Sudden (apparently) decline in gas mileage?

[nfpgasmask]

So, around the beginning of October, it seems my VX's gas mileage took a turn for the worse. Now, I could blame it all on winter blend fuel (ethanol) but it just seems to drastic, imo.

I always, always reset my tripometer when I fill up. And typically, I would get about 240 miles per tank. This was normal for me with the lift and roof rack, etc. But now my VX is right about 112,000 miles old, and I am getting about 190 miles or less per tank.

I'm still burning oil, no more or no less than "normal" and I am not feeling any performance loss or getting any CELs. The only thing different, is my VX doesn't like to start up like she used to. Normally, I would turn the key and vroom. Instantly my VX would fire up. Now, she labors a little on start up, a little skipping and then she runs smooth and normal. I recently got an alignment and my tire pressure is good. I will be needing new tires sometime soon but they are really ok for now.

So, what do you all think? I am thinking about changing the O2 sensors, possibly installing a new CAT, cleaning/changing my EGR and injectors. But with no CEL codes coming up, I feel like I could just be grasping at straws.

Or should I look at something else? Perhaps my brakes are dragging or something like that? I've checked my hub nuts and loosened them a little, as they were a little tight, but that has changed nothing.

Anyway, I just can't imagine ethanol is the reason, but maybe it is?

Bart

[MSHardeman]

Bart, I'm not really experiencing a drop in gas milage like you are, but Vicki is slow on starting up lately. Like you, I used to just turn the key and she would fire right up, but recently she turns over, sometimes for quite a while, until she catches and then runs like always. I did notice that if I turn the key on and wait for a while she will start right up as she used to. To me, and to some other that I asked for help here on the board, it sounds like a fuel issue; specifically a fuel pressure issue. When you turn the key on the fuel pump in the tank starts up and prime's the system and that is why I think she starts up after the key has been on for a few seconds.

Some of the suggestions where to replace the fuel filter and fuel pressure regulator (which I haven't done yet), and to check the valve on the fuel rail to see if it is pressurized after the car has been off for a while.
http://www.vehicross.info/forums/showthread.php?t=21852&highlight=cranks+start

Also, this weekend I got a CEL, and when I read the code it was a P1404 (I think), which translates to the EGR valve being stuck open. When that happened she REALLY didn't want to start, so I'll clean the EGR valve too to see if that helps any. I did knock on the EGR, lightly, and I think that jarred the valve loose because I haven't had the CEL show up again, and she has been starting easier.

That's my 10 cents. Hope some of it is helpful.

[Gussie2000]

Cold weather DOES play a role in this issue.

The colder weather is the demand for fuel is greater,especially when the VX has rest for several hours the fuel-air mixter during the engine pre-heat increases fuel use.

I noticed the increase in fuel use during these last 2-3 months,mostly when the temperture goes under 40~ i run the engine for no less then 10 minutes,during the first 5 minutes or so the high RPM can't accurately tell how much fuel were consumed during that short period of time until it goes down to normal but certainly is a good amount

[BigSwede]

Bart, I'm not really experiencing a drop in gas milage like you are, but Vicki is slow on starting up lately. Like you, I used to just turn the key and she would fire right up, but recently she turns over, sometimes for quite a while, until she catches and then runs like always. I did notice that if I turn the key on and wait for a while she will start right up as she used to. To me, and to some other that I asked for help here on the board, it sounds like a fuel issue; specifically a fuel pressure issue. When you turn the key on the fuel pump in the tank starts up and prime's the system and that is why I think she starts up after the key has been on for a few seconds.

Some of the suggestions where to replace the fuel filter and fuel pressure regulator (which I haven't done yet), and to check the valve on the fuel rail to see if it is pressurized after the car has been off for a while.
http://www.vehicross.info/forums/showthread.php?t=21852&highlight=cranks+start
.

Sure sounds like a fuel pressure regulator to me...

[BigSwede]

So, around the beginning of October, it seems my VX's gas mileage took a turn for the worse. Now, I could blame it all on winter blend fuel (ethanol) but it just seems to drastic, imo.

