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Old 09-12-2011, 01:10 PM   #1 (permalink)
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Default A misfiring spark plug would cause lean or rich?

As stated, if a spark plug didn't fire would the o2 sensor be lean or rich? All my fact digging and understanding says it would be outputting a lean signal.

Just a argument I'm having with a friend.
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Old 09-12-2011, 01:35 PM   #2 (permalink)
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Not sure how much affect a single non-firing cylinder would have on overall o2 readings, but in theory, if your plug didn't fire, nothing would burn the gas, hence a rich (more fuel than air) condition in the cylinder.

Again in theory, when something like this happens, the ECU would get a rich reading from the o2 sensor and start pulling fuel which would then cause a lean condition. But if its only one cylinder, I really can't say how that would affect your o2 readings.

You can try it out, go out and start you car, then remove one of the plug wires (or start it with one plug wire off). If it does run on 3 cyl., then look at the o2 readings.

Report back with your findings.
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Old 09-12-2011, 01:42 PM   #3 (permalink)
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But the o2 sensor detects oxygen not fuel. A cylinder full of unburnt oxygen just went down to the sensor. Yeah there's fuel but o2 sensor doesn't detect fuel. So it would tell the PCM it's lean, PCM would add fuel to compensate creating a rich running car.

I'm with you though, I don't know if a spark plug occasionally not firing would effect anything enough to cause the PCM to make a change.

Someone knows this. Hoping duster, turbo666, or some other internal combustion smarty pants will chime in.
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Old 09-12-2011, 01:49 PM   #4 (permalink)
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Quote: Originally Posted by dohc122acr View Post
But the o2 sensor detects oxygen not fuel. A cylinder full of unburnt oxygen just went down to the sensor. Yeah there's fuel but o2 sensor doesn't detect fuel. So it would tell the PCM it's lean, PCM would add fuel to compensate creating a rich running car.

I'm with you though, I don't know if a spark plug occasionally not firing would effect anything enough to cause the PCM to make a change.

^^ True about the sensors and it would read lean and it shouldn't effect it right away because the pcm runs off fuel trim but after a while the pcm will try to adapt to it
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Old 09-12-2011, 01:52 PM   #5 (permalink)
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Good point DOHC and it makes sense.

Air-fuel ratio (AFR) is the mass ratio of air to fuel present. Just figured that the fuel is still mixing with the air, even if it isn't a burn mixture, for sensor to read. More fuel = lower A/F ratio.

Now I'm curious.... lol
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Old 09-12-2011, 01:55 PM   #6 (permalink)
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Misfires are often Lean
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Old 09-12-2011, 01:56 PM   #7 (permalink)
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Quote: Originally Posted by Mopar McNeer View Post
Good point DOHC and it makes sense.

Air-fuel ratio (AFR) is the mass ratio of air to fuel present. Just figured that the fuel is still mixing with the air, even if it isn't a burn mixture, for sensor to read. More fuel = lower A/F ratio.

Now I'm curious.... lol

Your on my friends side of the argument, at least I thought this is what his thinking was. But it's all good, just some fun education.
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Old 09-12-2011, 02:11 PM   #8 (permalink)
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Quote: Originally Posted by duster360 View Post
Misfires are often Lean

So you are saying that a non-firing plug would cause a lean condtion Duster? I wasn't entirely sure how to interpret your comment there.

Quote: Originally Posted by dohc122acr View Post
Your on my friends side of the argument, at least I thought this is what his thinking was. But it's all good, just some fun education.

Definately a good one. Should learn something by the time this is done.
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Old 09-12-2011, 02:15 PM   #9 (permalink)
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Quote: Originally Posted by duster360 View Post
Misfires are often Lean

Just a clarification. Would a unburned cylinder be picked up by the o2 sensor as lean or rich. And would an occasional ignition miss cause the PCM to add fuel at idle and light loads?
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Old 09-12-2011, 03:15 PM   #10 (permalink)
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A misfire is a brief and random lean spike.

The car would have to be running pretty shitty for the PCM to want to make a fuel adjustment. You'd get a P0300 first.
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Old 09-12-2011, 03:29 PM   #11 (permalink)
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Thanks duster. Makes sense to me.
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