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Discussion Starter #1
Hello,

I am the owner of a brand new 2003 Cadillac Seville STS and I'm writing for advice from this large group of dedicated and wise members.

This is a wonderful vehicle, however, I am disappointed with the fuel efficiency of my new STS.

While I understand the limitations and caveats associated with the EPA estimates of 18 mpg city and 27 mpg highway, I believe my car is not operating efficiently.

My fuel efficiency is about 30 percent below the EPA standards as I'm experiencing about 13 mpg city and 21 mpg highway under the following conditions:

1. Driving without a "heavy" foot and avoiding "jack-rabbit" starts,
2. Extended highway travel with cruise control set to 65 mph for a 500 mile trip,
3. Tire pressure of 34 psi,
4. I have approximately 1600 miles on the odometer, and
5. I have switched to Mobil 1 5W-30 synthetic motor oil.

Although I have only 1600 miles on the vehicle, I have been assured that there is no engine break-in period.

Furthermore, as many of you know, the 2003 model includes two (2) changes "touted" to improve engine efficiency and longevity - the use of Teflon-coated pistons, and the new and lighter steel (instead of cast iron) crankshaft.

My fuel efficiency calculations are based on BOTH the car's built in trip computer, and my "manual" calculations from filling and refilling the gas tank myself and dividing gallons used into the amount of miles driven. I am pleased to see that my calculations are well within 10 percent of the car's trip computer.

The dealership has told me that my fuel efficiency is satisfactory and the EPA numbers are for "ideal" conditions that no one will actually experience - although I have read the EPA guidelines and feel that the dealership is wrong. Also, on all of my previous vehicles (not Cadillacs) I have managed to achieve the EPA estimates.

While I don't expect to regularly meet the EPA estimates, I feel that performing at 30 percent worse, indicates there is a problem.

Since the dealership cannot help me, I would appreciate any comments, including your personal experiences with gas mileage for this vehicle.

Thanx for wading through this lengthy message.

Regards,
Pat in Massachusetts
 

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My 2001 SLS averages 21.5-22.0 according to the trip computer. My calculations tend to support those numbers. Ive found I get around 27-28 on long trips, although ive only taken 2 trips over 250 miles. I would wait till you have more like 5-10k on the odometer before you worry to much. Welcome to the board
 

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Pat,
I would tend to agree with Brett. Even though there isn't a "break-in" for the engine, there IS a period of time that the car needs to "settle-in". I've seen it many times on many different cars, after several thousand miles, the car will not only improve on the gas mileage, but also get a little quicker. Years ago, GM used to install what was called an E-Cell in their cars. This was something that caused the engine management system to maintain a richer-than-normal air/fuel mix for the first few thousand miles and then it would go dead and return the system to a "normal" mix. I really can't say if GM still uses these things but it would tend to explain your situation.
 

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Discussion Starter #4
Thanx to Brett and Katshot for their informative replies.

I spoke to an engineer at Cadillac today and he stated that the STS will never meet the EPA estimates, although the SLS is more likely to meet them sometimes.

This is due to the final drive ratio of the SLS vs. the STS (3.71 vs 3.11). This translates into the SLS producing lower rpms at cruising speed compared to the STS; therefore better fuel economy. He also stated the SLS may achieve 5-8 mpg improvement over the STS.

In other words the EPA mileage estimates, as stated in all Cadillac literature and the new car window sticker are not true for the STS; but instead were based on the performance of the SLS.

A little bit of false advertising for quite a few years, perhaps?

At any rate, I'm afraid that I'll have to live with my less-than-expected fuel economy.
 

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Make sure you keep us posted, i wouldnt be surprised if it turns out a little better than you think
 

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PJM,

You might also keep in mind that the Northstar will learn your driving habits over a period of time, and your mileage may go up some. Mine is a much older Northstar, and my fuel economy isn't what other post. I average 22-23 mpg on the highway and 17-18 in town. Keep in mind that mine sits in the garage 90% of the time, and when I do drive it I tend to push it a bit. Just for fun I checked the mileage at the track the other night and got about 12.5mpg.

Don
 

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My '95 ETC used to get about 20mpg (I hope I remember correctly) as a whole. I drove it like a 95 year old man most of the time.. I suppose being v8's these cars are going to burn some gas. But, it should be known that the performance models are going to suck up some more gas. Hey, that's what forums like this are here for. Now we know..

I do tend to have a feeling that the gas mileage will go up, though. Being that the STS learns your driving habits (it'll shift differently) things should change - atleast to some extent...
 

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Discussion Starter #8
Sal,

Thanx for your reply. Yes, we should at least be told that Seville (and Deville) "performance" models (STS and DTS, respectively) will not achieve the EPA mileage estimates stated in EPA guide book and on the new car's window sticker.

