My wife has a 2008 4Runner with the v6. We used to get at a minimum 20+ on highway trips. Now it is close to 18 driving between 70-75.

I have cleaned the MAF and TB. Changed the air filter. Ran Seafoam through a vacuum line, fuel tank. Total mileage is around 80k. I run Amsoil in everything.

The one thing that I did not do when I cleaned the MAF was to reset the ECU by pulling the battery off.

Is it time to change the spark plugs and wires?

City driving is 18.5 and I thought that I used to be at least over 19 there too.

slow down to 55 and you will be impressed, you`ll get to your destination 10 minutes later, does it make any difference? I`m driving from Denver to Kansas city often, gps shows arrival time difference between 55 and 75 as 30 minutes at most.

Vic...that doesn't explain why I had the drop though. I have actually driven 60 mph for a little over 200 miles and still got the 18.5 mpg. Before I had the drop that I am experiencing now that would have gotten me closer to 22-23 mpg driving that speed.

No change in tires/tire pressure/tire wear and rotation? Type of gas? The obvious checking for frozen calipers?

First of all, cleaning the MAF is dangerous because you can screw up the sensor without really knowing it, and cleaning the TB does nothing. Dirt isn't going to kill MPGs.

I'm betting your cat(s) are getting clogged reducing flow, or your plugs need to be changed, or the quality of fuel you are using is getting crappy. O2 sensors in good shape?

Tire pressure is normal. Did not just have them rotated. Gas type before and after is all the same, all e10 mainly from Costco. I don't think there are any frozen calipers since there is no pulling to any side with or without the brake pressed.

WXman...if I killed the MAF wouldn't it show up with the CEL? I did clean it with the MAF cleaner that CRC has and let it fully dry.

How would anyone really know if the cats are clogged? I have not done anything with the O2 sensors, but again there is no CEL. Gas should be OK. Costco seems to have a truck there every few days and my Tundra gets it's normal mileage from the same gas.

Where are you getting your numbers from? The little optimism meter or pen and paper? what CRC product did you use to clean the MAF and did you actually clean the little wires deep inside the tube and not the bulb? I would maybe try resetting the ECU, but that may not correct the fuel mileage immediately.

A dirty MAF will provide improper air metering data that will affect fuel trim and thus potentially fuel mileage and performance, it can be severe enough to trip an MIL and sometimes subtle enough to be a problem but not trip a DTC for a while.

Carbon build up on the Throttle Body is something that could affect performance and ultimately fuel mileage as well depending on the severity and where the average opening distance of the throttle plate is. i.e. if the guy drives WOT all the time, probably not an issue, but if its just cracked in the 1/3~1/2 range at freeway speeds it might cause enough restriction and/or turbulence to be an issue.

I haven't heard of many Toyotas having issues with clogged cats, not that it isn't possible; but a clogged cat doesn't happen randomly. Catalysts don't really just up and die and/or clog up; there has to be an inherent issue that causes that condition, Cats are more than often a symptom not a problem in themselves.

Not trying to veer too far off topic, but….

I've made this trip quite a bit as well. Next time, try Highway 24 between the Manhattan/Colby Stretch. It's a lower speed limit, but a much prettier drive. No one else hardly on the road at all. If you're not in a hurry, it's a great way to go. I only saved around 2 mpg (going 65mph rathern than 85mph makes a huge difference) and it only takes an extra 45min-hour.

One of these days i'd like to try the hway 36 route, but diverting back down to 24 at Philipsburg.

Sorry for the hijack! FYI, when I changed the plugs on my V8 at 105k miles, I believe the MPG actually went down by 1 mpg or so. Got the good iridium plugs, Denso I believe, gapped correctly etc.

These trucks get thirsty, but they're so nice to drive….

Good luck!

All mileage has been hand calculated. I have learned that over time the optimism meter is around 2 mpg over what I am actually getting.

I would say that I drive somewhat conservatively. Not that I never get on the throttle, but not very often.

I used the CRC Air Mass Sensor Cleaner. All I cleaned was the bulb and I guess the wire that it connects to as far as I could see it. Got it back to the amber/orange color. I did not touch it at all.

75 vs 55, MPG easily 4mpg difference. steady speed on heavy vehicle shaped like a brick. I can drive like a grand ma and get 24 in my 2012. or get on it and mpg drops to 14 in town.

I doubt new spark plugs decreased your mpg, must be something else, bad gas, tire pressure, etc....

I was just going off experience over the years. Took a trip in a car once and ended up stranded once we started back home. Turned out that the cat was randomly clogged up. A new cat and she was running like new again.

And I've had throttle bodies that were so dirty they were black all over. I cleaned them up like new and saw absolutely zero change in MPGs or throttle response. I just don't think this is going to randomly change his mileage.

Something clearly has happened all of a sudden to his 4R and that's why I think he could have some plugs that are fouled, or a cat issue, or crappy fuel going into the tank. Getting gas from the same station doesn't mean it's the same gas. Stations change suppliers from time to time, and also the underground tanks can become contaminated.

So I guess the next logical thing to do is change out spark plugs? Is it really necessary to do the wires too?

[QUOTE=trf97;1451073]So I guess the next logical thing to do is change out spark plugs? Is it really necessary to do the wires too?[/QUOTE

bad spark plugs will cause misfire so you check engine light will be tripped, if it`s not than search somewhere else.