Gas Mileage is dropping.

[tarkus]

Nothing to do with misfiring. Heavily used spark plugs degrade engine performance.

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[Motzking1]

Originally Posted by tarkus

Nothing to do with misfiring. Heavily used spark plugs degrade engine performance.

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Heavily used? They haven't even reach the factory maintenance interval of a hundred twenty thousand miles? And I would like for you to explain the mechanism of action of how a used spark plug in any fashion that is working perfectly normal is going to have any adverse reaction at all on the engine. There is a federally mandated very sophisticated monitoring system to make sure if there's any issue whatsoever with the spark plugs that would cause any incomplete burn or anything all that would cause an incomplete burn in the cylinder that would eliminate a malfunction indicator lamp mil. The systems are extremely sophisticated and extremely effective. Hundreds of millions of dollars of research shared among all manufacturers has gone into designing these systems to be extremely effective. So you want me to disregard all of that and believe that you know better when to replace the spark plug?

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[alias06]

Originally Posted by Motzking1

Heavily used? They haven't even reach the factory maintenance interval of a hundred twenty thousand miles? And I would like for you to explain the mechanism of action of how a used spark plug in any fashion that is working perfectly normal is going to have any adverse reaction at all on the engine. There is a federally mandated very sophisticated monitoring system to make sure if there's any issue whatsoever with the spark plugs that would cause any incomplete burn or anything all that would cause an incomplete burn in the cylinder that would eliminate a malfunction indicator lamp mil. The systems are extremely sophisticated and extremely effective. Hundreds of millions of dollars of research shared among all manufacturers has gone into designing these systems to be extremely effective. So you want me to disregard all of that and believe that you know better when to replace the spark plug?

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If I am reading your post correctly you are referring to the O2 sensors giving a MIL when something goes wrong. As sophisticated as they are, they do not always tell the whole truth to the problem. There are many steps involved in the system before it will actually trigger the light. One example of this would be for an issue to happen more than once within a certain drive cycle such as if a single misfire was to happen within 500 miles of driving the light will not come on but it doesn't tell you that you had a misfire. But let's say that you have a misfire every mile you drive, that'll more likely trip the light. Just because you don't get a MIL doesn't necessarily mean everything is operating at full potential. Also OP stated that the mileage is at 105K miles, I wouldn't say that is too far off from the 120k you stated. Some mechanical items fail earlier and some lasts longer, its all subjective to abuse and maintenance.