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Strange Poor Gas Mileage Issue - HELP!


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So here is the deal. I was getting good gas mileage with my 99 Deville and now I'm getting pretty crappy mileage. I bought the car in April 2010 and I was getting 26-28 with it consistently and now I am getting 19-21 consistently. My RPM USED to run 1850 (+/- 50 RPM) at 64 MPH, now it's running about 1900 (+/- 50 RPM) at 60 MPH! I know for a fact the torque converter is locking up because I can feel it unlock (and watch the tach) when I tap my break and keep a steady speed, I can feel it lock up at 41, and I can feel it unlock on steep hills.

The plugs and wires are only 10,000 miles old (AC delco premium OEM) and I have checked them and they are good. My mileage problem happened shortly after I replaced the tires with Goodyear Integritys. I think that is just a coincidence though. The tires are the exact same size and I have them properly inflated. Air filter has been replaced not so long ago. Tranny fluid has been exchanged not so long ago. I am at a serious loss here. I drive this one same 240 mile highway stretch every week and I just watched it progressively get lower and lower. It is very depressing knowing that the car used to get that good mileage. I know it is cold out, but it shouldn't make THAT big of a difference. It's not from letting my car warm up either because I reset it when I am heading out of town.

I used to have a 2002 Seville STS with a P0741 code and I NEVER got this bad of mileage.

If anyone can help point me in the right direction, I drive over 500 miles a week and it's KILLING my bank account! I might as well drive my Silverado if I keep getting this bad of mileage.

HELP!

-Dusty-

2006 Cadillac DTS Glacier Gold Tri-Coat with 175,000 miles

1993 Cadillac Sixty Special Gold Mist with 185,000 miles

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26 to 28 average at highway speeds after a reset of all parameters moving at 60 mph you mean correct? if that is what you mean, I can confirm that my 96 Deville got the same mpg at highway speeds after a reset so that you were monitoring ONLY highway speeds

now you are getting 19 to 21? That IS quite a drop

Any codes?

How is the performance

The increase in RPM of 50 isnt that serious, if it were you would set a code, but its something to keep in mind

Same octane rating?, tire pressure good?, brakes dragging?

Have you looked at the intake to see if the throttle body and air filter are clean?, and nothing has been sucked in?

See this link

http://answers.yahoo.com/question/index?qid=20070604092014AA1Yj4S

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Tires can and sometimes "WILL" make a difference in mileage...but usually more in the range of 1 to 1.5 MPG.

I have never heard of tires making a 5 to 7 mpg difference. (I lost about 1 MPG with the last set of tires I put on my car.)

If your RPM is higher now and you "KNOW" the TC is locking up...the only thing I can think of is TC slippage.

But that should set a code.

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Thanks everyone for replying. BBF, my post is actually a little confusing about the RPM. It actually increased quite a bit. Before when I was getting good mileage I was running 1850 at 64, but now I'm running 1900 at 60. I haven't measured what it is now at 64, but I will later today. But you can see I was running 50 RPM less at 4 MPH more. So i'm actually probably running 300 RPM or so more, but I'll check later to be sure.

And yep, it was after reseting all parameters, but actually I was getting that mileage at about 64 MPH. And when I would go 70, I would get 23, which is fine with me. But now I can't get over 21, even if I drop all the way down to 60.

I replaced the air filter about 6000 miles ago and I cleaned the throttle body quite well about 4000 miles ago, so those should be fine.

Jim, that's exactly what I was thinking. That IF tires were going to make a difference, it wouldn't be that huge of a difference. And I was also thinking torque converter slippage, but that would more than likely throw a code if it was going to be that big of a mileage dump.

Ranger, I searched the forum a little bit and I saw someone mention to check the FPR. I was going to check it today, but I'll have to get nice and bundled up because it's -5 air temperature. Should it have just been running or does it matter that it has sat overnight?

-Dusty-

2006 Cadillac DTS Glacier Gold Tri-Coat with 175,000 miles

1993 Cadillac Sixty Special Gold Mist with 185,000 miles

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Ranger, I searched the forum a little bit and I saw someone mention to check the FPR. I was going to check it today, but I'll have to get nice and bundled up because it's -5 air temperature. Should it have just been running or does it matter that it has sat overnight?

