Poorish gas mileage 2004 LS 3.9L

[barryt83]

Thanks for all the help with my overheating issue!


Now onto my mileage. I have a slight miss at idle (most likely plugs and coils). Could this be causing my mileage issue? When filling my tank the range gauge said 201, I was able to get 141 actual miles. I put 8.41 gallons which equals 16.7 MPG average. 2/3 rds of that mileage was all on the freeway and the MPG showed over 24mpgs, but soon as I get in the street the mileage drops really low. Is this normal. It seems excessive to me our am I being crazy?

 

[joegr]

It mostly depends on your driving.
How could you not think that misfiring (putting fuel into a cylinder and then shooting it out the exhaust instead of burning it) would reduce your gas mileage? Of course it does.

 

[barryt83]

I assumed it would I was just checking if there were any common ls issues. Will a check engine light come on for low vacuum?

 

[joegr]

I assumed it would I was just checking if there were any common ls issues. Will a check engine light come on for low vacuum?

Do you think you have an intake leak, or have you actually measured intake vacuum?
A big leak will result in enough unmetered air coming in that you will run lean, so the light will come on for a lean condition. Failure of the MAP (part that measures intake pressure) would also trigger the light. High intake pressure (low vacuum) in itself with no other symptoms (like lean mixture) would probably not trigger the light.

 

[barryt83]

I have a vacuum gauge. What is a good place to T off of?

 

[RigsLS]

Even with fresh coils, plugs and fuel filter, you should see my mileage. But then again my LS doesn't really need a Throttle position sensor, it's either off or on.

 

[joegr]

There are no good places. You could pull the line to the fuel pressure sensor and plug it in there, but it's a hard line. Those hard lines will break if you just look at them wrong, let alone try to move one.
Why are you thinking low vacuum?
Why not replace all the coils and plugs and go from there?

 

[1LoudLS]

I don't even understand the logic of trying to test other things when you know you have a problem that affects millage...


fix the problem you know you have, and then IF the millage issue is not resolved, then start testing other things.




also for what its worth, while not the best, near 17MPG is not terrible millage for this car. I've never got great millage from any of my heavy V8 cars

 

[SoulSoak]

Even a virgin LS 3.9 with no issues whatsoever will get relatively poor gas mileage (depending on your definition of "poor"). I have new plugs, good cops, new fuel filter etc. And I only get 14.Xmpg city and maybe 20-23mpg highway.

 

[G-RELL]

Thanks for all the help with my overheating issue!


Now onto my mileage. I have a slight miss at idle (most likely plugs and coils). Could this be causing my mileage issue? When filling my tank the range gauge said 201, I was able to get 141 actual miles. I put 8.41 gallons which equals 16.7 MPG average. 2/3 rds of that mileage was all on the freeway and the MPG showed over 24mpgs, but soon as I get in the street the mileage drops really low. Is this normal. It seems excessive to me our am I being crazy?

Crazy...

14 city to 20hwy i would bet is average here... I average 17

 

[joegr]

I average in the upper 19s in mixed driving. My wife is getting about 17. We swapped cars for a little bit to verify that it's the driving situation, and not the car. (She tends to idle a lot.)

 

[LS4me]

Crazy...

14 city to 20hwy i would bet is average here... I average 17

I average in the upper 19s in mixed driving. My wife is getting about 17. We swapped cars for a little bit to verify that it's the driving situation, and not the car. (She tends to idle a lot.)

My wife is getting upper 13's, low 14's in pure around town, short drives. She put 1900 miles on the LS over a year.

 

[joegr]

My wife is getting upper 13's, low 14's in pure around town, short drives. She put 1900 miles on the LS over a year.

Well, I know you are doing considerably better than that.
I had to drive my daughter's 325i home from college last night. I noticed its average mpg was 24.5. She's mainly uses it for very short trips. I had to get it because the water pump is failing. It and the thermostat are the only cooling system parts I haven't replaced. I'll get both of those and a few more items now.

 

[LS4me]

Well, I know you are doing considerably better than that.
I had to drive my daughter's 325i home from college last night. I noticed its average mpg was 24.5. She's mainly uses it for very short trips. I had to get it because the water pump is failing. It and the thermostat are the only cooling system parts I haven't replaced. I'll get both of those and a few more items now.

