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1998 Cadillac Deville Sedan
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107 Posts
Discussion Starter #1 (Edited)
My car used to get 32 miles per gallon on highway but it only get 25MPG now. Is there any reason cause that other than the cold weather? Thanks!


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1998 Cadillac Sedan Devill, 220,000 miles

replaced blower motor in 2004
refilled A/C in 2004
rebuilt door switch in 2005
rebuilt TP sensor in 2005
 

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1992 Town Car Cartier & 2014 Accord LX MTX
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34,087 Posts
hmmmmm...:hmm:
You're gonna have to be more specific, what kind of Caddy, what speed on the highway we're you getting 32mpg at??
 

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1998 Cadillac Deville Sedan
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107 Posts
Discussion Starter #3 (Edited)
I~LUV~Caddys8792 said:
hmmmmm...

You're gonna have to be more specific, what kind of Caddy, what speed on the highway we're you getting 32mpg at??


It's 1998 Cadillac Sedan Devill, 220,000 miles It has a little bit oil leaking and oil burning since 2004. In the summer of 2004, it got 32MPG when I curise at 70MPH on highway and before I changed the spark plugs to some free Bosch plugs.
I checked the spark plugs this summer summer of 2005 and I found one had some oil and it got about 30MPG.
Now it's 25.
Thanks.
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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87,185 Posts
Cold weather will cause your milage to drop some. I'd dump the Bosch plugs and go with AC Delco 41-950's. Many people have reported problems with Bosch in the Northstar.

Additionally, check the FPR. Easy to do. Check the Tech Tips section.
 

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Cadillac Technician
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11,051 Posts
Yeah, I agree with that, usually the heat ranges don't match up right on the Bosch plugs and they won't burn right in just about any GM car.
Put Delcos back in, check your tire pressures and your fluid levels.
How's your trans holding up? At 220,000 you might have some tired old fluid or some slipping components.
Lots of things affect mileage. Also what method are you using to calculate?
That monitor in the dash needs to be reset periodically, especially if you are taking it for trips on the highway.
 

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1996 Cadillac Deville, 2004 Cadillac DTS, 2011 CTS AWD Coupe
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331 Posts
Thats low? lol my 96 deville gets 20mpg at a cruise of 74...and I have no clue why.
 

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1996 Cadillac Deville, 2004 Cadillac DTS, 2011 CTS AWD Coupe
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331 Posts
I have new delco wires/plugs in there from 5,000 miles ago, my fpr doesnt leak, my feul filter was replaced 5,000 miles ago- and my tires are just about right. Any other ideas? haha, Im a broke high school senior- saving me money in gas could be a big help
 

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Cadillac Technician
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Drive less... ;)
Change your driving habits. When you accelerate from stops don't do it like you are at the drag strip. There is a reason old men drive like old men. :D
Brake and accelerate gradually and evenly, just changing your driving habits can make a big difference in fuel mileage. Even though they sound cool when you floor it, that burns waaaaaaayy more gas.
If you want I can find a copy of a TSB that GM released on tips for better gas mileage.

WHAT NOT TO DO: Engine and Fuel Additives, Alternate Fuels, and "Miracle" Products
Various unproven products to improve vehicle fuel economy have been reported ranging from magnets that align molecules to chemical combustion improvers.

Most products claiming to provide benefits are based on unsubstantiated claims. Those that do present "scientific" results generally either have too little supporting data to be conclusive, have not conducted experiments in a controlled fashion, or cannot be substantiated by anyone else but the product's manufacturer.

The U.S. Federal Trade Commission summarizes results for products tested by the federal government at www.ftc.gov/bcp/conline/pubs/autos/gasave.htm. A review of the list shows that the majority did not work, and for those that showed some effect, the benefit was too small to be cost effective.


Harmful Ideas That May Damage Your Vehicle and Increase Emissions
One more recent poor idea to improve fuel economy that should not be attempted is to blend either kerosene or diesel fuel into gasoline. Why? Both kerosene and diesel fuel are distillate fuels meant for use in compression ignition engines, not spark ignition engines. They have very low octane and since they are heavier (higher density) than gasoline, they will cause heavy engine deposits and degradation of engine oil.

Notice: Never put Kerosene or Diesel Fuel in your Gasoline Engine vehicle. This may result in inconsistent performance and permanent damage to your vehicle that is not covered by your New Vehicle Warranty.