I'm not Ranger...but if I were checking it.. I would start the car for a few seconds and THEN check it.

By sitting overnight...in the cold and wind...any leaking gasoline could/would evaporate and it might LOOK good...even if it is NOT good.

Someone else will have a much better answer for you , I'm sure. :)

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Consider that the dense cold air to me always improves performance.

If you are slipping that much, I am surprised that its not setting a code. Let us know what your RPM is at 64. Have you tried different fuels?, maybe you are getting too much ethanol due to the winter blends

How is the engine performance, does it feel piggy at all?

Are you sure you are not starting in second gear? (not likely but just brain storming, you would get a code)

Smell your tranny fluid to see if its burning

try disconnecting your battery and letting the system go through a relearn

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

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Sorry about that, I completely forgot to mention about performance. My performance seems perfectly fine, I haven't noticed a difference.

I just checked the FPR and I did it while it was running and it was dry as a bone when I pulled off the vacuum line.

The RPM at 64 varied between 2000-2100. The road wasn't exactly perfectly level so I would say around 2050.

I've been filling up at the same gas station for years now and I never use any other station. Mostly because our town has a little monopoly going on and there is only one station that gets gas from a different supplier. The other stations use our local supplier which is extremely terrible gas.

It's not starting out in 2nd, I know that for sure. I had a Caprice that lost 1st gear. Man was that a dog!

I exchanged the tranny fluid back around May or June and I just smelled and checked it the other day and it looks like I just changed it. I had put Dexron VI in when I changed it.

I actually just disconnected the battery the other day for about 20 minutes and that didn't fix anything.

What about EGR? Could an EGR valve that isn't working properly cause a low mileage problem? It does have a quarter of a million miles and the orginal EGR. What do you think?

-Dusty-

2006 Cadillac DTS Glacier Gold Tri-Coat with 175,000 miles

1993 Cadillac Sixty Special Gold Mist with 185,000 miles

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What about EGR? Could an EGR valve that isn't working properly cause a low mileage problem?

Yes, but the EGR is a monitored device and should trip the SES light and set a DTC if it fails in any way.

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Darn...what if the pintel was sticking? Would it throw a code?

-Dusty-

2006 Cadillac DTS Glacier Gold Tri-Coat with 175,000 miles

1993 Cadillac Sixty Special Gold Mist with 185,000 miles

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Darn...what if the pintel was sticking? Would it throw a code?

Yes, you would get a pindle position code

I wonder if this is related to the Dextron VI?

How was the tranny fluid changed?, was a fluid exchange machine used (was it flushed with a machine)?

Interesting that you could be getting good performance and have such a drop in MPG, have you checked the MPG manually instead of doing it with the dash system?

I would reset the vehicles system on fill up, then check it manually the next time you fill up, (miles divided by gallons) then check it against the dash calculation.

We have seen drops in MPG in the winter months if I recall, but that is a big drop

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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In the very first post...it says...I just watched it progressively get lower and lower..

So...that indicates that whatever is causing the lower mileage, didn't happen all at once.

SOMETHING gradually changed over time.

Now, we just need to figure out WHAT changed.

The RPM change worries me.

If the TC isn't slipping..how can the RPM change??

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One time, just for kicks and giggles. I unplugged the EGR valve when the car was running. It didn't throw a code. I drove around for a couple of hours with it unplugged and it never threw a code. As soon as I shut if off and restarted it, then it threw the code.

So here is my theory: When the car is cold, maybe the EGR checks out. But when it gets to closed loop, it fails. But apparently the computer does not check it again after that intial startup. What do you think? I know it sounds strange, but you never know.

Yep, I have done it manually. Actually, I do it everytime to compare with the computer.

I was getting good mileage after I did the trasmission fluid exchange, so I think that would be ruled out. I did the exchange myself by taking off the cooler line and letting it drain out.

I agree, winter does kill mileage, but this is my 10th Cadillac and I haven't seen this much of a drop in mileage in any of them. Usually it's just 1-2 mpg, not 6 or more.

Jim, do you think maybe it could be slipping just enough to not throw a code? But then wouldn't you think after a few months of slipping, I'd notice it in my tranny fluid?

Oh! Just thought of another possible cause. Catalytic converter plugged? But then I would notice power difference, wouldn't I? And it would probably smell too, my 93's got plugged and it stunk something rotten.