Some readers may say that if I can afford to spend $50K+ on a new car, then I can afford poor gas mileage too. Well, I borrowed the most of the money (using the GM "0" percent loan), and I only wanted to get close to the gas mileage that I thought I was entitled to, according to both Cadillac and the EPA.

If you read about the EPA, you'll see that they only verify about 1 in every 10 vehicles, and must trust the manufacturers to tell the truth. I feel that Cadillac did not tell the truth by placing the erroneous EPA mileage estimates on the window sticker of my new STS.

It's really the truth-in-advertising problem with Cadillac that upsets me more than the poor gas mileage.
 

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Well I have been checking my mileage on my '99 STS and it seems to be hanging around 17.5 to 17.9 mpg and that includes the occassional heavy foot, OK OK OK more than a few occassions but not everday, well..... maybe only once or twice a day but not for extended periods of time. Anyway I'm getting at least 17MPG and I just filled the tank this morning with 91 octane so I will keep my foot out of it for the next 7 days and let you know what mileage I get. I don't think there is any major mechanical difference between the '99 and '03 models that would drastically affect the mileage between the two.

It's going be tough driving like my Grandpa (God Rest his Soul) for a week but anything for a forum member. :)
 

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Ya know? I'm jealous of you guys. I want your cars. :D Otherwise.. I bet in coming years there will be some disclaimer on the window stickers that say something like "gas mileage may vary on high performance models"...
 

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Discussion Starter #11
mnymaker,

Thanx for your reply and for the link to the great photos of your White Diamond 99 STS.

Regarding the 17 mpg you're experiencing; is that for city or city/highway driving? Either way, it's better than what I'm getting.

I'm averaging 15 mpg city/highway, using Mobil 1 5W-30 synthetic oil, and driving like an old man. I also use the K&N drop-in filter replacement.

However there are some differences between your 1999 STS and my 2003 STS that we should to consider:

1. In 2000, Cadillac reduced the compression ratio in all Northstar engines slightly to allow use of regular (87 octane) gasoline. (I am using regular gas.)

2. In 2002, Cadillac claims the use of factory fill GF3 oil spec. results in .5 to 1 percent in fuel economy and 40 percent reduction in oil consumption. (Of course, Mobil 1 5W-30 exceeds the GF3 spec.)

3. In 2003, the Northstar engine iron crankshaft was replaced by a new steel crankshaft to reduce weight, reduce engine noise, and improve efficiency. Also for 2003, the Northstar pistons now have a Gryphal polymer coating, to reduce friction, reduce engine noise, and improve efficiency.

With all of these efficiency "improvements" since 1999, I find it amusing that a 2003 STS gets poorer fuel economy that a 1999 STS!

Anyway, I look forward to hearing about your most recent gas mileage evaluation - however I'm on my way out of town for about 2 weeks, so I suppose I'll read about your results upon my return.

Once again, I am impressed with, and pleased to have access to, this terrific forum - thank you Sal!!!
 

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I find it interesting that ALL Northstar-equipped Cadillacs claim an EPA estimate of 18 city/ 27 highway regardless of model. Especially interesting is the fact that Cadillac expects the cars to get the same mileage even though there is a big difference in final drive gearing between the sport and non-sport models.
Even MORE interesting is that Cadillac owners seem mystified by it all ;)
 

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A couple of points:

My '02 STS averages approx 18 mpg if I don't beat on it too often, but that includes an 85 mile round trip 4 days a week which is not highway but not city either - kinda in between. I'm using a K&N air filter and Mobil 1. And I only use premium gas from one ot the "high btu" gas stations.

My mileage went up about 1 mpg average after the dealer flashed the updated programming into my PCM - my car was part of the recall. It may be that your's needs the update too. I could be wrong there - you're programming could well be up to date.

The steel crankshaft and coated pistons are for NVH (noise/vibration/harshness) reduction AFAIK, although a minute amount of efficiency may be gained. The Northstar Gen II incorporates these changes with a slew of others.

As stated by above, there may be some break-in involved - not the same as in earlier engines but over the first several thousand miles the moving parts and seals do take on a final set and surface characteristics. I'd expect the efficiency to go up a bit after 5,000 miles or so.

Although you can use lower octane fuels with the STS, I think you'll find a decrease in both performance and efficiency if you do so. True, it's not a big difference, and greatly depends on how you drive and how much you pay attention to such things. However, the computer does compensate for the lower octane with altered timing and who knows what else. Further, not all gasolines are equal. In a large study done by SuperChips, the gas sold at Citgo, Amoco, and Texaco had a higher btu content (which means more energy per gallon) than the others. This can vary by region, so they recommend you run 3 full tanks of each brand (not necessarily just the "big 3") through your vehicle and carefully record the mpg. The one which gives you the best mpg should then be used for peak efficiency.
 

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Discussion Starter #14
the Sandman,

Thanx for the comprehensive reply - you've provided lots of good info for me to digest.

