Sudden (apparently) decline in gas mileage?

[89Vette]

OK, I guess time flies when you are having fun!!!

I just pulled my log book. My VX has 111,234 miles on it right now.

I replaced the EGR Valve at 80700 miles.

I replaced the FPR at 68708 miles.

And I replaced the fuel filter at 46180 miles!!!

I must be thinking of the last time I did the fuel filter on the Trooper!! So I know what I am doing this week! I'll start with the fuel filter and see if that helps with start up at least! And, its been roughly 43,000 miles since I changed the FPR. So how long are they supposed to last? I guess I need to check the manual. Maybe another new one is in order.

Thanks folks,

Bart

Have you tried another brand of fuel? Maybe something happened where you've gotten more moisture in your fuel. Or someone cutting gasoline.

[BigSwede]

And, its been roughly 43,000 miles since I changed the FPR. So how long are they supposed to last?

My old 99 Trooper FPR went out at around 50K. I'm sure they are supposed to last longer than that, but I believe the FPR is one of those fine GM parts that Isuzu used here and there, so whaddya gonna do?

[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.

[nfpgasmask]

Have you tried another brand of fuel? Maybe something happened where you've gotten more moisture in your fuel. Or someone cutting gasoline.

I have. Normally I get gas across the street from work. But the last few fillups I have tried Chevron, Shell and Citgo with the same results.

My old 99 Trooper FPR went out at around 50K. I'm sure they are supposed to last longer than that, but I believe the FPR is one of those fine GM parts that Isuzu used here and there, so whaddya gonna do?

Yeah, I'm gonna go out and buy a fuel filter on lunch today and some of the Lucas stuff. I'll swap it this weekend and see if my startups get better and see if the Lucas treatment makes a change. If I have time I will pull my FPR and check the screen and replace it if needed.

Bart

[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.

[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]

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.

I saw the story about the successful complaint against Toyota. Bravo!

The tank is 22.5gal according to this forum's knowledgebase...
http://vehicross.info/modules.php?na...warticle&id=29

How low (much) fuel you burn before refilling is a good point. Because of these goofy, bouncing gas gauges, some might consider it empty at 1/4 tank. Others might blow it til fumes.

Mine has 16" wheels w/31" (street) tires -- which I thought would raise mpg. Maybe it doesn't. 4:30 gears seem awfully steep for optimal MPG.

[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

[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.