Sudden (apparently) decline in gas mileage?

[WormGod]

Aside from winter gas being the pure blame of bad MPG, there are many other points that people should remember. First off, the winter grade gas is about as close to a "high performance" than they will ever put in it. And you can imagine, it burns faster because of this. You may not feel the power, but there is a hair bit more there. Still not a reason to "get on it".

Also, engines run harder in cold weather to reach and maintain normal running temp. Higher idle RPM to get to maintain that temp means more fuel dumping. Even a slight 50-100 RPM (hard to register by eye) can cause a pretty visible change in MPG. Colder air through the intake is no help as you are pushing 25f air into an engine that wants to run at 180f.

Plus, MOST people tend to let their cars sit for a bit longer idling to "warm up". THIS, is the major culprit for bad mpg in winter. I think they do this without fully realizing that just sitting there, they are burning plenty of gas. USER ERROR. Just GO. It only takes about 10-20 secs for a freshly turned on vehicle to lube through well enough to get going without damage in cold weather. Your car will heat up faster under load, your HEAT will warm you up faster because of this, and you will not burn your gas up sitting there for 10 minutes while your car idles....

My STi goes from 25mpg to 21-22 every winter. Tends to run better at highway speeds since the IC stays nice and cool.

The VX goes from about 14mpg to about 11. She LOVES the cold however cause that S/C is a cold air huffer.

[circmand]

when was the last time you had the engine tuned? How often should it be tuned?

[nfpgasmask]

So last night I changed out the fuel filter. I also added some Lucas additive.

Now, to answer some questions, my spark plugs were changed just this past summer. Off the top of my head, I can't remember what brand. However, after I changed my plugs, I did not notice a marked decrease (or increase for that matter) in my gas mileage, so I doubt my plugs would be to blame.

Now, the O2 sensors and CAT I have been thinking about. With all that oil burning, they could potentially be full of soot. But, if an O2 sensors was flaking, wouldn't I throw a CEL code? I mean, I know our CPUs are intelligent and they "learn" to compensate for certain conditions, so maybe not?

My next plan of attack will be O2 sensors and FPR. I think we have two O2 sensors, right? One upstream and one downstream?

I will keep you all posted.

Bart

[89Vette]

With a CEL for two specific cylinders, shouldn't you concentrate on them?

I'm not familiar with the code/meaning that you're describing (or even if you've posted the exact msg). But, I would suspect the plugs or coil packs -- having gotten that code.

And, TBH, I haven't (yet) figured out how spark is distributed on a coil-pack ignition system. IOW, without a distributor, if there's any other component that could fail and cause this condition. (Obviously, the older systems would require you take a look at the distributor -- for cylinder misfires.)

[CoastieCosta567]

Thats nuts, I get around 350-390 miles per tank, depending on fuel quality and speed, and I got somewhere around 165,000 miles.

[nfpgasmask]

Thats nuts, I get around 350-390 miles per tank, depending on fuel quality and speed, and I got somewhere around 165,000 miles.

I've never seen more than 250 miles per tank. 350 would be a dream.

Bart

[CoastieCosta567]

Last Feb. i hit a personal record score of 20mpg, no lie, my wife was there and saw it, we where shocked but have never seen it that high before and today still haven't. But that was due to the fact that i was driving down hill the whole way from the smoky mountains. Its possible that people get those mpgs, the worse I got was 9 mpg, but that was cuz there was a plug in the engine compartment that was loose, and every time i hit a bump the car turned off, took me 6 months to figure out that it was a small and simple fix as just connecting the plug back on correctly. Worse part was that i drove cross country from florida to california like that back in 2007, gas was not that bad though, thank God. My only guess for that issue is that i think the mechanic that saw the VX before i left unplugged something by accident or worse on purpose. You might want to check all your plugs, it could be something that simple. The plug that affected my mpg was the main wire harness that is inside the engine compartment, next to the right fender, there is 3 big-o plugs one over the other, the plug that was loose on mine, was the bottom one, and that one had my temp. gauge off when the plug was not plugged correctly.

[89Vette]

The only other difference I can think of is fuel quality/octane -- as mentioned. Do these people getting 300-400miles/tank live in other countries/areas where fuel is just WAY better?

Also, I looked around again. It seems consistent that the stoimetric point of ethanol is 14.1 (vs 14.7 for gas). That means, for 1 part fuel, it takes 14.1 parts ethanol for complete burn. 14.7 parts gas for complete burn.

Switch that around and it takes 14.7/14.1....about 1.0425 (4.25%) more ethanol than gasoline. But that doesn't count for energy difference (and the amount of throttle) necessary to roll the vehicle along. IIRC, Jerry Lemond was saying 10% loss -- which I thought was a little exaggerated. 5% is what calculations imply. If it's less than 2%, none of us should be getting much less than the 15-19mpg that was the EPA measure for the VX.

