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So I am picking up a Forester on Saturday. I was told its been running on 3 cylinders for the last couple months but doesn't overheat or anything else. Would it be smart to start with a new coil pack and plugs and wires.

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I haven't seen many Coil Packs fail on these so it's probably a wire or plug. Start with pulling the spark plug wire for each cylinder while the engine is running. If the engine speed changes move on to the next and do that with each cylinder until you don't notice the engine speed change. Thats your problem cylinder. Hold that wire close to a grounded metal part on your vehicle and see if your spark jumps. If it does jump then its your spark plug or your injector. If it doesnt jump then remove the spark plug wire for that cylinder from your coil pack and hold it as close to that now open port on your coil and see if the spark jumps. If it doesn't then it's you pack, if it does then it's your plug wire.
If your wire checks out try removing the injector wire and plug it back in and wiggle it to see if that helps it. If it doesn't, pull your plug to make sure it isn't gummed up. If it's not , make sure you have compression in that cylinder. If so move to the injector.  My guess is its your wire. Although on my 99 I recently had a cylinder missing. Turns out my power steering pump was leaking and the fluid was pooling in my spark plug hole and the spark was grounding thru that. Good Luck.

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Find out which cylinder is not running/improperly. There will be a code. If no 4, the biggest possibility is a burnt exhaust valve. This will require a head job.

If you can do the work yourself, then it's not fatal. If you have to pay someone, then it would be a deal breaker.

 

O.

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