A miss is not good over the miles

Q: I have a Mercury Sable wagon. The engine is the 3.0L V6, with 113,000 miles. Automatic transmission. Consistently while accelerating between 35 and 45 mph, the engine shudders and bucks. At higher speeds the auto runs fine.

New plugs and wires and fuel filter did not help, nor did good injector cleaner in a few tanks of gasoline. The Mercury dealer's computer found a bad valve in cylinder 6, and they think this is the problem; I have doubts.

A: The first thing I'd do is to be certain the transmission is in the appropriate gear for the vehicle speed and engine load applied. If manually selecting a lower gear causes an engine rev, yet the shuddering still exists, we can concentrate on the engine.

If the engine truly runs smooth at certain times and rough at others, the cause isn't likely to be mechanical. A slightly bad valve in cylinder 6 is certainly something to keep an eye on, but this sounds more like an electrical demon. You've replaced plugs and wires, but could one of the spark plugs have been damaged during installation? Is the distributor cap free of cracks and carbon tracking? Does the ignition secondary system pass an available voltage test? Dirty fuel injector?

Identifying which cylinder is misfiring should be our next step. Can the misfire be replicated with the vehicle stationary, or revving against the brake? If so, each cylinder can be momentarily disabled, with the offender showing little or no difference in engine speed and shake. If a specific cylinder is identified, a very close look at all components supporting it should reveal the cause. Should cylinder 6 be the offender, a valve job may be the solution.

You mentioned using fuel injector cleaner. Have them test each injector's flow rate. Also, look at the engine control system's RPM signal and fuel injector commands, among other circuit signals, using an oscilloscope or graphing multimeter. If erratic, these could cause a sputtering, misfiring engine.

E-mail Brad Bergholdt at under-the-hood@juno.com