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Old 05-10-2008, 09:27 PM   #1
Scott Peterson
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1997 SL2
Default Fuel economy

What causes bad fuel economy ? my sl2 doesnt get 40 mpg like every one else so what causes this ? you can drive it like a granny and it still isint great.i would say im getting like 30 mpg highway. it has newer plugs less than 1 year old, and im putting some nice wires and a pcv valve on it monday im hopeing this will help.

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Old 05-10-2008, 09:47 PM   #2
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1997 SL2
Default Re: Fuel economy

Auto or Manual? Your .sig doesn't specify. From what I've seen, the ~40mpg goes with the manuals. I've found that ~30 hwy is par for my automatic (so far) after doing all the recommended things.

...
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Purchased 114k 10/07
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Old 05-10-2008, 09:49 PM   #3
Scott Peterson
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Default Re: Fuel economy

sorry its a auto ! what are the reccomended things ? havent some auto's got 40 mpg ?

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Old 05-10-2008, 09:52 PM   #4
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Wrench Re: Fuel economy

My understanding is that the DOHC engines (you did mention that you had an SL2) don't get nearly as good gas mileage as the SOHC engines no matter what you do.

I do 95% of my miles as highway miles and average around 36-37mpg (manual transmission). However if I do even 20-30% city or stop-and-go miles, it drops right down into the low 30's.

I believe that the DOHCs are geared a little higher as "sport" setups and perhaps tuned differently for a bit more power or torque. Both hurt gas mileage.

You might check your catalytic converter to see if it is plugged up. That has happened to me on other vehicles. Dragging brakes can also hurt mileage badly and you might not even realize it is happening except that they seem to wear real fast.

Just a thought....

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Old 05-10-2008, 10:01 PM   #5
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Default Re: Fuel economy

Meaning.. ECTS, Thermostat (1/4 temp problem), plugs & pcv (so far)... I managed to pull up the mpg by about 7 city (commute/mix) since I've been tracking in Dec (and have also seen oil consumption increase at the same time). When my son takes it on hwy trips I see it jump to from ~25 to ~30. Others may get up to 40 but I haven't seen anything better than a hair over 30 so far.

...
1997 - SL2, barebones, 161k, Auto - Rebuild complete and.. burnt a valve 20k later Arrgh!
Purchased 114k 10/07
Added OEM CC

Last edited by off-track; 05-10-2008 at 10:08 PM..

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Old 05-10-2008, 10:12 PM   #6
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Default Re: Fuel economy

off-track,
What octane gas are you putting into it? If you are burning the 10% ethanol blend then the mileage will be about 5% lower than you would get with straight gas.

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Old 05-10-2008, 10:31 PM   #7
Scott Peterson
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1997 SL2
Default Re: Fuel economy

I put just 87 in it, and i dont use any additives, i have newer plugs , coolant temp sensor, monday i will put in a pcv and plug wires. i do notice on hard throttle that it smokes blue.... normal saturn, and also a cloud of black comes out, running rich maybe ?

...
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Old 05-10-2008, 10:45 PM   #8
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Default Re: Fuel economy

I avoid the ethanol blends whenever I can in both the Sat (87) and the summer car (89). It's increasingly difficult here as there are only two brands that have straight gas. I don't know if my son does the same when he fills, but I doubt it? I'd say that the 100% gas has been ~80% so far though.

...
1997 - SL2, barebones, 161k, Auto - Rebuild complete and.. burnt a valve 20k later Arrgh!
Purchased 114k 10/07
Added OEM CC

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Old 05-10-2008, 11:33 PM   #9
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1996 SW2
Default Re: Fuel economy

The trannies mated to the DOHC engines are geared LOWER than their SOHC counterparts, thus, the engine revs HIGHER at a given speed in a given gear than the SOHC. Plus, the DOHC breathes more in on each cycle.

The forties are more the realm of the SOHC, esp. 5-speed, world. The only DOHC automatic person here getting 40-ish MPG that I can think of is Luke, and we all know he's up to something (he uses a sail on windy days, or his kids push the car the first mile, or something....). Def. do the steps recommended here, and if it gets your highway MPGs into the mid-30s, call it a good day.

