I have a 91 Ford Explorer 4.0 v-6 with Auto trans. It is a 4x4 with the front Warn hubs engaged. Most of my driving is not with the 4 wheel drive engaged. While it never has been a really good mileage producer, it recently has got worse. Used to be around 16 and now around 12. Seems to run fine with no misses at any speed that I can detect. By the way, 166,000 miles. It has a K&N Filter which I have cleaned. My question is this. If an engine is not missing, does it help to increase mileage by changing the spark plugs. If so, why? It has been awhile since they were changed. A couple of the spark plugs on this engine take a contortionist to change and at the price of spark plugs now, I don't want to waste my energy and money.

My bet is the K&N Air Filter has contaminated the MAF Sensor with oil, pollen, and dirt. This can radically affect gas mileage. Try removing the MAF Sensor, with a Security Torx T-20 screwdriver, and clean the sensor with MAF Sensor Cleaner:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-8.html#post1377134

Ford specifically advises against using oil impregnated air filters for this reason; therefore, install a standard pleated paper air filter. My advice is to only use the same Motorcraft plugs that came in the Explorer. Yes, plugs are a major factor affecting gas mileage. I would change platinum plugs every 50,000 miles, not 100,000 miles recommended by Ford.

Here are some additional tips to improve gas mileage:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-4.html#post293582

I will check that out. (The MAF sensor) The air filter was kind of dirty and a bit oily when I removed it. This is not my primary vehicle, but due to the snow load around here, I needed a 4x4 to get up a hill to our house. It has served me well for about 5 years. Due to the Holidays coming up, I won't get to it until after New Years. I plan on installing the platinum standard plugs and cleaning the MAF Sensor. I'll let you know the results. Thank you for the advice

One more thing... On the top of the Air Filter box, at the rear are two vacuum hoses that attach to the air filter box lid. One has a molded end and the other is just a plain vac hose. They can be interchanged. Does it make any difference which hose goes where? I also have a 93 explorer and they were the opposite. It gets around 18 mpg.

I not familiar with that setup. Can you consult a Haynes Manual?

I have a Haynes book I looked there first but found nothing. Both of my Explorers have the same set up. My neighbor has an Explorer also, but is about 5 years newer. I will look at theirs and see if it has the same setup. Actually what it is, is that there are 2 steel tubes that come out of the top rear portion of the Filter lid. Both are same size. They go to a round item inside the filter box. No wires or anything. The vac hoses are different for a reason. I don't know which way is correct. I can send a photo if that would help.