It seems I have successfully confused everyone just as much as me. Sorry about that! :blink:

-Dusty-

2006 Cadillac DTS Glacier Gold Tri-Coat with 175,000 miles

1993 Cadillac Sixty Special Gold Mist with 185,000 miles

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One time, just for kicks and giggles. I unplugged the EGR valve when the car was running. It didn't throw a code. I drove around for a couple of hours with it unplugged and it never threw a code. As soon as I shut if off and restarted it, then it threw the code.

So here is my theory: When the car is cold, maybe the EGR checks out. But when it gets to closed loop, it fails. But apparently the computer does not check it again after that intial startup. What do you think? I know it sounds strange, but you never know.

Just a thought... but if it never does actually "GO" to closed loop...that would account for the lower mileage.

Does your temp gauge get to normal temp?

Have you cleaned the EGR valve?

Have you checked for vacuum leaks?

Yep, I have done it manually. Actually, I do it everytime to compare with the computer.

I assume you mean resetting the trip computer / DIC settings.

I was getting good mileage after I did the trasmission fluid exchange, so I think that would be ruled out. I did the exchange myself by taking off the cooler line and letting it drain out.

I agree, winter does kill mileage, but this is my 10th Cadillac and I haven't seen this much of a drop in mileage in any of them. Usually it's just 1-2 mpg, not 6 or more.

Unless it is not going to CLOSED LOOP.

Jim, do you think maybe it could be slipping just enough to not throw a code? But then wouldn't you think after a few months of slipping, I'd notice it in my tranny fluid?

I don't think the slippage in the TC would ever affect the fluid.

The TC is a FLUID COUPLING...not like internal bands that would slip on a drum.

Oh! Just thought of another possible cause. Catalytic converter plugged? But then I would notice power difference, wouldn't I? And it would probably smell too, my 93's got plugged and it stunk something rotten.

Plugged CATS would definitely cause power loss.

It seems I have successfully confused everyone just as much as me. Sorry about that! :blink:

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ok dont know if this was mentioned but if the egr were not working properly it would definately set a code. on my 96 Deville it would come on once the engine waarmed up because it was stuck shut. same applies if it is stuck open.

second, there is winter blend gasoline. I know in the winter I get about 16mpg and I never let mine idle and drive it hard. Research winter blend gas and it might make a little sense

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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ok dont know if this was mentioned but if the egr were not working properly it would definately set a code. on my 96 Deville it would come on once the engine waarmed up because it was stuck shut. same applies if it is stuck open.

second, there is winter blend gasoline. I know in the winter I get about 16mpg and I never let mine idle and drive it hard. Research winter blend gas and it might make a little sense

You get 16 mpg on the highway? I know you can get worse mileage on the winter blend, but it shouldn't drop it as drastically as 6 or more MPG. I can kind of gauge how much it should go down because my father-in-law has his 93 that he takes on the road every week too and he was getting about 26 or 27 and now he's only down to about 25. So it seems to make about a 1 mpg difference. It's pretty sad that I'm getting worse MPG than a 4.9 going 70 when I am only going 64.

What about an o2 sensor on its way out? Maybe it still is in the recommended voltage, but it's still bad enough to mess with my mileage.

-Dusty-

2006 Cadillac DTS Glacier Gold Tri-Coat with 175,000 miles

1993 Cadillac Sixty Special Gold Mist with 185,000 miles

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No way the winter blend drops highway mileage from 25 to 27 down to 16. As I said above, we do see a drop in the winter, I believe due to the % of ethanol, but I dont think its that bad.

I suggested changing gas stations as a result of that thought, maybe you were getting low grade fuel

Check your tires COLD, not HOT, and mark the temps on a sheet of paper, go to the station and ADD the pressure you need to bring your tires up to the correct pressure COLD, its possible that your pressure in the winter is low causing a higher rolling resistance. Because you are checking your tire pressure hot

Are you using regular or premium?

Smell your exhaust, is it rich with fuel, is it black?

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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mine is 16 average mix, on the highway I get great but if I am not picky on where I get my premium fuel it makes a big difference. Like drinking water.

Could very well be an o2 sensor or more than 1.