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Old 05-10-2008, 11:39 PM   #10
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Default Re: Fuel economy

Quote:
Originally Posted by madpogue View Post
The trannies mated to the DOHC engines are geared LOWER than their SOHC counterparts, thus, the engine revs HIGHER at a given speed in a given gear than the SOHC. Plus, the DOHC breathes more in on each cycle.

The forties are more the realm of the SOHC, esp. 5-speed, world. The only DOHC automatic person here getting 40-ish MPG that I can think of is Luke, and we all know he's up to something (he uses a sail on windy days, or his kids push the car the first mile, or something....). Def. do the steps recommended here, and if it gets your highway MPGs into the mid-30s, call it a good day.
ya that luke im not sure what to think

...
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Old 05-11-2008, 12:24 AM   #11
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1997 SC2
Post Re: Fuel economy

Semantics. In both the hot rod and mechanical engineering worlds a 4.11:1 differential is referred to as being geared "higher" than a 3.23:1. Depends upon your viewpoint I guess. No matter.

Regardless, the point is that the engine turns over more times per mile in top gear on the twin cam setups as I understand it, so the engine is performing more combustion cycles per mile. Each combustion cycle requires a proper fuel-air mix to fire, so all other things equal, a higher gear ratio drive train will result in more fuel used.

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Old 05-11-2008, 12:38 AM   #12
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1996 SC2
2006 VUE 3.5L
Default Re: Fuel economy

Quote:
Originally Posted by Scott Peterson View Post
What causes bad fuel economy ? my sl2 doesnt get 40 mpg like every one else so what causes this ? you can drive it like a granny and it still isint great.i would say im getting like 30 mpg highway. it has newer plugs less than 1 year old, and im putting some nice wires and a pcv valve on it monday im hopeing this will help.
Me too. I believe only the guys with the SOHC get that great milage. We get the super zippy performance off the light but burn more gas. I'm happy when I can get 30 mpg in my SC2. I'm also MT.

-Robert

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Old 05-11-2008, 07:00 AM   #13
Scott Peterson
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Default Re: Fuel economy

I dont know if its just me. but i seem to have come across more sohc's in the junk yard than dohc's

...
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I love the greaters too...you can't even tell who the *** he is greating. Both of his eyes are looking different directions. You only go to walmart for comic relief.

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Old 05-11-2008, 07:18 AM   #14
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Default Re: Fuel economy

I saw improved fuel economy in my '02 SL2 when I had my throttle body flushed. I went from 30'ish to 35 mpg. That was over a year ago and it's still the same. I only use brand name gas and my cars seem to get better mileage with it.

I heard that Hero Hut gas has an ethanol blend so if you're trying to avoid that stuff.... It's funny that putting sugar in your tank used to be a bad thing. hmmmm lol

Anyway, I put new plugs, wires, and a coil pack in - 1 and 4 were mis-firing, and had a valve adjustment at 60k. I recently started adding Chevron's Techron fuel system cleaner to my tank each month. I'm up to 38 mpg now. It's cheaper at W Mart - the place where we dare not speak its name. lol

I hope this helps.

...
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02 SL2..."Wednesday"


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Old 05-11-2008, 07:30 AM   #15
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Default Re: Fuel economy

I think you have to be careful comparing your MPG results to people that state what they get on the Internet. Please tend to post pretty optimistic results. For all we know they got that mileage one time from a tank that was filled up at the top of the continental divide and calucated at the bottom.

I do know that highway MPG can varie significantly by what speed you're willing to travel. My SC2 5-speed consistantly gets around 30 MPG in city driving which is better than the new adjusted EPA estimates for the car, but I usually only get around 32 MPG on the highway which is lower. I attribute that the fact that I drive pretty fast on the highways. I usually keep my CC set at just under 15 MPH over the posted speed limit. I could drive 65 MPH and get much better results, but I doubt I'll ever do that. For city driving however, I do start off slowly and try to time lights.

