Sudden (apparently) decline in gas mileage?

[89Vette]

You're blaming the wrong culprit. It's the winter gas' high Butane level. I think it's more noticable this year due to new polution regs.

Well this is the first winter with E10 (where we didn't have a lot of snow). With all the warm weather, people should be seeing higher avg MPG than E10 might dictate. (FWIW, if you didn't know: Alcohol returns less energy (BTUs) for a given combustion cycle. So, there's no way we'll get the same MPG as the 100% gasoline days.)

Have you tried Lucas fuel treatment? Cleans the injectors and plenum and boosts mileage, noticably. My wife (she drives one, too) and I are averaging 330 miles per tank now, almost 400 in the summer without the Lucas then, to counteract the Butane which leaves us in the spring. Hope this helps.

330? 400? It seems fairly hard to believe these vehicles are getting 20mpg on E10 (or gasoline for that matter)? The only way I could see that being close is if on long, continuous highway runs. That's because I've never gotten much over 250/tank. Jerry Lemond says it's really impossible to improve MPG over what I'm getting.

If this is really true, please say more about tires/wheels and percentage of hwy miles being run. Even the speed on the hwy would be relevant.

Any other mods (or significant service) too! Intake gasket? fuel injector swap? tuning?

[BigSwede]

Well this is the first winter with E10 (where we didn't have a lot of snow). With all the warm weather, people should be seeing higher avg MPG than E10 might dictate. (FWIW, if you didn't know: Alcohol returns less energy (BTUs) for a given combustion cycle. So, there's no way we'll get the same MPG as the 100% gasoline days.)

It's not as big a difference as you might think.

Ethanol contains roughly 80% of the btus of gasoline. So if the mix is 10% ethanol, you still have 98% of the btu as before. So you can expect 2% less btu and hence, mpg. So if you were getting 16 mpg before with straight gasoline, it will "plummet" to 15.68 mpg with 10% ethanol.

[89Vette]

It's not as big a difference as you might think.

Ethanol contains roughly 80% of the btus of gasoline. So if the mix is 10% ethanol, you still have 98% of the btu as before. So you can expect 2% less btu and hence, mpg. So if you were getting 16 mpg before with straight gasoline, it will "plummet" to 15.68 mpg with 10% ethanol.

I understand your point....I'll have to go back and refresh on that one. Thru the vette forum, I'm a long-distance friend to an injector rebuilder/seller. He figures a 5% change (drop in mpg) due to ethanol. He's got a micro-drilling rig and injector response machine. Both were purchased to design "in-between" injectors as replacements.

For example, a car the came with a 22lb/hr injectors will be fitted with a 23lb/hr injector as an ethanol replacement (to maintain stoich balance). The mapping machine helps him determine/compensate for non-WOT fuel delivery.

[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.)