Misfire - Ignition and Injector Inspection

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.”


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. Wait for it… told you so.

Misfire: Gather Up the Usual Suspects
Consider the circumstances: 14-year old Toyota truck, 175K of 75% freeway use, plenty of time spent off-road in the last 25,000. That means lots of careworn 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 porky Toyota V6 got even slower.