Chemicals that are normally used as solvents also should not be used. These include acetone, ketones, and methanol. These solvents can be incompatible with your vehicles rubber or sealing components, and may dissolve the vehicle’s paint finish. In the case of methanol, corrosion of metal parts in the fuel system also may occur.

Notice: Never use acetone, ketones, or methanol additives in your vehicle. Some of these solvents may damage or corrode your fuel system. They are also very damaging to the painted surfaces of the vehicle if spilled.

WHAT TO DO: Maximizing Fuel Economy/Minimizing Costs
The best fuel economy possible is the direct result of proper maintenance and good driving habits. Listed below are GM's recommendations to achieve the best mileage possible. The first group are things to consider for your vehicle, while the second are tips relating to your driving habits.


Vehicle Considerations:


• Tire Pressure - One of the major contributors to poor fuel economy are under inflated tires. Tires low on pressure create drag that the vehicle’s powertrain must overcome, wasting dollars in fuel. Always keep your tires inflated to the proper pressure as shown on the vehicle placard. This not only serves to increase gas mileage but cuts down on tire wear, further decreasing your costs per mile.

• Air Filter - A vehicle that has a dirty air filter can’t efficiently draw air into the engine. This restriction forces the engine to expend energy to "breathe" wasting fuel in the process. Change recommendations are found in your vehicle Owner’s Manual.

• Proper Viscosity "Starburst" Rated Oil - Always use the proper viscosity oil in your engine. Oil that has a higher than required viscosity will create more drag on the internal components of the engine, causing more work for it, especially when cold. Each Owner’s Manual contains information on the proper type of oil for your vehicle. Look for the "starburst" symbol on the front of the bottle, and the SM rating on the API circle on the back label. If you are in doubt, stop by your dealer for an oil change, and any other services required. Most current GM vehicles are equipped with oil life monitors to further assist on the "when" to change your oil. (Aveo/Wave/Optra/Epica currently do not have oil life monitors).
Notice: GM Vehicles DO NOT require additional engine oil additives. Some additives may cause harmful effects to the internal seals and additionally void the terms of your vehicles New Car Warranty.


• Top Tier Fuels - Some fuel manufacturers provide gasoline advertised asTOP TIER DETERGENT GASOLINE (Chevron, Conoco, Phillips 66, Shell, Entec Stations, MFA , 76, Somerset Oil, QuikTrip, and Kwik Trip in the U.S. and Chevron in Canada. These fuels are preferable when and where available. They help to keep your fuel injectors and intake valves free of deposits. Clean engines provide optimal fuel economy, performance and reduced emissions. When Top Tier fuels are not available, consider a bottle of GM Fuel System treatment PLUS, P/N# 88861011 (in Canada, #88861012), at oil change time which will remove intake system and injector deposits. GM does not recommend any other fuel system cleaner.
Important: DO NOT confuse Top Tier Fuels with Higher Octane (Plus/Premium Grade Fuel) commonly sold at most all gas stations. Plus and Premium fuels are required in some high performance GM vehicles. However, they do not necessarily represent higher detergency present in TOP TIER Detergent Gasoline.


Important: For additional information regarding Top Tier fuels and availability, please refer to Corporate Bulletin Number 04-06-04-047E for U.S. or 05-06-04-022 for Canada.


Notice: E85 FUELS: Only vehicles designated for use with E85 should use E85 blended fuel. E85 compatibility is designated for vehicles that are certified to run on up to 85% ethanol and 15% gasoline. All other gasoline engines are designed to run on fuel that contains no more than 10% ethanol. Use of fuel containing greater than 10% ethanol in non-E85 designated vehicles can cause driveability issues, service engine soon indicators as well as increased fuel system corrosion. See Corporate Bulletin Number 05-06-04-035 for additional information.


• Use the Recommended Grade (Octane) Fuel Purchasing higher than required octane fuel is a waste of money. Using higher octane fuels in a vehicle that only required regular unleaded fuel will neither increase performance nor improve gas mileage. In all cases refer to your owners manual and ONLY use the octane rated fuel recommended for your vehicle.
Important: In high performance GM vehicles that DO require Premium (91 octane or higher) fuel, you MUST use fuels of at least this octane. Use of lower octane fuel may result in reduced performance, knocking, and/or permanent engine damage not covered under the terms of the New Vehicle Warranty.



• Check Engine/Service Engine Soon Light - Is the Check Engine/SES light on? When this light is on, the vehicles On-Board diagnostics computer has noticed that something is wrong. GM vehicles have many sensors that the computer uses to both control and sense actual fuel usage. When the computer lights the Check Engine/SES light it has lost some ability to run efficiently. This may result in increased fuel consumption, increased emissions, and/or driveability concerns.

