In my part of the world, business is cyclical, like the phases of the moon or the changing tides. Some days you are busy,
and some days you're slow. And sometimes, you are slammed. This Mustang came in on one of "those" days: I've got 10 cars on
my schedule, all waiting for me to diagnose a variety of complaints, and of course, they all want them back by the end of
the day. To say I was busy would be an understatement.
THE FIRST MISTAKE
The complaint was simple enough. As is my usual practice, I first verified the rough idle and stumble by taking a test drive.
When I got back to the shop, I hooked up my scan tool and found code P0301 (Cylinder Number One mis-fire) and P0316 (misfire
detected on start up) stored in the Engine Control Module (ECM).
This vehicle uses a Direct Ignition System (DIS) that fires two plugs simultaneously. Opening the hood, I could hear the distinctive
"tick" of a spark jumping to ground outside the cylinder. Looking a little more closely, I could see the spark jumping to
the valve cover on the Number One wire. The wires looked like original equipment, and a closer inspection revealed signs of
leakage in the others. I knew that bad wires were a common cause of coil failures, so I hooked up my scope to Number One and saw a low firing line
and spark line that looked an awful lot like what I am used to seeing when the coil fails.
On this type of coil, one plug is "positive," and one is "negative." When the coil discharges, current first travels to ground
thru the negative plug, then back to the coil through the positive plug. When the coil is stressed, the internal insulation
can fail, reducing total coil output. In this low state, there is just not enough voltage left to jump the gap on the second
plug, even though the first plug continues to run just fine. That's why it's possible to have a DIS coil with one dead plug.
OK, I got this one nailed. I order a new coil and ignition wires and move on to the next car on my list.
SHORTCUTTING A DIAGNOSIS
When the parts arrived later in the day, I pulled the Mustang back in to the bay. It is a simple installation and took no
time at all. I cleared the codes and went to verify the repair. Have you guessed yet? The miss was still there. I took it
for another test drive, and the original codes were back.
You would think that after all the time I've had in this business I would remember my personal rules regarding diagnostics:
Never take a shortcut, especially on a misfire code.
Relearning the Rules
VEHICLE: 2003 Ford Mustang
DRIVETRAIN: 3.8 V-6 Automatic transmission
MILEAGE: 65,261 miles
COMPLAINT: Vehicle idles rough, stumbles. MIL on.
Yes, I was busy. Yes, I had pressure on me to get the cars looked at and diagnosed. Yes, I was in a hurry and didn't follow
the rules.
FIGURE 1 I thought the low firing line indicated a common fault — a bad coil. I was wrong.
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A misfire code can be set by any condition that doesn't allow for complete combustion in the cylinder. My normal procedure
is to first do a relative compression test to ensure the engine is mechanically sound. Doing that test now indicated that
the Number One cylinder wasn't good.