We did a 3100+ mile trip to Texas and back and averaged 31mpg over the entire trip (I used ECO mode on the freeway and comfort and sport modes in town). Just filled-up and have ~425 miles DTE! One of the many things I prefer about the BMW..... More power and considerably better MPG.

 

[joegr]

Sadly, the 325i is definitely less power...

 

[roncoinc]

Using the message center only i have never gone below 20mpg ..
93 octane only.
motorcraft 5/20

 

[1LoudLS]

thats pretty good for an LS, I didnt get that with my V6!

 

[don-ohio]

On the road,no city driving,I get 28 mpgs easily on the V6 and 24 mpgs easily with the V8. That's not running 80,which I rarely do...........that's running 60-70 mph on the freeways. don-ohio ^)

 

[LS4me]

On the road,no city driving,I get 28 mpgs easily on the V6 and 24 mpgs easily with the V8. That's not running 80,which I rarely do...........that's running 60-70 mph on the freeways. don-ohio ^)

My 31 was running upper 70's in The people's Republik, low 80's in AZ & NM and upper 80's in TX. The best I ever got in the LS was 28.5 doing 80ish on the flats of Wyoming.

 

[don-ohio]

I get better mileage at 70mph and under. I don't use cruise unless it's fairly flat territory. Cruise wastes gas on uphills. Yeah, I let off a little.......maybe going down to 60 mph at the top of a long hill. Makes for better gas mileage.
I think if I had the 3:31 diff. gearing I might get a little better,since my tires are stock size. don-ohio ^)

 

[LS4me]

Cruise wastes gas on uphills. Yeah, I let off a little.......maybe going down to 60 mph at the top of a long hill. Makes for better gas mileage.
^)

And VERY, VERY ticked-off drivers behind you!!!! I thoroughly despise hyper-milers! The gas you save is really infinitesimal! You'll use that gas getting back up to speed.

Of course my 8-speed really helps.... 80 is about 1950 RPM...

 

[joegr]

And VERY, VERY ticked-off drivers behind you!!!! I thoroughly despise hyper-milers! The gas you save is really infinitesimal! You'll use that gas getting back up to speed.

Of course my 8-speed really helps.... 80 is about 1950 RPM...

Overall, he is wasting gas as all the drivers stuck behind him have to adjust speed to deal with it. One of my bigger traffic peeves is people who can't drive a steady speed. Of course those that are going faster than me and never vary to a speed slower than mine are not a problem at all. Those that are going slower and never vary to a speed faster than mine are only a problem for a short time. Those that vary from slower than me to faster than me are the real problem, particularly if they don't use the passing lane correctly (correctly = only when passing).

 

[FDR]

particularly if they don't use the passing lane correctly (correctly = only when passing).

I've been on a few stretches of road in rural parts of PA where the people dive right back into the right lane on two lane highways to stay out of the passing lane for too long. It almost seemed unsettling given normal NJ attitudes

I get better mileage at 70mph and under.

Most cars are geared to be most efficient around 55mph, a long-standing national (US) speed limit. It's only some recent vehicles that are moving the speed up to 65 and more through different gearing (number of transmission gears doesn't tell you much, need to know gear ratios). I took a newer Odyssey on a trip that seemed to do best at 75

 

[joegr]

...It's only some recent vehicles that are moving the speed up to 65 and more through different gearing (number of transmission gears doesn't tell you much, need to know gear ratios). I took a newer Odyssey on a trip that seemed to do best at 75

Aerodynamics are a really big part of it too.

 

[LS4me]

Most cars are geared to be most efficient around 55mph, a long-standing national (US) speed limit

That proved to be false. The 55mph limit really had very little effect...

The speed limit was enacted by Congress in March 1974 as part o f a package of measures dealing with the oil crisis mph, it was thought, was the most efficient use of an auto's engine and thus would save fuel. As it turned out, however, the energy actually saved was minimal-at best 1 percent of gasoline consumption or about the same amount a driver could realize by increasing the pressure of his radial tires from 24 to 26 pounds And state officials feel .that the NMSL is an Yet the 55 mph limit Cruising at 55 In the face of this data, speed limit advocates shifted ground.

We had a 1976 Plymouth Arrow that got better mileage at 70 than 55.