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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Ok, just got back from my weekly 240 highway mile jaunt and here are my stats:

Tire pressure (cold): 34 in front, 32 in back. (Lower in back to compensate for the less weight to get even tire wear)

Tire Brand: Goodyear Integrity

Mileage on Tires: 4500

Ambient Temperature: -5F

Cruising Speed: 64 MPH with cruise on

Cruising RPM: 2000 RPM or so

Check Engine Light On: No

Climate Control: 74F set on Econ

Instant MPG: Fluctuated between 17-21, even on level road.

Average MPG: 20.9

Average MPG (miles/gallons used): 20.5

Operating Temp: Consistently 190F

Miles on Oil: 30

Oil Used: Mobil High Mileage 5W-30 (just switched from Supertech Synthetic)

Wind Speed: 4 MPH East

Direction Traveled: 120 East, 120 West.

Keep in mind that this is ND and the road is extremely flat. There was a couple more things that I wrote down, but I left my paper at home.

My father-in-law with his 93 Deville with 195,000 miles headed out in the opposite direction at the same time and set his cruise at 64. He averaged 28 MPG. I know I shouldn't be comparing apples to oranges, but that's really odd. He fills up at the exact same station as I do.

I hope this information helps.

-Dusty-

2006 Cadillac DTS Glacier Gold Tri-Coat with 175,000 miles

1993 Cadillac Sixty Special Gold Mist with 185,000 miles

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Just got back from another 240 miles trip. Here are my results.

Tire pressure (cold): 34 in front, 32 in back. (Lower in back to compensate for the less weight to get even tire wear)

Tire Brand: Goodyear Integrity

Mileage on Tires: 4800

Ambient Temperature: -15F to -20F

Cruising Speed: 64 MPH with cruise on

Cruising RPM: 1900 going east, 2000 going west.

Check Engine Light On: No

Climate Control: 74F set on Econ

Instant MPG: Fluctuated between 17-21, even on level road.

Average MPG: 20.8

Operating Temp: Flucuated between 185-190F

Miles on Oil: 300

Oil Used: Mobil High Mileage 5W-30 (just switched from Supertech Synthetic)

Wind Speed: 11 MPH NW

Direction Traveled: 120 East, 120 West.

-Dusty-

2006 Cadillac DTS Glacier Gold Tri-Coat with 175,000 miles

1993 Cadillac Sixty Special Gold Mist with 185,000 miles

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Next time you do this record the average speed

Your system will provide that info

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Ok, I seperated each leg of the trip and got some different readings. I went a little slower and the temp was much higher. It seems that the temp is playing a huge part of my mileage. I wonder if putting cardboard in front of the radiator to hinder air movement will help a little.

Here are my results:

Going East:

RPM - 1800 +/- 50

Ambient Temp: 18-24 F (Above 0)

Wind - 18 MPH WNW

Speed - 62 MPH

Avg Speed - 59.6 MPH

Operating Temp - 190-194 F

Avg Econ - 26.4 MPG

Obviously it's quite abit higher because I had the wind helping me along.

Going West:

RPM - 1875 +/- 50

Temp - 25-30 F (Above 0)

Wind - 15 WNW

Speed - 62 MPH

Avg Speed - 58.7

Operating temp - 190-194 F

Avg Econ - 21.9 MPG

So if you were to average them completely, it would be about 24.15, which still isn't where it should be going 62 MPH. It still isn't where it should be (considering I got 28 or more consistently going 64 in the summer) but it is much better.

I have not changed a thing. The only thing that has changed is the temperature. But temp shouldn't make that big of a difference, should it? It never did on my 02 STS. Is there something that is causing the car to be affected more by ambient temp? Like a bad coolant sensor or something? Any thoughts?

-Dusty-

2006 Cadillac DTS Glacier Gold Tri-Coat with 175,000 miles

1993 Cadillac Sixty Special Gold Mist with 185,000 miles

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Are you clearing, the following?

Average MPG

Average Mile Per Hour

Fuel Used

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Are you clearing, the following?

Average MPG

Average Mile Per Hour

Fuel Used

Yessir!

-Dusty-

2006 Cadillac DTS Glacier Gold Tri-Coat with 175,000 miles

1993 Cadillac Sixty Special Gold Mist with 185,000 miles

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