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Old 11-08-2005, 11:15 AM
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My car has a misfiring cylinder...can I fix that myself?

Short story: My alternator died and when it got replaced, the mechanic noticed the check engine light was on. He used his fancy computer, and it told him that cylinder #2 was misfiring (or, at the very least, was misfiring in the past) He reset the computer, but the light came back on, so I guess it wasn't a one time thing.

What could be a probable cause? My limited car knowledge tells me one of the causes could be a bad spark plug, and a spark plug is fairly inexpensive, so as long as it's not hard to replace one (doesn't seem it from what I've read) I'll do that at the very least. But what else might cause it, and is there any way for me to tell by listening to the engine, or doing some other kind of test that doesn't require expensive equipment?

If there's nothing more I could do myself, what might be the cost to have it repaired (that is to say, what parts might need to be replaced and their cost, and approx. time needed? (I think the place has ~$50/hour labor cost))

For what it's worth, my car is a '98 Subaru Legacy GT (that's the version with the 2.5 L engine.)
Old 11-08-2005, 11:23 AM
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first thing i'd try is to replace the plugs and wires.
Old 11-08-2005, 11:50 AM
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Plugs & wires here, too. 2.5 is a 6-cylinder right? Careful, CAREFUL when replacing the wires that you do them one at a time to make sure the right wire goes to the right plug. You'll know immediately if you screw this up, but it cvan take a while to sort it out.

If that doesn't solve your problem then there's probably some magical gizmo under the hood that's malfunctioned and it'll take a mechanic to diagnose & fix.
Old 11-08-2005, 11:52 AM
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On a related note, what happens when a car doesn't fire on one cylinder (I heard this with reference to a 1980s Beemer 6 series on a car show)

Does unburnt fuel just get squirted straight out the exhaust pipe?
Old 11-08-2005, 11:55 AM
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Sure does. I f-ed up my plug wires on the Camry once and the fuel collected in the catalyitic converter. Penis ensued.
Old 11-08-2005, 01:44 PM
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Quote:
Originally Posted by Inigo Montoya
Plugs & wires here, too. 2.5 is a 6-cylinder right? Careful, CAREFUL when replacing the wires that you do them one at a time to make sure the right wire goes to the right plug. You'll know immediately if you screw this up, but it cvan take a while to sort it out.
Nope, still just 4 cylinders. And yeah, the few thngs I've read about replacing spark plugs ALL warn to only do one at a time. But since I only hve to do one, the chance of getting them mixed up drops even more.
Old 11-08-2005, 01:44 PM
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Quote:
Originally Posted by Pushkin
On a related note, what happens when a car doesn't fire on one cylinder...Does unburnt fuel just get squirted straight out the exhaust pipe?
Well, it doesn't squirt a stream, if that's what you mean. It's more of a mist at that point. Some of it stays in the cylinder and forms carbon deposits. Some of seeps past the rings and contaminates/dilutes the engine oil. The rest goes out the cylinder's exhaust port, into the exhaust system. Most of that will be burnt in the catalytic converter, which overheats the converter and shortens its life. What doesn't get burnt there will work its way out the tailpipe in gaseous form.

Back to the OP, I agree that faulty plugs and/or (especially) plug wires are the most common cause of a misfire. They're not the only cause. Other possibilities incude fuel injectors, engine mechanical problems, and electronic control system problems (sensors, computer, etc.). Accurate testing is difficult without proper test equipment.

The choice is to either have it tested, or play the odds and replace highly suspect parts in the hope that fixes it. If it doesn't, then it's testing time. Replacing the plugs and plug wires is a reasonable thing to do -- there's a good chance that will fix it, and even if it doesn't it's likely they're due for replacement on a maintenance basis. Just be aware it's not a sure bet that will fix it.
Old 11-08-2005, 01:47 PM
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If you are going to try parts replacement, I suggest replace all four plugs and the entire plug wire set. If one has failed now, the others are likely to follow suit before too long.
Old 11-08-2005, 04:01 PM
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Like I tell my students, if you are chasing a misfire, change the thing that makes the fire. (Sparkplugs) Simple cheap, and quite often fixes the problem. if money is an issue, I might try just plugs before changing the wires, but you might want to do both at the same time. Change the wires one at a time. Route them exactly where they werre before. Do not take any shortcuts. Some engineer who knows a hell of a lot more about Subarus than you do decided on that routing for a reason. Yes it is a pain in the ass to put it back the way it is supposed to be, but those wires are arranged that way for a reason.
IIRC it is very easy to cross thread a plug on a Subaru. Be very careful, and buying a spark plug starter for $5 might save you a bunch of grief. One suggestion, when screwing in the plug run it in with your fingers for at least 3 or 4 complete revolutions. If the plug won't turn that far by hand, chances are you got it cross threaded. Take it out and start over. Do not force it with the rachet. This is really an ounce of prevention vs pound of cure thing.
If this fails to fix it, I would suggest that you follow Gary T's recomondation and have a pro look it over.
Old 11-08-2005, 04:34 PM
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Quote:
Originally Posted by Gary T
If you are going to try parts replacement, I suggest replace all four plugs and the entire plug wire set. If one has failed now, the others are likely to follow suit before too long.
yup.

when replacing spark plugs, always thread them in by hand - never ever ever start them with a wrench. then use a torque wrench to get them to the prescribed torque.
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