Actually 22gals x 19mpg = 418 gallons possible for a tank of gas. Obviously, that would be for a highway trip. It seems pretty bad that many of use are in the 250mpg range, don't you think?

Sorry for the ramble....

[circmand]

The only other difference I can think of is fuel quality/octane -- as mentioned. Do these people getting 300-400miles/tank live in other countries/areas where fuel is just WAY better?

Also, I looked around again. It seems consistent that the stoimetric point of ethanol is 14.1 (vs 14.7 for gas). That means, for 1 part fuel, it takes 14.1 parts ethanol for complete burn. 14.7 parts gas for complete burn.

Switch that around and it takes 14.7/14.1....about 1.0425 (4.25%) more ethanol than gasoline. But that doesn't count for energy difference (and the amount of throttle) necessary to roll the vehicle along. IIRC, Jerry Lemond was saying 10% loss -- which I thought was a little exaggerated. 5% is what calculations imply. If it's less than 2%, none of us should be getting much less than the 15-19mpg that was the EPA measure for the VX.

Actually 22gals x 19mpg = 418 gallons possible for a tank of gas. Obviously, that would be for a highway trip. It seems pretty bad that many of use are in the 250mpg range, don't you think?

Sorry for the ramble....

The government mpgs have very little to do with what you can expect. They allow you to add mpgs for a hybrid that you will never see. A lady recently sued Toyots because she never saw half the mpg the government allowed them to claim. I think the tank is 21 gallons. Also I don't know about you but I never get the tank completely empty before I refuel.

[blacksambo]

Have you ever deviated from the stock engine and components? I mean it! Air filter, plugs etc.??? If you have, you will get less mileage. How I get 20 mpg in summer is simple. No engine change items. (Tried a K&N air filter once and watched mileage plummet...really!) In winter, I run higher tire pressure due to lower ambient temperture not causing over inflation. Take the roof rack off, not only quieter but better mileage. The old addage more HP=less mileage is true. Stick to 215hp. Also, synthetics in drive line and engine cut resistance and boost mileage. That's all for now, I'll think of more later.

[89Vette]

Have you ever deviated from the stock engine and components? I mean it! Air filter, plugs etc.??? If you have, you will get less mileage. How I get 20 mpg in summer is simple. No engine change items. (Tried a K&N air filter once and watched mileage plummet...really!) In winter, I run higher tire pressure due to lower ambient temperture not causing over inflation. Take the roof rack off, not only quieter but better mileage. The old addage more HP=less mileage is true. Stick to 215hp. Also, synthetics in drive line and engine cut resistance and boost mileage. That's all for now, I'll think of more later.

Are you the person posting improvement with Royal Purple? What speed to you drive in the hwy?

One of these days I'll try that sea foam stuff. I assume it cleans the valve for better atomization? (I've heard water vapor works for that too.)

I tried injector cleaning but that only got me .5 mpg. (Hard to imagine fuel cleaner would help.) Pumping up the tires helped a bit tho, Mine are (street-tread) 31's. I wonder if that oversize helps, hurts, or is a wash?

Many moons ago, I tried 14" wheels and larger tires on an early generation Honda CVCC. (lightest and smallest car I ever owned.) The mileage went down with the larger tires. Quite a bit IIRC.

I've never driven on the hwy long enough to figure out the max mileage. My trips are usually 5-10 miles. Can't help that its running in open loop such a high percentage of the time.

[MSHardeman]

Bart, wanted to give you a heads up. Don't know if this will help your gas mileage issue, but I was changing my FPR out today due to the VX taking a few more cranks than usual to get started, and I noticed that the rubber tube connected to the FPR was cracked and wasn't getting a good seal. I haven't started the VX after it has sat for a while to see if my slow start issue has been taken care of, but I'm wondering if my FPR was just fine and I just needed to trim the tube down so it could get a better seal.

[nfpgasmask]

Mark!! Thanks very much, I will have a look at this. I still need to replace mine again. I have a few things that need attention, my brakes as well, up front, which could be dragging a little. Anyway, I will definitely check this out.

Thankfully, I have been roaming around Reno in my new to me Trooper RS. I'm enjoying the little 4 banger and getting about 300 miles per tank! This little goat hauls!! Its got some issues that will need attention, but so far I love it.

Thanks - Bart

[blacksambo]

How are your oxygen sensors? Have you checked them today? Number one cause of mileage decrease if they are off.

[Vendetta]

How are your oxygen sensors? Have you checked them today? Number one cause of mileage decrease if they are off.

This is helpful, I'm getting CEL for sensors on banks 1 & 2. Also experiencing diminished mileage. Unfortunately though, I just upsized my tires, so it'll be tough to reconcile cause and effect here.

-V