Sudden drop in MPG

I have a 2011 Tacoma with towing pkg, 4.0L, auto. Have been towing a 4,000 trailer with consistent 13-14mpg. On the last 900 mile tow, it suddenly began struggling, and stays in 3-4 gear. Mileage with to 8-9 mpg and near the exhaust is sooted up. Have no error code registered or pending. Have tested A/F sensors, MAF, CTS, all seem to work normally. Cold idle is now 2000-2500 RPM for 10 minutes. Monitoring the A/F sensors, they hover at 14.5:1 until you push the throttle 25%, then they go to flat out rich. What else can I check ?

 

What type of gas are you using?

Have you ever heard of 'summer gas vs winter gas'?? There is a difference.
I don't know if its your problem or not...but....I get 2mpg less gas mileage when I start using winter gas. I don't tow anything.

https://blog.gasbuddy.com/posts/U-S-moves-back-to-winter-gasoline/1715-584804-2663.aspx

https://itisscience.wordpress.com/2011/11/28/winter-gasoline-yes-there-is-a-difference/

It just depends on your state and/or where you're located. I tried searching for maps but came up blank.

 

O2 Sensor? Although that should throw up an error code (I think)... I don't know, but that would be my guess, had something like that happen on an old Chevy Astro my parents gave me to drive, don't remember if there was an error code or not.

 

Something isn't right. Cold start idle should be 1500-1700 rpms.

 

Might want to just do a tune up. Check the plugs, air filter, change the oil, check all other fluids. MAF could be dirty.

 

Check plugs, new, air cleaners ( both paper and charcoal, vacuum less than 5" at WOT. Pulled A/F sensors, checked exhaust pressure at WOT, less than 1 PSI, so Cats are open. Checked IAT sensor, OK. Changed oil, trans fluid, diff oils.

 

Subbed. No ideas beyond what you've done, which sounds pretty thorough. It's obviously going full rich, the question is why.

Keep an eye on the engine oil; running super rich can cause gas mix with it in some circumstances.

 

Thanks Dave. Also saw that the fuel pressure regulator hose goes into the air cleaner box ahead of the throttle ? So unless the air cleaner was completely clogged, the fuel pressure will never change. That does not seem right.

 

How's the Engine temp/thermostat??
Ebrake cable sticking/ rear brakes dragging?
Calipers dragging?
How does it coast w/o trail? WITH trailer?
Trailer brakes / wheel bearings/wheels dragging?
Trailer e brake engaged?
Auto tranny fluid Ok?
M tranny slipping?
Radiator fluid going low? (HEad gasket)
Compression test??

All I can think of . Some may not apply...