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Old 04-22-2015, 08:51 AM
snapon20r snapon20r is offline
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Default Misfire Questions

Hi guys, just a question here, 2003 chev 2500 hd 8.1 L, had a misfire under load on # 6 cylinder, had a cracked spark plug, so we figured out the problem on why it was misfiring. My question is when I was able to get the engine to misfire, the Bank 2 LT Fuel trim would go to 25, B2S1 would richen- so computer is richening for a lean condition. Based on theory wouldn't you think it would be the opposite, LT fuel trim going negative because of unburned fuel going through the exhaust?? Maybe I am missing something. Just wondering what your thoughts are. Thanks.
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Old 04-22-2015, 09:06 AM
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B2S1 is an oxygen sensor, not an unburned fuel sensor.

When you have an ignition miss like this, you dont burn the oxygen in that cylinder. Unburned oxygen is also the result of a lean mixture as far as fueling goes, so the PCM tries to burn up the oxygen by adding more fuel.
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Old 04-22-2015, 09:23 AM
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I lost many of my hairs and 2 hrs while fighting a misfire on a 2003 Ford Windstar 3.8. Had a bad coil and fuel trims that were thru the ceiling. Got fed up and replaced coil and they went normal.... I was and am still baffled why a PCM will do what it does when it does it. Maybe the OEM programmers do it to mess with the aftermarket! Who knows.
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Old 04-22-2015, 11:20 AM
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Quote:
Originally Posted by snapon20r View Post
Hi guys, just a question here, 2003 chev 2500 hd 8.1 L, had a misfire under load on # 6 cylinder, had a cracked spark plug, so we figured out the problem on why it was misfiring. My question is when I was able to get the engine to misfire, the Bank 2 LT Fuel trim would go to 25, B2S1 would richen- so computer is richening for a lean condition. Based on theory wouldn't you think it would be the opposite, LT fuel trim going negative because of unburned fuel going through the exhaust?? Maybe I am missing something. Just wondering what your thoughts are. Thanks.
From one perspective this seems to make sense. Remember your stoichiometric ratio? 14.7 parts of air to 1 part of fuel. If combustion does not occur, that same ratio comes down the pipe, hence the lean condition.
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Old 04-22-2015, 12:03 PM
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Quote:
Originally Posted by maven View Post
B2S1 is an oxygen sensor, not an unburned fuel sensor.

When you have an ignition miss like this, you dont burn the oxygen in that cylinder. Unburned oxygen is also the result of a lean mixture as far as fueling goes, so the PCM tries to burn up the oxygen by adding more fuel.
good explaining Maven, never thought of it that way. actually, never even crossed my mind to look at lt fuel during a missfire event.
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Old 04-22-2015, 12:21 PM
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It makes sense to a lot of people, and in a certain line of thought, but only if you think of O2 sensors as hydrocarbon detectors, which is what MANY techs do, they assume that since theres unburned fuel in the exhaust the PCM should somehow sense this with the upstream sensors and pull fuel, like a rich condition existed. problem is ECMs cant tell there is fuel there. All they know is there is oxygen. Just remember its an OXYGEN sensor and youll usually be fine.

If youre comparing pre and post cat sensors its alot easier to get a quick handle on whats going on. If youve get a "lean" condition that is really a mechanical or ignition related incomplete or non existant fuel burn, and youre dumping fuel into the exhaust youll see the post cat (S2) sensor typically much higher than normal voltage. You can easily bury S2 at 1000mV with an dead ignition system in a cylinder, OR you will see S2 voltage reading well down below 200mV, often times in single digits if youve got a TRUE lean condition where the engine just cant deliver enough fuel to burn all the air.


Try this experiment, kill the ignition system on a warmed up, closed loop operation engine and watch the post cat voltages, then restore the ignition system, now create a vacuum leak and watch the voltage. Both scenarios will show as a "lean" condition on S1, but S2 will help tell the real story when used in conjuction with what S1 has to report.
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Old 04-22-2015, 12:49 PM
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I really appreciate all your responses, it really makes sense now, I just wasn't thinking deep enough. Thanks again guys.
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Old 04-22-2015, 03:37 PM
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Maven; You beat me to it. Fundamentals guys.

OXYGEN sensors sense OXYGEN.

The thing to keep in mind is WHY is there oxygen that the sensor is seeing.

Ideally, all the oxygen will be used up during combustion.

Post-Cat sensors will see less oxygen as the catalyst completes the burning of available fuels and OXYGEN.
We need three things for combustion; Fuel, Oxygen and enough heat to ignite.
Add in the three "T's" of Time, Temperature and Turbulence that any combustion uses and it can be figured out from fundamentals.
Even the woodstove or campfire uses the same principles.

Just remember, OXYGEN sensors may INDICATE a lean or rich condition but they are sensing OXYGEN.
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Old 04-22-2015, 04:27 PM
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You might want to add to the mix, when diagnosing misfires, that depending on the vehicle and if the ECM can identify the cylinder, the ECM may shut off that injector to protect the cat from over heating..
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