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HT4100 4.1, 4.5, 4.9 Discussion, 4.1,4.5,4.9..Which You Prefer? in Cadillac Engine Technical Discussion; The larger injectors are fooling the computer into believing that the car is going farther on a given amount of ...
  1. #61
    Sevillian273's Avatar
    Sevillian273 is offline Cadillac Owners Connoisseur
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    Re: 4.1,4.5,4.9..Which You Prefer?

    The larger injectors are fooling the computer into believing that the car is going farther on a given amount of fuel. It expects a certain flow rate (stock injectors) and calculates based on that. I've run into this myself when I was using 19lb injectors over the stock 16lb units. I was mystified by the discrepancy between actual and displayed mileage until last year when drewsdeville pointed out to me that the computer was not taking the un-metered fuel into account. ('un-metered' fuel being the difference between the flow rate of the stock units and that of the larger units.) Calculate the actual mileage using the odometer and you will be unpleasantly surprised!

  2. #62
    drewsdeville is offline Cadillac Owners Master
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    Re: 4.1,4.5,4.9..Which You Prefer?

    ^^^That.

    You just installed injectors that flow about 15% more fuel over time. The PCM doesn't know this - all of it's algorithms are static. Now dynamic fuel adjustments made by the PCM will be off by about the same percentage, as pulse width is being reduced to accommodate.

    15% reduction from the false 28-31mpg's is 23.8-26.3mpg's - right where it's supposed to be.

  3. #63
    sprocket is offline Cadillac Owners Member
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    Re: 4.1,4.5,4.9..Which You Prefer?

    Well thanks for destroying my fantasy guys! Yeah, I thought the fuel gauge looked like it was dropping too fast for 29 mpg. Anyway, the car runs pretty good and is very responsive. The idle isn't perfectly smooth but maybe there is still some things I can do to improve it more. These are used, and very old injectors. Cleaning them might help. How about an adjustable fuel pressure regulator?

    Really, the idle is acceptable to me. It's not that bad.

  4. #64
    Sevillian273's Avatar
    Sevillian273 is offline Cadillac Owners Connoisseur
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    Re: 4.1,4.5,4.9..Which You Prefer?

    An adjustable FPR probably isnt necessary. After all, the computer is compensating for the extra fuel flow. Lowering the fuel pressure can degrade the atomization of the fuel from the injectors.

  5. #65
    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    Re: 4.1,4.5,4.9..Which You Prefer?

    Buying known-good injectors would yield the best results. A couple of bottles of Lucas fuel system cleaner would probably be a big help too but injectors can fail for a variety of reasons and fuel cleaner only addresses gummed passages

  6. #66
    still_walkin's Avatar
    still_walkin is offline Cadillac Owners Connoisseur
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    Re: 4.1,4.5,4.9..Which You Prefer?

    i owned two devilles 86sdv and 93 sdv. heres my good and bad. the 86 the power sucks the coolant system isnt all that great but the engine is easy to work on . its gets better gas than the 4.9 buyut i overall like the classic look of it but i hate the hubcaps on the 86. and the 93 more power uses more gas the engine is a bit harder to work on and i love the SIR system. the extra space is great and i love the fact that it still looks elegent from todays cars. however in looks great on vogues
    jojostar25 and jojostar25 like this.

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