Differtiating between ignition vs valvetrain issues
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Joined: Jul 2003
Posts: 213
Differtiating between ignition vs valvetrain issues
I've got what I think is an "advanced" topic: being able to differentiate between a high RPM miss attributable to ignition problems vs valvetrain, and I'd like to hear other people's thoughts on it.
I've got a high RPM, random miss. I noticed that when it happens you can see the AFR swing lean, and I'm trying to figure out if that would correctly rule out valvetrain.
If its an ignition problem, the mixture isn't ignited so you get a bunch of raw fuel and air dumped into the exhaust. Since the O2 sensor only measures oxygen and not fuel, it considers this unburned mixture to be lean, because it sees the extra available oxygen.
What I'd like to know is what the O2 sensor would do in the case of valve float:
- If the intake doesn't close, the mixture is pushed back into the intake manifold. Reduced power, but whatever does remain to burn should still be right AFR.
- If the exhaust doesn't close, the unburned mixture is pushed out into the exhaust, so it looks very much like an ignition problem, I'd think.
So my conclusion is that both ignition and exhaust valve float will cause a lean read from the O2, but intake valve float won't. So in the end its not much of a clue at all.... right?
Are there any other indicators you can use to distinguish between the two possible causes?
I've got a high RPM, random miss. I noticed that when it happens you can see the AFR swing lean, and I'm trying to figure out if that would correctly rule out valvetrain.
If its an ignition problem, the mixture isn't ignited so you get a bunch of raw fuel and air dumped into the exhaust. Since the O2 sensor only measures oxygen and not fuel, it considers this unburned mixture to be lean, because it sees the extra available oxygen.
What I'd like to know is what the O2 sensor would do in the case of valve float:
- If the intake doesn't close, the mixture is pushed back into the intake manifold. Reduced power, but whatever does remain to burn should still be right AFR.
- If the exhaust doesn't close, the unburned mixture is pushed out into the exhaust, so it looks very much like an ignition problem, I'd think.
So my conclusion is that both ignition and exhaust valve float will cause a lean read from the O2, but intake valve float won't. So in the end its not much of a clue at all.... right?
Are there any other indicators you can use to distinguish between the two possible causes?
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Joined: May 2003
Posts: 6,661
From: LA (lower Alabama)
Re: Differtiating between ignition vs valvetrain issues
Start with reading your plugs-- then if OK check fuel pressure with guage taped to windshield and read when it's doing it.
Valve float should be ALL the time at a certain RPM when at WOT.It would not do it sometimes and not others.
Narrow your plug gap to .035 and see if that helps. Seen more than one miss from wide plug gap.
Valve float should be ALL the time at a certain RPM when at WOT.It would not do it sometimes and not others.
Narrow your plug gap to .035 and see if that helps. Seen more than one miss from wide plug gap.
Registered User
Joined: Aug 2001
Posts: 152
From: Memphis
Re: Differtiating between ignition vs valvetrain issues
Dave, are you still running an opti? I can remember some discussions on the edit list a while back and that may be your issue. On some of the dynos I've seen float, opti problems and spark issues on a boosted setup can show the same staggered plt.
I don't think you can see a difference with random misfires of the three on the AF plot
Scan for for misfire counts. On a sun machine, you can get the real answer you want since it can analyze the spark itself. These are an art though, I was sold as a teen when a fellow state with 100% confidence that my mother's car had a rolled lobe on %7 in about 10 seconds. Spark energy, vacuum and o2 will give up the secret.
BTW, what is the merc running now?
I don't think you can see a difference with random misfires of the three on the AF plot
Scan for for misfire counts. On a sun machine, you can get the real answer you want since it can analyze the spark itself. These are an art though, I was sold as a teen when a fellow state with 100% confidence that my mother's car had a rolled lobe on %7 in about 10 seconds. Spark energy, vacuum and o2 will give up the secret.
BTW, what is the merc running now?
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