I always, always reset my tripometer when I fill up. And typically, I would get about 240 miles per tank. This was normal for me with the lift and roof rack, etc. But now my VX is right about 112,000 miles old, and I am getting about 190 miles or less per tank.

I'm still burning oil, no more or no less than "normal" and I am not feeling any performance loss or getting any CELs. The only thing different, is my VX doesn't like to start up like she used to. Normally, I would turn the key and vroom. Instantly my VX would fire up. Now, she labors a little on start up, a little skipping and then she runs smooth and normal. I recently got an alignment and my tire pressure is good. I will be needing new tires sometime soon but they are really ok for now.

So, what do you all think? I am thinking about changing the O2 sensors, possibly installing a new CAT, cleaning/changing my EGR and injectors. But with no CEL codes coming up, I feel like I could just be grasping at straws.

My buddy's 2000 Trooper has recently dropped drastically in mpg also. I suggest pretty much the same things you have, but it is difficult to pin one thing down.

10% ethanol will drop gas mileage slightly, but not like you are reporting. An we have ethanol year round, here, so that doesn't explain my buddy's Trooper.

[89Vette]

Sure sounds like a fuel pressure regulator to me...

:_iamwiths

[Ldub]

mine always goes from 230-240 mptank, down to 200 mptank every winter.

[nfpgasmask]

Well, my fuel pressure regulator was changed out probably 35,000 miles ago. Same with my EGR. I replaced it, not cleaned it.

Now Mark, I think you are on to something, because I notice the same thing. If I put the key in and turn it but not start up, I wait until the beep, beep, beep, beep is over and then usually it fires right up. Meaning the fuel pump has activated and got the right amount of fuel pumped up front. I sometimes wonder if it could be a tired fuel pump. The start up issues may or may not be related to my fuel consumption woes, but who knows?

I suppose I could pull my FPR and see if the screen has any holes in it, but again, I changed it a couple summers ago. I'll have to look back in my log book and find out exactly how long ago it was. Same goes for my EGR.

I was thinking though, when you burn a lot of oil, you are pumping a lot of soot through the exhaust system. So wouldn't my O2 sensors and my cat be pretty dirty? I mean, you would think I would throw a code if there was an issue, or at least experience some performance issues, which I am not, so I am kinda stumped.

Anyway, thanks for the thoughts.

Bart

[Ldub]

Well, my fuel pressure regulator was changed out probably 35,000 miles ago. Same with my EGR. I replaced it, not cleaned it.

Now Mark, I think you are on to something, because I notice the same thing. If I put the key in and turn it but not start up, I wait until the beep, beep, beep, beep is over and then usually it fires right up. Meaning the fuel pump has activated and got the right amount of fuel pumped up front. I sometimes wonder if it could be a tired fuel pump. The start up issues may or may not be related to my fuel consumption woes, but who knows?

I suppose I could pull my FPR and see if the screen has any holes in it, but again, I changed it a couple summers ago. I'll have to look back in my log book and find out exactly how long ago it was. Same goes for my EGR.


Bart

How long since you changed the fuel filter? That helped mine light up quicker, & is the cheapest / easiest / & maybe least fun solution to try first...:_wrench:

[VX crazy]

I remember Tone and I replacing his fuel pump in his vx back in 2002.......huge PITA....thank goodness I have small hands! But I always thought fuel pumps just go, either work or don't?

[blacksambo]

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.

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.

[nfpgasmask]

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

[nfpgasmask]

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.

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.

I'll try it! I just did a Seafoam treatment will little noticeable difference. We kinda got talking about two problems, one being the gas mileage, and the other being the slow start-ups. I bet changing my fuel filter will help with that a lot.

When I go buy a fuel filter, I will get some Lucas Fuel Treatment at the same time and see if things get better.

Thanks again,

Bart

[Mile High VX]

I use Lucas at almost every fillup...I'm getting about 15-16 MPG and typically have about 250-275 on the odo when I get gas. If I run it until the fuel light comes on it will always be over 300.

[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?

:_thinking

[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?name=Encyclopedia&op=viewarticle&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. ;)

[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.)

[BigSwede]

You should have 4 O2 sensors, one before and one after each cat. My understanding is the ones after the cats are not particularly important, it's the two before the cats that are key.

[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. :p

[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