Fuel Type?
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HT4100 4.1, 4.5, 4.9 Discussion, Fuel Type? in Cadillac Engine Technical Discussion; I'm still green behind the ears here when it comes to mechanical stuff.......but I was wondering.....everything I've read seems to ...
  1. #1
    EldoFan's Avatar
    EldoFan is offline Cadillac Owners Member
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    Question Fuel Type?

    I'm still green behind the ears here when it comes to mechanical stuff.......but I was wondering.....everything I've read seems to indicate the 4.9 requires premium gas...is that right?? Also, about the 4.5, does that require regular or premium?

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    Chicano-Mexicano's Avatar
    Chicano-Mexicano is offline Cadillac Owners Fanatic
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    Re: Fuel Type?

    Quote Originally Posted by EldoFan View Post
    I'm still green behind the ears here when it comes to mechanical stuff.......but I was wondering.....everything I've read seems to indicate the 4.9 requires premium gas...is that right?? Also, about the 4.5, does that require regular or premium?
    Yes the 4.9 requires premium to operate correctly the 4.5 on the other hand as long as its TBI can use regular but if you have an Allante with the Port injected 4.5 then you must use premium as well.

  4. #3
    kdcing is offline Cadillac Owners Fanatic
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    Re: Fuel Type?

    The User's Manual for the 1993 Eldorado indicates that fuel w/ 91 octane or higher is to be used. It goes on to say that fuel w/ an octane rating as low as 87 may be able to be used in an emergency, as long as heavy knocking does not occur.

    Basically, the decision comes down engine pinging/kock. The 4.9L does NOT have a knock sensor and cannot control the ignition timing to prevent knock. Knock typically occurs during high temp conditions (i.e. summer driving) or during high cylinder pressure conditions (i.e. acceleration). If you have an ear that is calibrated to listen/hear engine knock, then you may be able to test the use of lower octanes fuels. Fill up your tank w/ 89...if you do not hear knock, then your engine is probably ok w/ the use of 89. Repeat w/ 87. If knock is heard at any time, then the tuned condition of your engine probably prevents the use of 89 or 87 octane. Again, realize that different conditions (i.e. high temp, high cylinder pressures, lower altitudes) will increase the likelihood of engine knock.

    Prolonged engine knock will kill an engine...it can shatter pistons. So if you have uncertainty as to what engine knock sounds like, then use 91 octane as a cheap insurance policy. If you understand what knock is, and can sense it, lower octane fuels can be used successfully. My '93 Eldorado (4.9L) has been fed 87 octane for at least the last 7 years (140k miles) without any engine damage. My engine currently has 205k miles (original engine...never replaced or rebuilt). On several occassions during hot summer days, my engine has knocked under heavy acceleration. To prevent engine damage, I controlled the engine knock by getting off the gas.....fueling w/ higher octane fuels during the hotter days also prevented the knock.

    If you want additional information, take 4 days off of work and put a tank of coffee on the burner. Then do a search for "Fuel", "Octane" or similar.....there are a gazzillion posts - and even more opinions - on the subject.

  5. #4
    Chicano-Mexicano's Avatar
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    Re: Fuel Type?

    You are able to use it in emergencies but it should only be used in emergencies. Like when I was forced to use 87 on the way back from Michigan (the highest they had was 87 the lowest they had was 70 something!!!!!!!!!!!!!) she ran fine but after logging some miles in town I noticed that she would rev uncontrollably for about a minute on start up and didn't seam to be happy at all with running 87 (her millage also tanked in town) but once I filled her with premium she idled smooth a steady and has been running happy for over 3 months know so its my recommendation that you just run premium fuel in the 4.9 it will be a lot cheaper in the long run that replacing or rebuilding an engine.

  6. #5
    EldoRedo's Avatar
    EldoRedo is offline Cadillac Owners Fanatic
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    Re: Fuel Type?

    Just to mention the other option is to advance the timing 3 degrees to take advantage of 91's increased power. That plus the usual 4.9 HP package of a less-restrictive muffler and a high-output ignition and I've increased my mpg by 3. Then it comes down to doing the math to decide if it's better than switching to 87!

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