Regular vs. Premium Fuel in a '99 STS
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Cadillac Seville / Cadillac Eldorado Forum Discussion, Regular vs. Premium Fuel in a '99 STS in Past Cadillac Vehicle Discussion; BTW, premium is NOT required for a '99. Either it or '98 was the first year when it became only ...
  1. #1
    SwampeastMike is offline Cadillac Owners Enthusiast
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    Regular vs. Premium Fuel in a '99 STS

    BTW, premium is NOT required for a '99. Either it or '98 was the first year when it became only "suggested for greatest performance".

    Anyway, for the first time and due to non-availability I used a tank of 87 octane.

    It seemed to last forever in almost exclusive city (very hilly) driving! While I do often drive as if there's an egg between my foot and the accelerator (and brake) I have zero problem punching it for left turns onto main roads or perhaps just to hear the sound on a long uphill stretch just by my house I won't swear but the acceleration didn't seem as neck-jerking as normal

    Ran it down to "low fuel warning", put in the premium I normally use and the gas gauge seems to be dropping far more rapidly despite no other changes.

    I'm not of the inclination to do any real testing, but must ask:

    Has anyone else noticed this--especially those with a '99 STS?

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  3. #2
    basscatt's Avatar
    basscatt is offline Cadillac Owners 10000+ Posts
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    Re: Regular vs. Premium Fuel in a '99 STS

    under "normal" driving conditions -
    I wouldn't expect worse gas mileage with premium -
    but I wouldn't expect better mileage either -

    I would expect better mileage using premium
    if towing a trailer - or climbing mountains -
    or possibly a balls-to-wall run across the Bonneville saltflats -

    generally speaking - anything above 87 is wasting money -

  4. #3
    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: Regular vs. Premium Fuel in a '99 STS

    ??? Most of us are anal enough that we keep pretty accurate written logs of fuel and oil mileage. A few tanks of this and that plus a calculator and pencil will tell, and one tank of gas is the WORST way to jump to a snap conclusion, octane be damned............ given today's variable fuel pump (gas station) flow rates, who says he pump clicks off at exactly the same tank level every time........... that 1.5 gallon airspace in the tank top fills and bleeds differently with different pump fill rates.............

  5. #4
    SwampeastMike is offline Cadillac Owners Enthusiast
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    Re: Regular vs. Premium Fuel in a '99 STS

    I agree that one take of gas is a TERRIBLE way to make any sort of conclusion even if the difference did seem stark.

    For years I kept extremely accurate, comprehensive datalogs and made a great number of exhaustive tests controlled to the best of my ability on the hot water space heating system in my old home updated with a German "super boiler" and thermostatic radiator valves (every radiator is effectively its own thermostat) and really burned myself out with the record keeping/testing thing. It was enough to convince me just how little we understand about the least understood method of heat transfer known to science--radiation.

    This odd mind of mind admittedly went a bit manic with possibilities and theories even if I did have the president of the U.S. division of an extremely conservative German manufacturer who "never does these sort of things" fully prepared to provide me with thousands of dollars worth of hardware and collaborative access to their Canadian software engineering department for a solar/conventional integration experiment that even the German engineering staff considered "interesting". Unfortunately peculiarities of my home, lot and public trees required the cooperation of the public university who owns the vacant lot next door that will remain vacant unless they bulldoze my home. My drawings and plan were approved by the facilities maintenance chief and chief engineer to be nixed by the legal department for unspecified "liability" reasons despite a letter from my insurance company assuring that my coverage would be extended to include.

    Sorry for the aside and semi-rant.

    I was just curious if anyone seemed to notice a similar occurrence. I've always sort of wondered if the unusually high head gasket failure in model year '99 may have something to do with PCM tuning with regards to fuel/performance that managed to exacerbate a problem with insufficiently aggressive threads/tapping that couldn't handle the full potential of the engine

    Am heading off soon to Turkey to return the day before the November election so definitely no time now to experiment and when I return the weather will be cooler which DEFINITELY reduces fuel economy for the short, city drives that are most common for me when driving the Seville.

