DIY Engine Misfire Diagnosis And Repair | Mobil™

By Justin Fort, automedia.com Difficulty: Moderate Estimated time: 180 minutes

An engine misfires. It’s a sensation you instantly recognize but just as quickly block out. The engine stumbles for a moment and then regains its pace. Just as soon as the rpm settle down, though, the engine 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?”

Engine misfires 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.

Engine misfires: 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 the sake of engine diagnostics). 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 engine misfire didn’t arrive only under load: It could as well show up at idle as when accelerating. Of course, a misfire while accelerating meant the Toyota V6 got even slower.

The sensible method is to gather available knowledge about the engine 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, will permit you to jack into the engine-control unit (ECU) and get a dialog 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 an engine misfire in Cylinder #4. Okay, six cylinders of potential problems have just been narrowed to one.

Had we not been computer controlled, studying the spark plugs would have helped focus on possible sources of a misfire. 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.

Before you get started, however, be sure to follow all the car maintenance safety protocols with goggles, gloves and whatever else is needed.

Diagnose and investigate: Ignition Choose your plan of attack – cheap to expensive, easy to difficult – and stick to it. It’s cheap and easy to start with ignition items, so we went to the spark plugs. Because the P0304 had repeated, the #4 plug came out first. It read lean (a gray-brown, not bad but trending hot and fuel-starved), predictive of a fuel problem rather than a spark problem.

The repair and replacement (R and R) of fuel injectors is a bigger project than the plugs, so we stuck to the plan and stored the knowledge in case ignition repairs failed to fix things. The other plugs had been replaced about 20,000 miles earlier and looked almost ideal. Every one was in good shape, short of the lean read on #4. We cleaned them up and swapped the plug on Hole #4 with #2. If the problem were the spark plug, the misfire would move to #2. It didn’t. P0304 came back.