Poor fuel economy on 4.6, ideas?

[jonesy20]

It's a 2011 4.6 72k miles, I've had it about 2 weeks. I just did about 5 miles, city streets, with very light traffic. I kept rpm under 2k and throttle was never more than about 25%. Set cruise control when I could and coasted. It was practically hypermiling. Just got 10mpg doing this. I've replaced the air filter and oil with synthetic, no check engine, idles well, runs smooth.

I had an 06 CLS55 AMG (5.5L with supercharger) a few years ago and would get 13 in that so something seems wrong here.

Spark plugs? O2 sensor? MAF?

Looking at this link 10 is definitely a significant outlier. 2011 Hyundai Genesis MPG - Actual MPG from 5 2011 Hyundai Genesis owners

 

[Easy]

Okay. So top off the tank. Drive as long or as much as you like; miles/days, etc., or just an afternoon cruise. Top off again and do the calculation for MPG yourself and compare to what the car is telling you.

 

[AgentSmith007]

It's a 2011 4.6 72k miles, I've had it about 2 weeks. I just did about 5 miles, city streets, with very light traffic. I kept rpm under 2k and throttle was never more than about 25%. Set cruise control when I could and coasted. It was practically hypermiling. Just got 10mpg doing this.

jonesy,

I can do 20 mpg in conditions and with techniques you described with my 2011 4.6 with 79,XXX miles.

Did you do this test right after you filled in your gas tank?

 

[EdP]

It's a 2011 4.6 72k miles, I've had it about 2 weeks. I just did about 5 miles, city streets, with very light traffic. I kept rpm under 2k and throttle was never more than about 25%. Set cruise control when I could and coasted. It was practically hypermiling. Just got 10mpg doing this. I've replaced the air filter and oil with synthetic, no check engine, idles well, runs smooth.

I had an 06 CLS55 AMG (5.5L with supercharger) a few years ago and would get 13 in that so something seems wrong here.

Spark plugs? O2 sensor? MAF?

Looking at this link 10 is definitely a significant outlier. 2011 Hyundai Genesis MPG - Actual MPG from 5 2011 Hyundai Genesis owners

Took my wife to the doctor and according to the computer, got 8.6 mpg. That is a one mile trip.

I don't know what traffic and how many stoplights you encountered, but city traffic is terrible on large displacement engines on heavy cars. After getting the 8.6 we went to another place 30 miles away and highway driving of 75 to 80 and ended up with 23 mpg.

Before you piss away money in fixes that won't help, do some more driving and under different conditions. One five mile trip in city traffic in a car that probably was still cold does not tell you very much. Takes about 10 miles for the oil to get to temperature. From my experience though, I'd not expect a lot more. Oh, an I don't think the computer is very accurate on short trips too. I'd use it more as a way of comparison, not true mpg.

 

[66nexus]

^Agree with all above. One thing I'd like to point out that wouldn't hurt is to take out your MAF assy and spray it down with MAF cleaner. It'll literally only cost you the price of the can. It might not solve anything, but won't hurt either.

As far as the mpg on the dash, I ignore because, like EdP, my dash mpg changes daily based on my trip history. The computer isn't the best guide for judging your overall mpg (and not just the last 20 minutes or so).

 

[Rspected]

Open your hood and listen for vacuum leaks, listen for exhaust leaks, clean your MAF and throttle body, set the car in eco mode instead of regular. I have the 5.0 R spec and I get an average of about 17mpg without eco mode consistently, stop and go or not. I'd also have your fuel injectors tested for functionality.

 

[jonesy20]

I was just using the dash mpg meter. I'll do a proper calculation on the next fill up. How many gallons is the tank?
No risk with cleaning the MAF?

 

[Rspected]

I was just using the dash mpg meter. I'll do a proper calculation on the next fill up. How many gallons is the tank?
No risk with cleaning the MAF?