    Who knows--maybe it was the weather as with that tank it was so freaking hot that after an overnight sit in the garage, I swear that the temp gauge moved one notch above "C" by the time I backed down the driveway and up the incline required for the turn, down to the stop sign and made a left onto the level main street--what? 150 yards? Now when the weather here is typically miserable it has been pleasant and I have to go through the next stop sign (about 0.2 mile either direction) before the temp gauge moves. Such is the sort of thing that makes control of experiments so notoriously difficult in the real world

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    Re: Regular vs. Premium Fuel in a '99 STS

    We've been saying for years that they run just about as well on 87 as 93, but I seriously doubt either has anything to do with the HG failures.

  7. #6
    JimD is offline Cadillac Owners Connoisseur
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    Re: Regular vs. Premium Fuel in a '99 STS

    Quote Originally Posted by SwampeastMike View Post
    BTW, premium is NOT required for a '99. Either it or '98 was the first year when it became only "suggested for greatest performance".
    I'm nitpicking, OK? The Internet hearsay and guesses just ring my bell from time to time. And this thread rang my bell.

    The 1998 Seville Owner's Manual states this about fuel (pg 6-1):
    "FUEL
    Use premium unleaded gasoline rated at 91 octane or higher for best performance. You may use middle grade or regular unleaded gasolines, but your vehicle may not accelerate as well."

    Unfortunately, the Owner's Manual offers no explanation/elaboration of the octane ratings for "middle grade" or "regular unleaded" gasolines.

    Where I buy 90+% of my gasoline, the pump selector offers 85 octane, 87 octane, and 93 octane.


    Fast forward to the 2000+ 4.6 L FWD Northstar.
    Many design changes were incorporated for these engines. Not the least of which was roller cam followers which has nothing to do with octane recommendations.
    The combustion chamber was redesigned to control flame front propagation and the compression ratio was dropped from 10.3:1 to 10.0:1.

    The 2004 Deville Owner's Manual states this about fuel (pg 5-4):
    "Gasoline Octane
    Use regular unleaded gasoline with a posted octane of 87 or higher. However, for best performance and for trailer towing, you may wish to use middle grade or premium unleaded gasoline...."

    Again, the Owner's Manual offers no explanation/elaboration of the octane ratings for "middle grade" or "premium grade" gasolines.
    I'm not of the inclination to do any real testing....
    Enough said.

  8. #7
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    Re: Regular vs. Premium Fuel in a '99 STS

    If you want to run regular on a 93-99 STS, you can, but why? You bought a car for the performance, so enjoy it to the max.

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    ThumperPup's Avatar
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    Re: Regular vs. Premium Fuel in a '99 STS

    when it comes toe to toe or head to head or what ever just down to it
    if your worried about gas i know for me i i am worried about gas prices that is the day i stop driving my seville in local driving and get some light weight good on gas carola or something lol

  10. #9
    LaidbackSTS is offline Cadillac Owners Member
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    Re: Regular vs. Premium Fuel in a '99 STS

    I have a 98 SLS and a 99 STS. I have always ran 93 Octane in both. At one time we (wife and I) decided to try 87 octane for a few tanks and we both could tell a HUGE difference in performance. I may get a few more miles out of a tank of higher octane, but I cant keep my foot out of it long enough to really tell. If im wasting money on the higher grade of fuel, then so be it. I like the difference in power and the throttle response is much better with the 93 octane.

  11. #10
    RippyPartsDept's Avatar
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    Re: Regular vs. Premium Fuel in a '99 STS

    'accellerate better'
    'difference in power'
    'throttle response'

    ... this is due to the knock sensors pulling timing when sensing that lower octane gas is used
    the above mentioned 'seat of the pants sensors' are the only way to tell
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