Gas mileage down the drain???

[Mazda]

On a V6/V8 you should only have to change the "upstream" O2 sensors, on the exhaust manifolds.

The "downstream" sensors are for testing if the catalytic converts are working, so wouldn't effect MPG

Thanks Ron I did not now this. That's a big savings if you don't need to change all 4.

Bryan

 

[RonD]

I actually just put tranny fluid in, I didn't look or smell wierd when I checked it last week, just low, and what exactly do you mean by ethanol content? As in what I use at the pump? Cuz I just use the regular stuff, all I can afford.

Ethanol has less "energy" per gallon than gasoline, just like diesel fuel has more "energy" per gallon than gasoline, which is why diesels have better MPG numbers.

So if you use a 10% ethanol, 90% gasoline mix you can expect less MPG than 100% gasoline, but it's not a direct relationship, i.e. you wouldn't get 10% less MPG in the above mix example.

Just FYI, Octane is not an "energy" rating, Octane is a heat rating, a higher octane number means the fuel will "self ignite" at a higher temperature.
High compression engines need to run a higher octane number because lower octane fuel will pre-ignite with the higher compression, so pinging and knocking.

Reports of better MPG with higher octane fuel is a myth, BUT...big but, that is if you are talking about 100% gasoline in the compared fuel.
Often Regular gas has ethanol added, and Premium doesn't, so in that case the ethanol would lower the MPG in the regular gas, it isn't different enough to justify the cost difference but it is real in that respect.
Premium fuel "burns cleaner", another myth, Premium fuel has additives to "clean" fuel delivery systems, which helps your engine run more efficiently, but a can of Seafoam($7) once a year will do the same thing.

In BTUs
Ethanol 84,600 BTU/us gallon
Gasoline125,000 BTU/us gallon(regular or premium)
Diesel 138,700 BTU/us gallon

A 10% ethanol mix would be about a 2% drop in MPG, so if you got 20mpg with 100% gas, you would get 19.6mpg with 10/90 mix.
so Ethanol has less energy but does increase Octane level when added to gasoline, gas that is 87 octane will be 90 octane with 10% ethanol added, this helps prevent pinging and knocking.
Ethanol also absorbs water from the fuel so helps prevent fuel line freezing in cold weather.

 

[01BaRanger]

Ethanol has less "energy" per gallon than gasoline, just like diesel fuel has more "energy" per gallon than gasoline, which is why diesels have better MPG numbers.

So if you use a 10% ethanol, 90% gasoline mix you can expect less MPG than 100% gasoline, but it's not a direct relationship, i.e. you wouldn't get 10% less MPG in the above mix example.

Just FYI, Octane is not an "energy" rating, Octane is a heat rating, a higher octane number means the fuel will "self ignite" at a higher temperature.
High compression engines need to run a higher octane number because lower octane fuel will pre-ignite with the higher compression, so pinging and knocking.

Reports of better MPG with higher octane fuel is a myth, BUT...big but, that is if you are talking about 100% gasoline in the compared fuel.
Often Regular gas has ethanol added, and Premium doesn't, so in that case the ethanol would lower the MPG in the regular gas, it isn't different enough to justify the cost difference but it is real in that respect.
Premium fuel "burns cleaner", another myth, Premium fuel has additives to "clean" fuel delivery systems, which helps your engine run more efficiently, but a can of Seafoam($7) once a year will do the same thing.

In BTUs
Ethanol 84,600 BTU/us gallon
Gasoline125,000 BTU/us gallon(regular or premium)
Diesel 138,700 BTU/us gallon

A 10% ethanol mix would be about a 2% drop in MPG, so if you got 20mpg with 100% gas, you would get 19.6mpg with 10/90 mix.
so Ethanol has less energy but does increase Octane level when added to gasoline, gas that is 87 octane will be 90 octane with 10% ethanol added, this helps prevent pinging and knocking.
Ethanol also absorbs water from the fuel so helps prevent fuel line freezing in cold weather.

