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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
Diagnose and Investigate: Ignition
Choose your plan of attack—cheap to expensive, easy to difficult—and stick to it.  (At the same time, be sure to follow all the safety protocols with goggles, gloves and more.)  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, not a spark problem. The R&R of fuel injectors is a bigger project than the plugs, though, so we stuck to the plan and stored the knowledge for if ignition repairs failed to fix things. The other plugs had been replaced about 20K earlier, and looked almost ideal. Every one was in good shape short of the lean read on #4. Sticking to sensible method, 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. The plugs did, but the problem didn’t, and P0304 came back.
Photo 1: Plug read: ash-brown with a hint of green—good mixture on late-model engine. Touch of grey from hot, hard run up and down Black Mountain. White flecks, bad gas?
Reading Spark Plugs is Easy
Plugs good, plug wires not? The born-on dating for plug wires is easy to find on Toyota products—it’s stamped on the wire. Those on this truck were as old as the truck itself and probably original, so even though they ohmed out fine and looked good, if dusty, at 175K it was not hard to justify a new set. With a new set of wires in place, the P0304 came right back, so it wasn’t a wire problem. On the flipside, now we’ve got what looks to be a good backup set, and a new set on the motor that’s got to be worth at least 100K (it’s a Toyota, so we’ll let you know).
There are a few easy ways to test plug wires. Examine them in the dark, engine running, and watch for sparks jumping. Next, mist the wires with water and see if this causes any sparks (in the same dark environs). You can remove a wire and gently bend it to see if the rubber sheathing cracks. All of these indicate failing wires (and don’t worry, you’re looking for small sparks).
Photo 2: The bend test: even with 175,000 miles on them, the OEM plug wires take a curve without cracking. Quality OEM product right there.
Spark Plug Wires Curve Without Cracking
With the spark throwers and spark carriers cleared of responsibility for code P0304, we moved along to the spark makers. On this Toyota, three coil packs live on the #1, 3, and 5 cylinders, and each power a plug there and on the opposite side of the engine, at #2, 4, and 6. The system is called waste-spark: The coil shoots two sparks at once, and the plug fires twice in the combustion cycle, once to fire the cylinder, and once more to clean up the leftovers in the exhaust stroke. Other vehicles can use a single coil firing through a distributor, or a single coil on each cylinder, but your job’s the same. Find the problem and solve it. We’d tested plug and wire, so it’s on to the coil. Using a multimeter, you can test the ohm ratings of both primary and secondary outlets on the waste-spark coil, and all of those on this truck tested well (between .67 and 1.05 ohms at the primary, 9.3K to 16K ohms for the secondary). Consult your repair manual or factory service manual (FSM) for all the test ratings. With no indication of a bad coil, a return to the swap methodology (sensible method) had us switch the #1 and #3 coils, but the miss at #4 remained.
Photo 3: One of three waste-spark coils on the Toyota V6. They don’t usually fail until plus 200,000 miles, so expect to find them dirty and untouched.
A Waste-Spark Coil
Photo 4: If you don’t own one, experience for yourself why the multimeter gets called the Ten Buck-O-Meter.
A Multimeter
With P0304 continuing despite plug, wire and coil checks, we were done with easy fixes. Shucks. On to the next suspect, one indicated by the plugs (lean read on #4), misfire behaviors (intermittent, heat-related, and occurring at consistent RPMs), and the elimination of other suspects: fuel. Though an injector problem had been suggested earlier, it was best to rule out the ignition parts before moving on to a set of injectors that required some real wrenching to access.
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