No. My model doesnt have an MAF but as long as you do it correctly and dont touch the MAF with your oily fingers, you should be fine. Pick up CRC cleaner from autozone specific to mass air flow sensors and just spray the sensor a few times and let it dissolve. I highly doubt this would get you from 8mpg to a normal number, but it might help. Sounds like you have a much larger issue at hand. Watch your live fuel usage screen instead of the average and see where it usually sits while you drive. No way you're only getting 8mpg. You'd have to be 3/4th on the throttle at all times to have that low

 

[66nexus]

I was just using the dash mpg meter. I'll do a proper calculation on the next fill up. How many gallons is the tank?
No risk with cleaning the MAF?

I've cleaned mine twice so far in almost 7 years of ownership. I usually do more on my cars but my cars in the past were usually older.

The only risk I see to cleaning it is the removal part. There's a plastic clip that's inserted into the harness plug which itself is inserted into the MAF. The clip is not difficult to remove, but you have to take care to do it since it is indeed plastic.

 

[Gunkk]

IMO, You're far more likely to mess up your MAF than fix it mess unless you really know what you're doing.

My dash mpg indicator reads about 10% happy (validated over a few years using Simply Auto app aka the old Fuel Buddy). This last 2 weeks spent I half a tank in my 2012 4.6 with an easy pace of daily urban/suburban work commute commute on 30-60mph roads, with stop lights every 1/2mi to 1mi, 10 miles each way... yielding 22.6 mpg on the dash... so maybe 20 mpg in actuality. Normally I get about 18mpg average long term on the dash (so 16-17 IRL).

In my experience with two 4.6's over 8+ years I can easily drop the average mileage to under 15 with a heavy right foot. Also the mileage on my car seems to be 1-2mpg better with summer blend fuel.

YM... as they say... MV.

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[kevgen]

Could it possibly be a lazy oxygen sensor?

 

[daviderle]

I was just using the dash mpg meter. I'll do a proper calculation on the next fill up. How many gallons is the tank?
No risk with cleaning the MAF?

If my memory serves, it is 20.1 us gal.

I drove my 2008 4.6 for just about $110k miles without mucking around with the MAF sensor,
On Average city driving I used to get around 16mg with heavy foot (17 on the computer), My record was 27.6mpg on a long trip from Atlanta to Miami with feather foot driving with minimal traffic.
IMHO, on the next fill up, put the good stuff with some chevron full system cleaner, get on the highway, open her up a bit for a while and don’t refill until fuel tank is finished.
Refill the tank and try again.

 

[jonesy20]

Could it possibly be a lazy oxygen sensor?

Is there any easy way to test them? For example connect to a volt meter and look for a certain resistance value?

 

[kevgen]

Any method of obtaining readings for comparison with a working o2 sensor should work, but a scanning tool is invaluable.

Is there any easy way to test them? For example connect to a volt meter and look for a certain resistance value?

 

[OUsig]

It's a 2011 4.6 72k miles, I've had it about 2 weeks. I just did about 5 miles, city streets, with very light traffic. I kept rpm under 2k and throttle was never more than about 25%. Set cruise control when I could and coasted. It was practically hypermiling. Just got 10mpg doing this. I've replaced the air filter and oil with synthetic, no check engine, idles well, runs smooth.

I had an 06 CLS55 AMG (5.5L with supercharger) a few years ago and would get 13 in that so something seems wrong here.

Spark plugs? O2 sensor? MAF?

Looking at this link 10 is definitely a significant outlier. 2011 Hyundai Genesis MPG - Actual MPG from 5 2011 Hyundai Genesis owners

you cant drive 5 miles and get any sort of accurate reading I dont care if you have a 600hp or 150hp 4 banger
Especially on a cold engine
That is why Hybrid vehicles are so popular - most people drive short distances and therefore the battery can take care of that

 

[rstlne]

you cant drive 5 miles and get any sort of accurate reading I dont care if you have a 600hp or 150hp 4 banger
Especially on a cold engine
That is why Hybrid vehicles are so popular - most people drive short distances and therefore the battery can take care of that

This.

Okay. So top off the tank. Drive as long or as much as you like; miles/days, etc., or just an afternoon cruise. Top off again and do the calculation for MPG yourself and compare to what the car is telling you.

And this.

I think you're just waaaaaaay overanalyzing right now. It will normalize upon fill-up and a run-through.

 

[homeofstone]

evap fuel tank air filter should be replaced but should not effect mpg.