A/C and heater causing mpg to drop to 11mpg from 19mpg...normal?

My rig - 2009 Tacoma 4x4, 67,000 miles.

Hi, after racking my brain for the last 6 months where I have seen my mpg drop from about 19-20 highway to about 11-12 highway and doing everything I could find recommended on this forum including maintaining tire pressure, oil changes (synthetic), changing my spark plugs, changing the air filter (and in-cabin filter too), cleaning the MAF, running 2 cans of Seafoam, a can of Duralube injector cleaner, buying non-ethanol gas, non-winterblend gas, only running premium for a few tanks, etc. I couldn't figure out why I was getting such bad mileage and gave up for the last month.

I drive a lot. Like, all year round, all seasons. This truck sat virtually unused for 2 years while I had a company truck so all those miles were fairly recent and I monitor my gas mileage constantly, virtually every tank.

Today I decided to drive without using my air conditioner since I was mostly in the cooler mountains and bam, I was up to 20mpg again. Drove 390 miles today total so got a pretty good gauge.

First question, is it normal to have the gas mileage drop to 11mpg when using the A/C on a 2nd gen Tacoma? It seems incredibly excessive to me. Seems like it was dropping maybe 4-5mpg max (usually less) for most the time I owned the truck, not 10mpg.

Second question: I'm still getting like 11-12 (sometimes as high as 14) in the winter when I'm not using A/C at all. Since the in-cabin air filter is new (and I keep it clean), why is the heater causing my gas mileage to go down almost as much as the A/C does?

Thanks for the great resource! I've been lurking the threads for years, anytime I google a question I usually end up here. Can't find an answer to this one though.

 

Either defrost setting cycles the AC

 

I have an 09 4x4 regular cab and I have my ac on 95% of the time , even if it's on low with the window (I have cranks and am too lazy to reach across most of the tiem) down. I get a consistent 19-22mpg. Something's not right about your mileage.

 

Ok thanks, I figured the mpg was dropping way too low to be normal.

I should also add I had my catalytic converter checked and was ok. And also, I don't seem to be experiencing any large (if any at all) drop in power, no engine sputtering, rough idle, or anything out of the ordinary. Just horrendous gas mileage until today.

Also, sometimes when I have the a/c or heater on when I start my truck there is a quick grinding sound when I first start driving off, coming from somewhere near the center of the engine compartment slightly off to the drivers side and it drags the truck power way down for about 1/2 a second and then I'm fine for the rest of the day.

I guess my question now would be, can anyone think of a reason running the heater or a/c would cause such a huge drop? It doesn't make sense to me. I'm at a loss for things to check at this point and hoping someone could provide some direction for a guy who's truck knowledge is entirely composed of from what I learn on the internet.

Thanks!

 

something's definitely wrong. i run my ac 24/7 usually and get 18-20 on the highway.
now whats wrong? I'm not sure..

 

Brake dragging? My old car did that, couldn't figure out wtf was wrong for the longest time.

 

The noise you heard could be the A/C compressor grinding, and a problem with the compressor could explain why the mpg drop. Did you notice the noise when you start to drive with the A/C turned off?

 

While this write-up is about a FJ Cruiser it may also apply to a Tacoma.
http://www.fjcruiserforums.com/forums/engine-performance/119773-fuel-mileage.html

 

Sounds like time for a new A/C compressor....

 

WOW

 

When using just the heater the AC compressor doesn't or shouldn't cycle on and off so it should not have any impact on fuel mileage. The defroster uses the AC to dehumidify. The heater alone does not use AC.

In theory using AC on the highway with windows closed will provide better fuel mileage than no AC with windows open.

At highway speeds are you feeling a drop in power when the AC compressor cycles on?

 

Just checked mine. First time in a couple of months that I have not towed boat. 4000 on ODO and 21.1 mpg. Mainly hwy back and worth to work. 70 mph with AC on 1/2 the time.

 

Bingo

True, technically. However I have learned over the years that when someone says they "turned on the heater" they just assumed that any time the temp knob was in the red, it was the heater. Perhaps the OP can give us more information.

OP, when you say you use the heater in the winter, what settings do you have it on? Defrost, or dash/floor heat only?


I would say that if it is truly the AC system reducing your mileage that much, I would have the compressor and pulley inspected to be sure that the compressor is not trying to seize up or drag.

 

That is very interesting. I did experience a similar problem with a Ford Windstar one time when the thermostat was replaced with a different, lower temp, rated one than the original.

But in that case the car would skip and run rough when it was extremely cold then a code was thrown indicating the engine coolant temp was dropping too low.

 

I turn mine off manually most of the time. The only time I really need the A/C to run with defrost is right after start up on damp mornings when the inside is fogged.

 

I agree, you would think there would be. but a compressor that perhaps is causing an additional 10% drag might not make a noise, but would certainly cause a serious lack of performance and reduced fuel economy. Of course we are ALL armchair-diagnosing things here. lmao.

 

wut

 

Perhaps the engine serpentine belt tensioner is on it's way out. Sunoco cheapo fuel, stay off the throttle and Granny shift is all I can come up with for the fuel economy dilemma. Good luck.

 

great read.

 

Great read!

OP I would try this def. I may replace mine when I get home just for the hell of it.