So I shouldn't really worry about the kind of gas then? Cuz honestly 2% I can live with, but I've dropped drastically in the time of about 8 months,

 

[01BaRanger]

Do all the tuneup stuff, then get a voltmeter and learn how to test your sensors that tell the computer how to tweak/control the engine. Pull the codes to see if there is a hard fault set in the memory. If you're running regular old 10% diluted gas (10% ethanol) that will cost you 3-5 mpg as opposed to real non-diluted gas. There are websites that tell you where you can get real gas in your area. Some places are running E-15, up to 15% ethanol/gas blend...which sux in terms of mpg.

Well I have given her new tranny fluid, air filter, fuel filter, and spark plugs, next is an oil filter change, but I'll be doing a full oil change at the same time, the oil it has a barely 4 months old

 

[tmcalavy]

Based on what you're saying, I'm guessing fuel pressure regulator or a sensor or two out of whack that is giving wrong info to the computer, ie. fuel/air mixture. If there are any codes it should help you narrow it down.

 

[01BaRanger]

Based on what you're saying, I'm guessing fuel pressure regulator or a sensor or two out of whack that is giving wrong info to the computer, ie. fuel/air mixture. If there are any codes it should help you narrow it down.

Ok thanks! And thank you to everyone else!

 

[01BaRanger]

Alright well I was finally able to get it checked for codes and she didn't give me anything, I have not gotten a chance to change/check the O2 sensors yet though, if they aren't what's wrong does anyone have any ideas? Oh btw I calculated exactly what I'm getting and I get 11.5 mpg bone stock and I WAS getting 20. Once again thanks for any and all help

 

[RonD]

11MPG is way off and in the area of a fuel leak or a good amount of black smoke while driving.
Black smoke is a sign of rich fuel mix, so you should see it if it is burning in the engine, and you would have rich codes from OBD check.

The filler neck and vent tube on the gas tank often rust out but they will only leak when tank is near full or when going around corners, these can be visually inspected, also the gas tank should have pressure inside when you remove the cap, if you haven't noticed this in awhile then you have a hole somewhere, could be return line as well.

Do you park on a clean dry surface so would notice any leak spots, gas evaporates very fast but does leave a residue.
If not head to a covered parking garage(mall) for lunch, park in a very clean spot.
Go have lunch.

When you get back put a pencil/pen...whatever in front of each front tire, start engine and back out.
Using pencil/pens as reference points look for any leaks, oil or gas.
Take picture, including the pencil/pens for later use if you see any leaks

If no visible leaks on the ground then open hood with engine cold and off, run fingers over any exposed gas lines, fuel rail, injectors, feed and return lines, smell fingers often for gas smell.
Turn on key, count to 5, turn off key, repeat 3 times, do not start engine, this will prime the fuel system to it's maximum pressure, now repeat above test to see if there is any leaking fuel.

 

[01BaRanger]

11MPG is way off and in the area of a fuel leak or a good amount of black smoke while driving.
Black smoke is a sign of rich fuel mix, so you should see it if it is burning in the engine, and you would have rich codes from OBD check.

The filler neck and vent tube on the gas tank often rust out but they will only leak when tank is near full or when going around corners, these can be visually inspected, also the gas tank should have pressure inside when you remove the cap, if you haven't noticed this in awhile then you have a hole somewhere, could be return line as well.

Do you park on a clean dry surface so would notice any leak spots, gas evaporates very fast but does leave a residue.
If not head to a covered parking garage(mall) for lunch, park in a very clean spot.
Go have lunch.

When you get back put a pencil/pen...whatever in front of each front tire, start engine and back out.
Using pencil/pens as reference points look for any leaks, oil or gas.
Take picture, including the pencil/pens for later use if you see any leaks

If no visible leaks on the ground then open hood with engine cold and off, run fingers over any exposed gas lines, fuel rail, injectors, feed and return lines, smell fingers often for gas smell.
Turn on key, count to 5, turn off key, repeat 3 times, do not start engine, this will prime the fuel system to it's maximum pressure, now repeat above test to see if there is any leaking fuel.

