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2007 Silver FJ ~130k
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As a follow up to my post about installing irridiums in my '07 FJ on 11/13/20 at 120k, I now have 133k on the vehicle and have definitely noticed a drop of about 2 mpg over that time period. Don't really know why. I think all the other maintenance and variables remained the same. The irridiums look consistent but perhaps too rich. What do the members think? I replaced them with the correct OEM copper plugs. I will monitor MPG and update this post.
Glass bottle Font Spark plug Office supplies Metal
Glass bottle Font Spark plug Office supplies Metal

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As a follow up to my post about installing irridiums in my '07 FJ on 11/13/20 at 120k, I now have 133k on the vehicle and have definitely noticed a drop of about 2 mpg over that time period. Don't really know why. I think all the other maintenance and variables remained the same. The irridiums look consistent but perhaps too rich. What do the members think? I replaced them with the correct OEM copper plugs. I will monitor MPG and update this post. View attachment 1220878
Doesn't look that rich to me. There would be carbon flaking off the plugs if there was a major issue.
 

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Those plugs certainly don't show ANY hint of running rich, which would exhibit black, fluffy carbon covering the insulator. They do look fairly typical of modern engines with properly functioning fuel-feedback mixture control systems that keep the air-fuel mixture very close to 14.7:1, and aren't burning any oil.

Seeing a 2 MPG decrease in fuel consumption is a very significant drop, but I can pretty much guarantee that it has nothing to do with iridium spark plugs.

Two types of sensors provide all the information the engine ECM uses for fuel-air mixture control: the mass airflow sensor, and the fuel-air sensors.

A far more likely cause of your decreased MPG is one (or both) of the following:

1. Your mass airflow sensor in the engine intake tract is dirty and no longer providing accurate data on the volume of air entering the engine ... this data is critical to maintaining proper fuel-air ratio. When was the last time you cleaned the MAF sensor? Cleaning at 30K mile intervals is reasonable, especially if you do much driving in dusty conditions. If you are using a cleanable, oil-wetted air filter (not recommended for engines using hot-wire MAF sensors) the cleaning interval may need to be much shorter.

2. Your fuel-air sensors in the exhaust system have simply reached end-of-life, and are providing inaccurate mixture information to the engine ECM. These sensors have a useful life of 85 - 120K miles, and you are definitely past that. These sensors slowly degrade over time, and become progressively less accurate and their response time lengthens. They can degrade severely enough to affect fuel consumption without triggering the check engine light.

Clean your MAF sensor and replace your fuel-air sensors (~$120 each) and I can just about guarantee that your MPG will improve, throttle response will improve, idle smoothness will improve, and overall engine performance will improve.
 
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