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It's difficult to say if the hesitation/power loss symptom and the stalling are caused by one issue, but it's possible.
It's difficult to say if the hesitation/power loss symptom and the stalling are caused by one issue, but it's possible. (Dreamstime/TNS)

Q: Over the past year, my 2010 Subaru Forester intermittently stalls and hesitates while I’m driving. By hesitating, I mean the car loses power and the check-engine light comes on and flashes. I can limp the car along at about 40 km/hr until it “kicks” in and lurches ahead at which point the check-engine light goes off and the car returns to normal.

At other times, the car will stall when I’m stopped. There doesn’t seem to be any rhyme or reason to when the stalling or hesitating happens, but these episodes are becoming more frequent.

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I have taken the car into several different mechanics (including the Subaru dealership) to have the car scanned, but no one has been able to diagnose or fix the problem. Of course, when I take the car in, it runs normally. A recent scan indicated the catalytic converter needs replacing (but the problem existed well before this), and other scans indicate misfires.

Mechanics have cleaned the fuel injection system, changed the wiring to the O2 sensor, changed the EGR valve, changed spark plugs … all to no avail. I’m at a loss. Any ideas what could be causing this? Any suggestions would be greatly appreciated.

—Linda

A: In a follow-up message Linda indicated there was mention during two repair attempts of diagnostic trouble codes pointing to cylinder 1 and 2 misfires, but it seems the repairs were largely focused in other areas. This would be consistent with the flashing check engine light, which indicates a catalyst threatening misfire is occurring (continued driving is highly discouraged).

It’s difficult to say if the hesitation/power loss symptom and the stalling are caused by one issue, but it’s possible. I’m thinking there may be a problem with either the ignition coil or one of the spark plug wires. On this 2.5 L non-turbo engine Subaru employs a waste spark ignition system, meaning one ignition coil fires two cylinder’s spark plugs in a loop configuration. Both coils and the ignition control module are combined into a single unit. Looking at the cylinder groupings, cylinders 1 and 2 share one coil, and 3 and 4 share the other. This is just too large of a clue to not run with!

When it’s time for either cylinder 1 or 2 to receive spark, the coil is fired and spark travels the loop, firing both spark plugs, one in the forward direction, one in reverse, even though only one of the cylinders needed it. That’s where the term “waste spark” comes in; one of the sparks is unneeded at that time. It’s a slick system, not as cool as individual coils, but it’s vulnerable to spark leakage, which can take out both cylinders. I wonder if that’s what’s happening, as spark leakage is as fickle as can be, depending on conditions!

Perhaps the ignition coil has a tiny insulation fault, or one of the spark plug wires has a crack or split, or is chaffing against sharp or hot metal. Under certain operating conditions the desired spark path is more favorable, and during others, perhaps hot/under moderate engine load, the leakage path is momentarily preferred — causing two cylinders to misfire.

I’d try wrapping aluminum foil tightly around the ignition coil (also touching engine metal), encouraging spark to leak (the insulation fault could also be internal, passing this test). If the symptom worsens, replace the coil! Carefully inspecting/rerouting the spark plug wires may point out a problem there. A head gasket failure between cylinders 1 and 2 is also possible but not likely, as the symptom is intermittent.

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