Will do thank you

 

[01BaRanger]

11MPG is way off and in the area of a fuel leak or a good amount of black smoke while driving.
Black smoke is a sign of rich fuel mix, so you should see it if it is burning in the engine, and you would have rich codes from OBD check.

The filler neck and vent tube on the gas tank often rust out but they will only leak when tank is near full or when going around corners, these can be visually inspected, also the gas tank should have pressure inside when you remove the cap, if you haven't noticed this in awhile then you have a hole somewhere, could be return line as well.

Do you park on a clean dry surface so would notice any leak spots, gas evaporates very fast but does leave a residue.
If not head to a covered parking garage(mall) for lunch, park in a very clean spot.
Go have lunch.

When you get back put a pencil/pen...whatever in front of each front tire, start engine and back out.
Using pencil/pens as reference points look for any leaks, oil or gas.
Take picture, including the pencil/pens for later use if you see any leaks

If no visible leaks on the ground then open hood with engine cold and off, run fingers over any exposed gas lines, fuel rail, injectors, feed and return lines, smell fingers often for gas smell.
Turn on key, count to 5, turn off key, repeat 3 times, do not start engine, this will prime the fuel system to it's maximum pressure, now repeat above test to see if there is any leaking fuel.

Could this be why I have better fuel response when I have a full tank?

 

[RonD]

Probably not, it would mean the fuel pump might be on it's way out or there is a leak in the line going from fuel pump to top of the tank(internal leak).

"A pint is a pound the world around"
8 pints = 1 gallon
This means each gallon of gas weights 8lbs(a little less because gas is lighter than water)

So if there is 10 gallons of gas in the tank that means about 80lbs of pressure at the fuel pump pick up(bottom of tank)
This can help a weak pump keep pressure up at fuel rail.
Also puts pressure on a leaky connection in the tank.

 

[01BaRanger]

Ok I'll check the lines by hand today, I put a bone dry piece of plywood under my truck after I got home last night I checked it periodically and there was no leakage and It didn't smell like gas this morning, and I have no black smoke,

 

[Chris_North]

.
So if there is 10 gallons of gas in the tank that means about 80lbs of pressure at the fuel pump pick up(bottom of tank)
This can help a weak pump keep pressure up at fuel rail.
Also puts pressure on a leaky connection in the tank.

That's not really accurate.

Pressure of a fluid (in this case gasoline) is exclusively dependent on depth, not its amount. Fill a tank 30 feet tall with water and the PSI at the base will be the same as if it were only a 1/2 inch pipe filled with water. IIRC for every foot of elevation there will be an increase of .42 PSI (don't quote me on that though). No clue what gasoline is.

So basically, there could be what weighs 4000 pounds of water in a pan one inch deep, and the pressure in any one square inch (assuming the pan is level) will be 0.42 PSI.


The other thing would be that the fuel pump is at the bottom of the the fuel tank, way below the fuel lines. No matter what the pressure the gas exerts at the bottom of the fuel tank, it will never exert any pressure of its own at the top. That's a big reason for having the pump. At least, before fuel injection, back when vehicles were carbuerated it was pretty much the only reason.

 

[01BaRanger]

Ok well I looked over the fuel lines and no leaks, stains, drips, or residue that I can tell.
So what are my options now? I haven't gotten a chance to look at my injectors but that's it, IAC isn't dirty, and neither is MAF, any more ideas?

 

[Chris_North]

I drive mostly city but it's a distance of like two miles twice a day....and even my hwy mpgs went down the drain, a 60 mile trip costs me a quarter tank

Was this always the case? Typically short trips like that kill MPG. The engine doesn't get a chance to warm up to operating temp before you arrive. I have a 3.0 and 60 miles on the highway will typically cost me about a quarter tank or so. According to EPA estimates (for whatever they're worth) the 3.0 and 4.0 get about the same mileage.


Have you ever checked your brakes to make sure they're not dragging? If you have an infrared temp gun you can point at the rims or calipers if you can see them and if they're dragging they'll be a big difference between left/right. You'd also notice a pull to one side when driving straight. There will also be a big difference between front/back, especially if you have rear drums.