• Spark Plugs - Even though current GM vehicles have 160,000 km (100,000 mi) service intervals for spark plugs if your vehicle is at that point in it's life, have the spark plugs changed to assure proper running and continued efficient, trouble free operation.


Changes In Driving Habits:


• Slow Down, Drive Smoothly - Avoid quick/full throttle acceleration from a standstill in town and high cruising speeds on the interstates. While the optimum MPG for highway cruising speed varies from vehicle to vehicle, faster is almost always worse. If your vehicle is equipped with a Driver Information Center that displays Instant Fuel Economy, select that read out and vary your cruising speed while on the highway. The display will change continuously with uphill and downhill sections but you should quickly be able to identify on level ground the speed range that your vehicle does the best in.

• Empty Your Trunk - Avoid leaving unnecessary items in your trunk. It takes power to move increased weight and that means more gasoline consumption and reduced performance. While the change may be slight, multiplied by thousands of miles, it all adds up.

• Avoid Extended Idling - There is no need to idle your engine till it reaches operating temperature. Idling wastes fuel.

• Combine Trips - Your vehicle uses much more fuel when the engine is cold. This is especially true in the winter months when the engine will take the longest to warm up. Combine errands or trips so that the vehicle only needs to warm up once to encompass many different stops.
 

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1992 Town Car Cartier & 2014 Accord LX MTX
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hmmm...My '92 deVille with the trustworthy 4.9 gets about 22mpg at 75mph...thats around 2100rpm I believe IIRC.
 

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1996 Cadillac Deville, 2004 Cadillac DTS, 2011 CTS AWD Coupe
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331 Posts
how could you know if overdrive is working properly? my rps are low i would guess, about 2400 for 75mph
 

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White Diamond '03 DHS (with DTS floor shift)
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87,185 Posts
Drive the car in "Drive" and watch the RPM's. Then shift into "Overdrive". The RPM's should fall.
 

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White Diamond '03 DHS (with DTS floor shift)
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I meant drop it from D to 3, you should see the RPM increase. 3 to D will see an increase. D = overdrive
 

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2010 CTS
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I would be happy if I got 25 mpg. The best I have ever gotten was on a 5,500 trip last month at it was 24.5. And, I was really trying....... Many times the DIC indicates more than 25-even 70 coasting downhill. But, the overall average is around 24.5.
 

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Ranger said:
Drive the car in "Drive" and watch the RPM's. Then shift into "Overdrive". The RPM's should fall.
You also want to make sure the torque convertor clutch locks in 4th, that can cause a drop in MPGs and a rise in RPMs. But that should cause the SES light!
 

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1996 Cadillac Deville, 2004 Cadillac DTS, 2011 CTS AWD Coupe
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331 Posts
Well Overdrive works, checked my tire pressure, each was about 25 (should be 30) so I made them 30...could that really have caused it?
 

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White Diamond '03 DHS (with DTS floor shift)
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Yeah, the TCC not locking up could cause a MPG loss I would suspect. Low tire pressure will lower it some as well. It all addes up.
 

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2004 DTS - 1989 Coupe de Ville (since sold)
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My loss came from a new set of tires. You'd think they'd be better in some cases. And maybe only drop a mile or two if not, but this time it was about 4-5 mpg that I lost. Bummer.
I drive the same way, every day, and check my mileage on every tankful (just for fun of course) but I knew something was wrong the minute I drove out of the lot of the tire dealer. Took more pedal to make it move, felt sluggish overall. Very noticeable. The next day my freeway mileage had dropped from 26/27 to 22/23. Ugh.
Glad the crappy old tires are gone, but sure do miss the gas.
Hmmm, maybe they crushed an exhaust tube on the lift or something...

Paul
 

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32 mpg was wrong there was something wrong with your car before calculating the mpg incorrectly the devilles are rated for 26mpg epa highway and they're tests are flawless meaning you won't get above this in any real world driving

I'm betting your 32mpg was NOT an avg. mpg over a 500 mile trip it was just a 10 mile drive etc.

Your car is now calculating correctly and thus your 25 is perfectly normal any 20-25 is as good as it gets.

Anything aboe 26 and your car is malfunctioning btw instant mpg isn't entirely accurate at first. Mine on instant can mark upto 40mpg for a minute or so then it goes down etc.
 
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