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Aiming for a Miss: Ignition and Injector Inspection
By Justin Fort/
Printable version
Degree of DifficultyModerate
Estimated Time180 minutes
180 minutes
Simple Steps for Misfire Diagnosis and Repair -
A misfire. It’s a sensation you instantly recognize, but just as quickly block out. The engine stumbles for a moment then regains its pace. Just as soon as the RPMs settle down, though, the misfire reappears, and you’re stuck with the sinking feeling that accompanies all automotive problems beyond the shadow of your wisdom: “Something’s wrong.”
Diagnosing and Repairing Engine Misfires
The sinking feeling is often followed by either “This is going to be expensive,” or “Why me/now/here?” All expected, but reasonable? What we recommend instead is “How can I fix it?”
A miss can be caused by a list of faults, but there are a few suspects that occur more than others. The primary villains are simple—spark or fuel—usually manifesting in spark plugs, plug wires, the coil(s), or the fuel-delivery system. There are other more dire causes: computer or wiring problems, breakage in the rotating mass (pistons, rods, crank bearings), valves and the heads can fail or distort, cooling difficulties might permit overheating, and any number of gaskets could have pushed. Most are rare and, importantly, most of the scary stuff was probably caused by your failure to address simpler problems in the ignition or injection.
Misfire: Gather Up the Usual Suspects
Consider the circumstances: 14-year old Toyota truck, 175,000 miles of 75 percent freeway use, plenty of time spent off-road in the last 25,000. That means lots of mechanicals being used hard and showing their age. Yes, it’s our fault: Parts that were wearing out on-schedule are more likely to do so sooner now, rather than the preferred later. It’s the anticipatable wave of maintenance that comes with new ownership of used vehicles. Don’t get lazy—just keep ahead of the curve.
While our miss was inconsistent, there were some notable details (always keep track of details for diagnoses sake). The miss came when the truck had been operated at a consistent speed (like freeway driving). It didn’t happen when the truck was cold, but would show up when it had warmed up. This miss didn’t arrive only under load: It could as well show up at idle as when accelerating. Of course, a miss while accelerating meant the Toyota V6 got even slower.
The sensible method is to gather available knowledge about the misfire, focus on steps necessary to eliminate suspects, and let the process guide you to its cause. Call it scientific method, with some sensible leaps. As for knowledge, if your car or truck is computer controlled, the place to start is to plug in. A code reader, available from parts stores for a bill or so, will permit you to jack into the engine-control unit (popularly known as the ECU) and get a dialogue of what’s up, what’s wrong and where it’s happening. The ECU can’t always tell you what specific part is broken, but in the case of our truck, it had stored data indicating there was a miss in Cylinder #4. Okay, six cylinders of potential problems has just been narrowed to one. Good start.
Had we not been computer controlled, studying the spark plugs would have helped focus on possible sources of a miss. It’s not hard to read the plugs: with a little attention and a good guide, such as those available in Chilton’s and Hayne’s manuals, plugs will indicate with clarity where problems are, if cylinders are out of tune, and if they’re lean or fat. Everything has its signs.
Before you get started however, be sure to follow all the safety protocols with goggles, gloves and whatever else is needed.
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