Fuel, Air, Spark

“It stalls”. My gas mileage is terrible.” There’s no power anymore”. Driveability issues generate similar complaints from Canadian motorists, but unlike the symptoms, causes can be as simple as a fuse, or as complex as a computer chip. Finding the trouble and repairing it quickly is still the name of the game, and if you’re using a special diagnostic rate, sends the customer a signal about the quality of your shop.

Where do you start? It depends on what you’re looking for, and why. OBD is a common starting point, and with the newer scan tools, hookup is fast enough to make pulling a code the logical starting point even where experience suggests the solution as the customer drives up. What the scan tool won’t do, however, is solve the problem. As Jeff Elder, marketing manager for Blue Streak Electronics says, “many people buy a scan tool on the assumption that it’s going to fix the car for them. It’s a knowledge tool to let them see into the ECM. It doesn’t have artificial intelligence. It can give a value to the technician, but there are many reasons why that value is such. That’s where it takes a good technician who understands what he or she is looking at. Many read codes and assume that they need to replace a part. There can be many codes set that aren’t erroneous, but can mask other problems. The scan tool is just a tool.” Pulling the code is a great start, but delivering fuel, air and spark to a tight cylinder at just the right time is the real issue; trouble codes help begin the diagnostic, but should rarely be used by themselves for fault finding.

Spark plugs are still at the heart of the combustion process

Sensors can detect specific performance parameters, but just as a doctor still uses a thermometer under the patient’s tongue, spark plugs are still an excellent, low effort way to determine if an engine is “under the weather”. Ironically, it’s the ECM’s ability to regulate ignition timing and engine knock that drives mean combustion chamber temperatures higher, great for performance and efficiency, but hard on spark plugs when there’s a problem. According to Doug Morrison, technical service manager for NGK, even a small head gasket issue can be diagnosed by reading the plugs: “Antifreeze going into your chamber will dissolve the lubrication aspect of the oil, rapidly increasing the cylinder temperature. That’s why one plug shatters and the technician asks “what’s going on with this?” You end up with half the insulator still on, and (the fracture) goes all the way down to the bottom. Since it’s a nice clean cut, it looks like a manufacturer’s defect. In extreme cases, both halves will cut and all you’re left with is the centre core. If the plug isn’t torqued down properly, you get blow by and the same thing happens.”

Another factor is built into the design of many distributorless ignition systems: current flow. Many DIS systems are designed to “waste fire” one plug on the exhaust stroke, allowing a single coil to fire two cylinders. While that’s convenient and cost effective for the manufacturer, a close look at the current path will shows that in some systems, current flow is reversed through the “off” plug, with the spark jumping nominally from the ground wire to the centre electrode. If the plug uses platinum or some other durable electrode material on one side only, such as on the ground electrode, accelerated wear can occur. DIS is a case where downgrading from full platinum to conventional plugs is rarely a good idea.

Do-it-yourself tune-ups and cleanup work from less competent shops are another source of troubleshooting difficulty. Poor running with new plugs and wires? According to NGK’s Morrison, the problem is often too much dielectric grease applied on installation. “That’s one of the biggest mistakes even the technicians are making nowadays”, says Morrison. “They put the stuff right in there. All you’re supposed to have is a thin bead around the edge of the boot so it slides over the insulator, giving it a light coat. That’s why you see wire companies that give you a little tube. A lot of technicians think that that’s not enough.” Morrison also warns of overuse of the compound at the distributor or coil end, and notes that if the terminal is stainless steel, as in many Japanese models, the wires can go on “dry”.

Oxygen sensors are “plugs” too

Oxygen sensors are the “other plug”, and like spark plugs, can feed critical information to the technician about the combustion process. Even a dead sensor can often tell a story. The life span of an oxygen sensor is a good indicator of general engine condition. According to Nosa Akamatsu, group product manager, oxygen sensor and fuel injection products, Robert Bosch Corporation, sensor life depends on type, but in ether case, it should be lengthy: “It’s wear and time that causes failure, depending on the type of sensor and system. Unheated sensors may wear out at 30,000 to 50,000 miles. Heated sensors will survive 100,000 to 120,000 miles. Heated sensors last longer because they are exposed to (uncontrolled) exhaust gases for a shorter time than unheated units.”

Akamatsu notes that the period before the sensor heats to operating temperature is a vulnerable time for oxygen sensors, which are exposed to contaminants from rich mixtures and possibly crankcase blow-by gases or coolant. Heated sensors come on-line faster, controlling mixtures sooner, reducing contamination. Regardless of which type a technician is dealing with, manufacturing defects rarely cause premature failures, although a significant number of techs swap out suspect sensors almost automatically. While that’s necessary for a contaminated unit, it’s important to think about the cause of the failure in the first place. “Any one of the components of the fuel system may cause O2 sensor failure, declares Akamatsu, who adds, “For example, a clogged fuel line can alter the mixture ratio and shorten the oxygen sensor lifetime. Similarly, too much fuel will cause a shortened sensor life. Failure of oxygen sensors can also cause failures of the catalytic converter. The air fuel mixture becomes unstable, damaging the converter and causing poor fuel mileage.”

In an unfortunate number of cases, the sensor hasn’t failed, but the installer has. One problem involves, again, silicone dielectric grease. Doing without the silicone is also a good idea around O2 sensors. By their nature, O2 sensors generate very small voltages, about half a volt in most cases, so contact resistance is crucial to reliable operation.

Connectors must be clean, tight, and installed according to the manufacturer’s recommendation to get a circuit with the correct resistance for accurate sensing. The ground path through the exhaust is equally important, but a surprising number of technicians are still using insulating sealant on the threads. Universal sensors require extra care on modern heated units. Cut and connect one wire at a time, or double-check the wiring diagram after installation. A crossed wire here will result in a non-functional sensor at best. It happens more often than you think.

With multiple sensors, actuators and computers controlling automotive systems, it’s easy to blame the “black box” when diagnosing driveability issues. Despite their reputations, the electronics are more reliable than many think. Checking out the wear items such as plugs, wires and oxygen sensors are a great way to peek inside the combustion process and are worth the time even where a scan tool and OBD leaves a trail.