Failed Ignition coil - Engine Damage
 

2001 Subaru Legacy 2.5L Petrol @ 90K

My ignition coil started to go bad, this caused an engine misfire in one cylinder with the engine warning light on. I took it to the garage as soon as I could, (after driving a few hours unfortunately) the misfire wasn’t too bad with only a few jolts when idling and slight hesitation under load.

I had a new coil put in, this was working fine for a few days but then failed with 2 cylinders dropping out. Mechanic said it was a dodgy part from new. It was running very rough and struggling with the engine check light flashing. I limped it 5 mins round the corner to the garage A new coil was fitted again under warranty.

After a few days later, my engine warning light has come on. This is currently being looked at, I suspect it’s the O2 sensor and a ruined catalytic converter due to raw fuel passing though and melting it?

I’m also concerned with fouled spark plugs, deposits on piston + valves and fuel getting into the crankcase and contaminating oil and causing problems. I’m also concerned with possible damage to cylinders with additional wear caused by fuel taking away the protective film of oil etc…

My questions:

• Is there any other damage that could have been caused?
• What steps could be taken now to repair as much as possible any damage that may have occurred?

I’m thinking clean any fouled spark plugs, oil change, probably new cat and O2 sensor?

I feel stupid for not stopping and getting towed by the RAC, it was difficult with my work and travel situations though - difficult to avoid.

Cheers

Many years ago on another type car I had a coil pack fail which ended up wrecking the cat due to the unburnt fuel, so that is a distinct possibility with yours. However, I wouldn't expect damage to cylinders and pistons from a misfire.

As you suggest, a change of cat, O2 sensor, plugs and oil should be enough to get you going again. Obviously the cat will be wallet emptying item, so hopefully someone on here can offer you a decent second hand unit.

Spark plugs changed yet?

Plugs should always be changed with coil packs, as a fault with one can cause issues with the other; Excessive gap or damaged insulator can cause spark energy to jump somewhere it shouldn't...like in the coil pack itself (and damage it as a result). A dodgy pack can cause plugs to get fouled. So belt and braces; New coil = new plugs.

If it's a 'dead' misfire (misfire under all conditions), usually the ECU will shut off fuel to that cylinder until the next engine start to protect the cat etc. However if it's a partial misfire (i.e under heavy load or at certain rpms) this may not trigger.

Thanks for the reply davemar, I'll have an ask around for a 2nd hand cat. Good to hear you don't think cylinders + pistons would have been damaged. Cheers

Thanks Ali-B, I'll take the plugs out this weekend, they were only a few months old and had fairly new leads on there as well. Do you think the old failing ignition coil could have damaged these newish plugs? Could they just be cleaned up?

Good to know the ECU shuts off fuel to the cylinder, I think thats what happened when it was running very rough with the new (faulty coil) and only on 2 cylinders I think. Previous to that with the old coil it was more like you say with the partial misfire, under load and at certain rpms.

Cheers