough handling of some types of supressor cables can cause a break in the inner carbon-type conductor, says the paper, and some of these cables have very high resistance in any case. COIL POLARITY

Incorrect coil polarity (terminals connected the wrong way round) is a common but not obvious cause of misfiring, the paper says. Experiments have shown that at 1700 r.p.m. one third more voltage than normal is required with the wrong coil polarity. Wrong polarity causes misfiring through lack of voltage above about 3500 r.p.m., and also causes hard starting.

A guide is that if the battery is positive earthed, the positive terminal on the coil should be connected to the condenser terminal on the side of the distributor.

A fifth cause of misfiring is plug tracking, which, in effect, retards the ignition.

HEAT VALUES The paper says that the heat range of plugs is not fully understood by many mechanics, and that they do not understand each manufacturer makes sufficient plug types to cure trouble whatever the cause: cold-running fouling, or electrode burning. Instead, mechanics often jump from brand to brand, when moving one notch up or down the heat range could overcome the problem. “The plug recommendation chart is not to be taken as gospel.” The hotter the plug the more resistant it is to cold carbon fouling, because the electrode works at higher temperature. The colder the plug, the lower its oil resistance. Such plugs are used to eliminate electrode burning at high working temperatures. RUN ON

A selection of too high a heat value can cause highspeed pre-ignition, which is a dangerous condition to be avoided at all times. Dieseling, or run-on, is not caused by spark plugs. Research has shown it is caused by one or more of these reasons: too fast an idling speed —too much charge in the cylinder at shut-off; too low a fuel octane rating for the compression ratio; or siezure of the exhaust manifold heater valve, causing overheating of the ingoing charge. Today compression readings of more than 150 p.s.i. and over are common, and an octane rating of 96 or higher is a must to obtain the performance intended by the manufacturer, the paper says. This octane rating can only be achieved by the use of additives in the fuel. The additives form the basic salts which cause deposit formation and spark plug fouling. Other additives, known as scavengers, greatly help the problem by forming compounds which go through a series of changes as temperatures rise under acceleration.

City driving demands slow driving followed by bursts of

acceleration, and under these conditions plug insulator tip temperatures can rise from 600 degrees Fahrenheit to 1400 degrees in 20 seconds. Under these circumstances

the compounds in the fuel may break down, producing the misfire-causing deposits. This is chemical fouling, which can be remedied by the methods mentioned earlier. NEW PLUGS

The greatest single advance in the battle against plug fouling has been the introduction of the new turbo-action projected nose-cone spark plugs, which have long centre electrodes and an extended insulator tip, so that they run hot in the low-speed range, the paper says. However, on open throttle the temperature of the electrode is lowered by the coolness of the incoming charge, and the plug runs cold on fierce acceleration and high speed. The expansion and contraction of the electrodes in these plugs cracks off the deposits and the plugs are largely self-cleaning. In some engines such plugs advance the ignition, and five degrees of retard is needed.

PLUG CHOICE Heat value must be considered, and in really high-com-pression engines a risk is involved in fitting the projec-ting-core plug with the maximum heat value. The cooling effect is not instantaneous, and in this time high-speed pre-ignition can occur. “Be guided by usage,” the paper says. “On an almost 100 per cent town car with high compression the use of a projected plug with high top heat is permissible. In country or travellers’ cars fit the projected plug with the low heat values.”