Moral of the story is that you are the biggest factor in determining your MPG results.

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Old 05-11-2008, 01:52 PM   #16
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1995 SL2
Default Re: Fuel economy

Quote:
Originally Posted by Scott Peterson View Post
sorry its a auto ! what are the reccomended things ? havent some auto's got 40 mpg ?
Didn't read the whole thread, but I think 40 is about max for an sl2 auto. I have a '95 SL2 (auto) and my best one-tank average was almost exactly 40, which was almost completely highway, with an average speed a bit below 60 and very little traffic and only a few stops. That also includes some cruise control, very slow acceleration, and slowing down a bit going up hills (when the cruise wasn't on). Also, on this particular trip there was a total elevation drop of approx 500 feet (although I doubt that is enough to be significant).

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Old 05-11-2008, 04:39 PM   #17
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1997 SL
Default Re: Fuel economy

I have a 97 SL manual tranny and 40 is about the max on the highway.
One trip I got 42 though on one tank.
About 33 in town.

"Others" don't get in the 40's, they get maybe 40 on a good day.

Start with basics- pump tires to 34 or so, feather the gas pedal, don't make quick starts, try to keep moving, avoid congestion and jams, etc.
Keep it tuned well. Avoid high speeds. Wind resistance kills MPG really fast, past a certain point. Some say 45 mph. Coast when you can.

I think 40 is pretty good for a 2500 lb car.

(My 997 cc Kawasaki got 45 at best, it weighed "only" 600 lbs. Of course, it was 16 valves and 100 HP and 0-60 time of right now.)

...
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Old 05-11-2008, 07:31 PM   #18
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Default Re: Fuel economy

Its driving style , since we have the same car , in good running cond .
For a whole tank , accelerate slowly , if you drive city .
If you can't do that , then be glad you drive a Saturn , rather than
a Chevy Tahoe .

I drive mostly highway , and get no worse than 35mpg ..with winter tires .
In the summer , I routinely get 39mpg for a 300 mi trip .
This is 65mph , no AC on , windows up , driving with traffic .

I once topped 41mpg for a month , with all the tricks I could muster .
Coasting , slow starts...etc .
39 , 41 , same thing to me .

FYI: I drive my wife's Saab 9000 , I get 25mpg , she gets 20 .

Wayne

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Old 05-15-2008, 10:00 PM   #19
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Default Re: Fuel economy

AARGH! The wife sent me to fill the Sat tonight (for the son's needs). Off I go to the local non-ethanol station that I patronize for just that reason. As I plug in the hose I notice the dreaded "contain up to 10% ethanol" sticker!!! WTF!!! I'm screwed! That leaves only one brand here that's "real gas". What can be done to stop this insanity? We're guaranteed >$4/gal soon (I already see that as $4.29) but why the heck must we accept lower mileage (and higher prices for everything else) along with it? Sorry, /rant-off

...
1997 - SL2, barebones, 161k, Auto - Rebuild complete and.. burnt a valve 20k later Arrgh!
Purchased 114k 10/07
Added OEM CC

Last edited by off-track; 05-15-2008 at 10:14 PM..

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Old 05-15-2008, 10:42 PM   #20
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Default Re: Fuel economy

Quote:
Originally Posted by off-track View Post
AARGH! The wife sent me to fill the Sat tonight (for the son's needs). Off I go to the local non-ethanol station that I patronize for just that reason. As I plug in the hose I notice the dreaded "contain up to 10% ethanol" sticker!!! WTF!!! I'm screwed! That leaves only one brand here that's "real gas". What can be done to stop this insanity? We're guaranteed >$4/gal soon (I already see that as $4.29) but why the heck must we accept lower mileage (and higher prices for everything else) along with it? Sorry, /rant-off
i also hear that guys with carburated vehicles are in even more trouble with this corn gas thats what i hear anyways.

...
1997 sl2, auto



I love the greaters too...you can't even tell who the *** he is greating. Both of his eyes are looking different directions. You only go to walmart for comic relief.

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