1998 Deville Fuel Economy Issue
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Cadillac DeVille 1985 to 2005 including:
1985-1992 Fleetwood, 1993 Sixty Special, All FWD Forum Discussion, 1998 Deville Fuel Economy Issue in Past Cadillac Vehicle Discussion; Okay, so I own a 98 Deville base model with the northstar engine and 195k miles. It has a couple ...
  1. #1
    flyingcadillac is offline Cadillac Owners Member
    Automobile(s): 1988 Allante, 1998 Deville
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    1998 Deville Fuel Economy Issue

    Okay, so I own a 98 Deville base model with the northstar engine and 195k miles. It has a couple of problems that kinda baffle me. 30k miles ago I did a timesert job on the heads myself, and from then on it ran great averaging about 21.8 mpg. About 2 weeks ago when I was driving back home very late at night one of my coolant hoses popped off...all the coolant spilled out. I was only 6 miles or so from home, on a road that was mainly downhill, so I decided to try and drive the car home, thinking that if I turned the engine off and coasted when I could, I wouldn't over heat the engine and cause long term damage. It went into limp home mode around 265 degrees once, but for the most part stayed around 255-260 degrees..anyway, after the whole ordeal I fixed the hose, filled it up with coolant, and changed the oil. No cooling problems have shown up so far. I didnt change the transfluid, but I checked it and it smelled/ looked fine. It was changed when I did the heads.

    However, I've noticed that the average MPG of the car has decreased from 21.8 to 20.8 since the ordeal. What could I have done to cause that?

    Ever since I bought the car, its thrown a code p0507 for high idle speed. It only does that when its up to temperature and left in park for about a minute, so its normally not a problem as I don't ever really aimlessly idle the car. I've tried pretty much everything to fix it, but i'm out of ideas. It also has had (for a long time) a power steering pump sound like there is air in it. I've done my best to bleed it, but it still makes the sound when turning quickly. I also know it needs a new A/C compressor at some point. The current one has a bad a/c clutch bearing and makes an annoying sound when the a/c is engaged. My point is, its had these problems previously, so I dont believe they would cause the MPG drop now. The tires are properly inflated. About two days before the hose popped off I changed one of the front strut mounts out, as it was bad. The car probably needs an front end alignment now. The engine doesn't burn an abnormal amount of oil.

    Anyway, thats pretty much everything I could think of that could decrease the fuel economy. Thoughts as to what it could be lowering it? Pretty much all of its other problems it had before I drove the engine hot.

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  3. #2
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
    Automobile(s): White Diamond '03 DHS (with DTS floor shift)
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    Re: 1998 Deville Fuel Economy Issue

    Did you clean the TB when it was apart?
    Have you checked for a leaky FPR?

  4. #3
    Slipgate is offline Cadillac Owners Enthusiast
    Automobile(s): 1998 Deville
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    Re: 1998 Deville Fuel Economy Issue

    Mine (also 98) has been steadily losing fuel economy over the past around a year. Starting at 21.5 or so and now is at 18. The HG looks to have blown in mine. Hopefully not yours.

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