Defective spark plug wire = LEAN exhaust?

[movindirt]

These are questions from a sample ASE L1 cert test:

1. A high O2S voltage could be due to______.

a. A rich exhaust
b. A lean exhaust
c. A defective spark plug wire
d. Both a and c

2. A low O2S voltage could be due to______.

a. A rich exhaust
b. A lean exhaust
c. A defective spark plug wire
d. Both b and c

I answered 1:d and 2:b

The correct answers according to the test are 1:a and 2:d

Sometimes I read things wrong but I have looked this over and over and still don't get how a defective spark plug wire will create a LEAN condition? I thought for the most part a defective plug wire would equal no spark, what am I missing??

[22REKid]

The L1 certification exam is tricky. It mostly pertains to OBDII. When a plug wire is bad and no spark gets to to plug, the cylinder will not fire causing a misfire. OBDII will recognize a misfire, make the check engine light flash, warning the driver of an immanent catalytic converter failure. O2 sensors will recognize an incomplete burn cycle through the voltage they relay to the computer. So even though raw fuel is essentially dumping into the exhaust, the O2 sensor will not get the reading that it needs due to an incomplete combustion of the fuel via bad plug wire. The O2 voltage doesn't really have anything to do with being lean or rich, it is just purely a voltage. O2 sensors are not smart enough to know what is rich and what is lean. No fuel=low voltage. Unburned fuel=low voltage. My dad is an L1. If you want, you can PM me and I can get you in contact with him. He is like the master of OBDII.

[Triaged]

O2 sensors sense oxygen (duh!). With normal combustion there will be very little oxygen in the exhaust. With a mis-fire there will be more oxygen in the exhaust than there would be if proper combustion had taken place because it is not consumed in combustion.

[rharriman]

O2 sensors sense oxygen (duh!). With normal combustion there will be very little oxygen in the exhaust. With a mis-fire there will be more oxygen in the exhaust than there would be if proper combustion had taken place because it is not consumed in combustion.


So you would have answered the same as dirt correct?? Assuming :( that a defective wire would result in a misfire??

Seems to me that in question 1 a faulty spark plug wire would create a rich exhaust so "a" and "c" both make sense. Not sure if I understand kids explanation either, because I don't know what is meant by:


the O2 sensor will not get the reading that it needs due to an incomplete combustion of the fuel via bad plug wire.

But he is right it does seam Tricky. :D

[rharriman]

For what its worth

Link to an article about the function of 02 Sensors (http://www.asashop.org/autoinc/dec2002/mech.htm)


I think what is confusing is the term "rich" or "lean" exhausts. From what I understand from the article, a rich exhaust doen't mean that the exhaust is rich in fuel content but rather that it is the result of a rich air fuel combustion mixture resulting in a lower O2 levels. Is this correct? Just trying to understand what I read and hope it helps.

[movindirt]

The L1 certification exam is tricky. It mostly pertains to OBDII. When a plug wire is bad and no spark gets to to plug, the cylinder will not fire causing a misfire. OBDII will recognize a misfire, make the check engine light flash, warning the driver of an immanent catalytic converter failure. O2 sensors will recognize an incomplete burn cycle through the voltage they relay to the computer. So even though raw fuel is essentially dumping into the exhaust, the O2 sensor will not get the reading that it needs due to an incomplete combustion of the fuel via bad plug wire. The O2 voltage doesn't really have anything to do with being lean or rich, it is just purely a voltage. O2 sensors are not smart enough to know what is rich and what is lean. No fuel=low voltage. Unburned fuel=low voltage. My dad is an L1. If you want, you can PM me and I can get you in contact with him. He is like the master of OBDII.

Thanks for the offer. I might PM you later if I don't get an answer today in class.


O2 sensors sense oxygen (duh!). With normal combustion there will be very little oxygen in the exhaust. With a mis-fire there will be more oxygen in the exhaust than there would be if proper combustion had taken place because it is not consumed in combustion.

I was thinking about O not being consumed. But I can find nothing in the text that these very questions come from that explains this.


Would not the most correct answer be the one that is not even there which is the 02S in faulty since it is stuck either high or low? Even with a misfire an ODBII system is going to keep adjusting to achieve a 14.7 to 1 A/F ratio.