A few questions since you also use Mobil 1: I use the weight of 5W-30; what about you? And what oil change intervals are you using?

According to Mobil 1, we both could be using the 0W-30 weight for more efficiency - what do you think? They told me there would be only advantages and no disadvantages in using 0W-30 instead of 5W-30 - what do you think?

I also use the Mobil 1 oil filter.

Thank you again for your assistance.
 

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Katshot said:
I find it interesting that ALL Northstar-equipped Cadillacs claim an EPA estimate of 18 city/ 27 highway regardless of model. Especially interesting is the fact that Cadillac expects the cars to get the same mileage even though there is a big difference in final drive gearing between the sport and non-sport models.
Even MORE interesting is that Cadillac owners seem mystified by it all ;)
I think it's a bit naive to think GM doesn't know *exactly* what the mpg for each model/level/drivetrain is, and more importantly what the EPA rules for reporting mpg are. I would venture to say they are within the EPA guidelines, and that it's up to the consumer to learn to decipher the information given. BTW does anyone have a scan of a "sport model" Caddy window sticker so we can see what the exact wording is?

Wait, on my '02 STS window sticker it says:
"Actual mileage will vary with options, driving conditions, driving habits, and vehicle condition. Results reported to EPA indicate that the majority of vehicles with these estimates will achieve between 15 and 21 mpg in the city and 22 and 32 mpg on the highway."

Now, technically the rear axle ratio on the STS is not an option. However, I'd say that both the STS and SLS are Sevilles, that Seville is the "model", STS and SLS are "levels", and that the reported mpg is a figure which represents a weighted average of mileage for both levels of Seville based on drivetrain and number of units of each sold. I'd also say that an SLS could probably beat the 18/27 EPA mpg listed on it's sticker if driven nicely.

It also appears that PJM's mpg values are very slightly below what the EPA reports is expected of a vehicle with the estimate given the STS. Based on the new condition, low octane fuel, and low miles tested, it would seem that he's probably not getting unacceptable mileage - further testing and tweaking are indicated.
 

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pjm said:
A few questions since you also use Mobil 1: I use the weight of 5W-30; what about you? And what oil change intervals are you using?

According to Mobil 1, we both could be using the 0W-30 weight for more efficiency - what do you think? They told me there would be only advantages and no disadvantages in using 0W-30 instead of 5W-30 - what do you think?

I also use the Mobil 1 oil filter.
I also use the 5W-30. I have the maintenance agreement with the dealer for 2 years, and strangely enough, they recommend an interval of every 3 months or 3,000 miles and pay no attention at all to the "engine oil life" reported on the DIC. So I supply the Mobil 1 and they do the dirty work.

I wouldn't switch to 0W-30, there would be *very* little difference in efficiency when cold, and as I'm sure you've noticed, the coolant temp stays cold for all of a minute or two. Granted oil temps will come up a bit more slowly but suffice it to say the transverse mounted FWD Northstar cars do not run cool for long. It's more advantageous to use the factory recommended oil and document it each time so that in the event of a failure your tail is covered.

You should get the Bilstein oil flush (click link for details) done every so often, maybe once every 4 - 8 oil changes or so - most Cadillac dealers have the machine.

Also, you can consider changing to synthetic transmission fluid. This should slightly improve efficiency as well as help the tranny run a bit cooler and last a bit longer. If you go this route, wait until 10,000 to 20,000 miles, and have the Cadillac dealer make the change after flushing the trans. This way you have a nice, clean transmission, synth fluid, and your still 100% covered by factory warranty - don't let anyone else but the dealer touch that trans while it's still covered.
 

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FWIW, I just came back from a business trip with my 94 STS. It was 220+ miles each way. On the way there I had the cruise set fairly steady at 75 mph. I reset the fuel mileage reading just when I hit the road Monday. It recorded 25 mpg on the drive there.

I was in a hurry to get home today & I drove an average of 80 mph (non-cruise) & the mileage was 22 mpg.

The car has 95000 miles on it & I was running 92 octane on the trip out and 93 on the trip home. These figures are for US gallons, not imperial gallons.
 

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Well after 120 miles or so of city based driving this week and a little less than a half a tank burned (maybe 3/8ths) I am still averaging around 17.5 mpg.

OK, OK I admit I put my foot in it a couple of times after saying I wouldn't but that little old lady at the light tried to stare me down, and I had to take her. Well maybe she was admiring the Diamond Pearl paint job after the car wash, hmmmmmmm well I couldn't take the chance that she would get me out of the blocks so I gunned it :D

I will keep you posted on the rest of the tank as I finish it off and all the other old bitties out there looking for a race :D
 

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No one start complaining about bad gas mileage, because the Q could get the worst ever!!!!! i get around 14-16 in the city (depending on how short my trips are, and how heavy my foot is) and about 20 on the highway (estimated, i havent really been on the highway too much)
 
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