Figure 3 The Ford's EGR valve is readily accessible.

I removed the EGR valve and found carbon stuck between the pintle and the seat, so I opened up the valve and blew it out with some shop air. I reinstalled it with a new gasket and verified the repair. I felt pretty good that this was the rough idle cause, but I didn't think that it was also responsible for the stumble I felt under acceleration.

TIME FOR THE SCOPE

Figure 4 While this scope pattern of the ignition secondary looks normal at first, notice how high the firing line (vertical line) is.

While the pattern looks normal at first glance, the firing line of the pattern was higher than normal. This portion of the pattern represents the voltage required to ionize the gap(s) in the secondary and allow spark to occur. Typically, this is affected by the size of the gaps, the pressure the gaps are under and the amount of hydrocarbons available in the gaps promoting conductivity.

The high firing lines were common to all cylinders. The engine had more than 60,000 miles on it, and I decided to check the spark plug gaps for wear. The specification for plug gap on this engine is 0.052 to 0.056 inches. I measured gaps on all the left bank plugs at roughly 0.070 (a 0.060 wire gauge fit easily while a 0.080 gauge was a no-go). OK, if firing voltage is high at idle, what would I expect it to be under load, when cylinder pressures are highest and demand on the system greatest? A stumble.

MODE $06 HELP?

There was one more source of information I thought might be helpful in telling me if I was on the right track, and that was the Mode $06 information stored in Global OBD2. I reconnected the scanner, accessed Mode $06 and looked for Test ID (TID) $53.