Foreign Service The snap-throttle test can be an effective diagnostic tool when tracking down ignition misfires on Japanese engines with extra long in-head spark plug boots.

he spark plugs are recessed deep jority of these arcing boot problems occur on poor- inside the cylinder head(s) on mil- quality replacement plug wires. Obviously, there’s a lions of Japanese engines. Conse- big difference between high-quality and low-buck quently, the spark plug boots on replacement wires. The high-quality wires are the Dan these engines are extremely long. proper lengths, and the top of each boot fits snugly Marinucci Longer boots aren’t necessarily onto the opening in the valve cover. With a poor- trouble, but they can and do fail. Also, some plug quality wire set, it’s not uncommon to see the top of 1 wire failures may fool you if you’re not paying at- the boot sitting about ⁄4 to 1 inch above the valve Ttention. Here’s what to look for on these systems. cover (see photo). No wonder the engine runs Experience shows that secondary ignition poorly when it rains! voltage may arc inside these long plug boots. Furthermore, no matter how carefully you try to Occasionally, the voltage arcs outside the boot remove them, some of these cheap plug boots tear to the head. In some situations, the resulting easily during a routine spark plug R&R. Under- misfire may occur all the time. But other standably, some techs refuse to work on these jobs times the driver notices a problem only during until the customer authorizes them to replace these acceleration or wet weather. In its earlier poor-fitting wires with a quality product. stages, this arcing may cause a surging sensa- Of course, the wires on a recessed-spark plug tion during light-throttle driving or steady Japanese engine may look fine but still be arcing cruise conditions. Stay alert, because several inside the boots. I’ve seen arcing plug wires drive sharp technicians have told us they initially techs nuts because they didn’t do some basic but mistook this surging sensation for a torque essential secondary ignition tests. converter clutch problem! Some guys like to analyze the secondary As with other problems we’ve all tackled, an with a power-graphing meter such as Snap- accurate vehi- on’s Vantage equipped with an optional igni- cle history is tion test kit. Another, more traditional tech- immensely nique is a snap-throttle test of secondary firing helpful. Look voltage while watching the results on a scope closely at the or bar graph display. Snap-throttle simply plug wires means flooring the gas pedal as quickly as pos- during your sible, then releasing the throttle as quickly as visual inspec- you opened it. Always check for a smooth op- tion and/or erating throttle linkage before doing this test. question the What’s more, releasing the throttle as quickly customer as possible is essential to a safe, successful carefully. If test. With some practice, you’ll find that a the drive- proper snap-throttle test doesn’t increase en- ability symp- gine speed very much—certainly not to unsafe tom(s) began rpm. If your snap-throttle test sounds like a occurring af- drag engine comin’ off the line, you’re not re- ter the plug leasing the throttle soon enough! wires were re- Interpreting secondary ignition readings is a

Photo & scope patterns: Dan Marinucci & scope patterns: Photo placed, check really big topic, but concentrating on a few de- I intentionally dislodged this good plug wire to make a point. If there first. tails will help you confirm those arcing plug the boot of a replacement plug wire sits high above the valve Matter of boots. Required firing voltage, which is some- cover (arrow), it doesn’t fit, and ignition misfires may result. fact, the ma- times called kV (kilovolt) demand, is voltage continued on page 14 February 2003 13 Foreign Service

Fig. 1 Fig. 2

Firing Line

Spark Line

needed to start the arc across the Now we’ll look at the real thing, first on in the late ’70s, are a practical alterna- spark plug gap. Required firing volt- a traditional scope pattern and then on a tive for people who don’t have much age at idle is relatively low, often less bar graph display. In Fig. 1 above, we’re scope training or experience. Like the than 10kV, because operating condi- watching secondary ignition parade dur- traditional scope pattern in Fig. 1, the tions at idle aren’t too demanding. ing a snap-throttle test. By the way, this bar graph display of the snap-throttle But slamming the throttle open engine had at least two problems: arcing test in Fig. 2 shows a relatively low fir- causes a sudden, severe increase in inside No. 3 spark plug boot as well as a ing voltage on cylinder 3. If you repeat- cylinder pressure and temperature, worn spark plug in cylinder 2. Hopefully, ed the snap-throttle test while watching along with a corresponding increase even inexperienced scope users can see a bar graph display of burn time, which in required firing voltage. Depending the details on the firing (vertical) line and represents the length of the spark line, on the ignition system, snapping the the spark (horizontal) line. you’d see a noticeably longer burn time throttle open could push firing volt- The firing line represents required fir- on cylinder 3. This combination of bar ages into the 15- to 20kV range. All ing voltage. One clue that the spark is graph readings hints that the spark’s kV readings should increase during a arcing outside the cylinder is the unusu- arcing outside the cylinder. snap-throttle test. Furthermore, the ally low cylinder 3 firing line. In fact, this The Interro, Vetronix and other ig- peak firing voltages should be within firing line barely increased when we nition analyzers offer a choice of tra- a few kV of each other. nailed the throttle. ditional scope patterns or bar graphs. Okay, suppose secondary ignition For another clue, compare cylinder There are also scopeless ignition ana- voltage is arcing inside the plug boot 3’s longer, smoother spark line to lyzers—Ferret Instruments’ Models on cylinder 3. Or, cracked insulation those of the other cylinders. First of 54 and 55 are good examples—that on the No. 3 plug wire is allowing all, whenever the firing line gets let you read firing voltage and burn voltage to arc to the engine. If the shorter, the spark line has to get time via bar graph or raw numbers. voltage is arcing outside cylinder 3, longer, and vice versa. Second, nor- But if you go scopeless, be sure the then a severe pressure/temperature mal combustion chamber activity al- tester shows live readings as well as change inside the cylinder won’t af- ways causes a fluctuation or move- minimums and maximums simultane- fect it. Bottom line: A snap-throttle ment of some kind in the spark line. ously. Even a simple, scopeless tester test will cause little or no increase in Increasing cylinder pressure via the can show you a combination of the cylinder 3’s required firing voltage. snap-throttle test dramatically in- lowest required voltage and longest Never assume you’ll see a notice- creases the amount of “turbulence” in burn time during a snap-throttle able difference in required voltage the spark line. Cylinder 3’s spark line test—a combo that often confirms between cylinder 3 and the other is very smooth, while there’s consid- arcing outside the cylinder. cylinders at idle. Typically, the only erable turbulence on the other spark So there’s a gamut of diagnoses, way to see the problem is to stress lines. The reason cylinder 3’s spark from simply replacing suspect plug the ignition with a snap-throttle test. line is so smooth is that its spark is wires to using a little snippet of sec- Also, a combination of low peak firing arcing outside the cylinder, unaffect- ondary ignition testing. Regardless, voltage during the snap-throttle test ed by the sudden increase in cylinder you’ll be better prepared the next and misfiring under load usually indi- pressure. time you suspect arcing inside those cates a problem outside the combus- Secondary ignition bar graphs, which loooong Japanese plug boots. Good tion chamber. first appeared on engine analyzers back luck, and I’ll see you next month!

14 February 2003