81 fleetwood brougham
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500, 472, 425, 368 Discussion, 81 fleetwood brougham in Cadillac Engine Discussion; Just wanted to know the good and the bad about the 8-6-4 motor. How much mpg do you get too....
  1. #1
    uso
    uso is offline Cadillac Owners Member
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    81 fleetwood brougham

    Just wanted to know the good and the bad about the 8-6-4 motor. How much mpg do you get too.


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  3. #2
    drmenard is offline Cadillac Owners Connoisseur
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    Re: 81 fleetwood brougham

    the 8-6-4 or 6.0 or 368 motor is a dependable low power motor.. The best gas mileage I ever got on the highway was 19mpg...But let me say that was with the 8-6-4 disconnected, thats running on all 8 all the time... around town 15 at best... but that was years ago...

  4. #3
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: 81 fleetwood brougham

    A friend said he saw 21 mpg on the road, with 8-6-4 functioning. Bruce Roe

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  6. #4
    uso
    uso is offline Cadillac Owners Member
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    Thread Starter
    Oh ok thanks guys. I just bought my 81 and can't wait to get on the road with it. I think I'm just goin to keep the 8-6-4 functional

  7. #5
    The Ape Man's Avatar
    The Ape Man is offline Cadillac Owners Connoisseur
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    Re: 81 fleetwood brougham

    Bruce has the correct info.

    There isn't much of a downside. The 6 cylinder mode vibrates but is only a transition between 8 and 4.

    All the original complaints with these cars EXCEPT the 6 cylinder vibration were DEFI related and later accepted as normal.

  8. #6
    uso
    uso is offline Cadillac Owners Member
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    How do I know if the 864 is turned off cuzz I keep checking it on the electronc gage that tells u how many cylinders are running and its always on 8.

  9. #7
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: 81 fleetwood brougham

    The orange wire from the transmission is likely cut.

  10. #8
    The Ape Man's Avatar
    The Ape Man is offline Cadillac Owners Connoisseur
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    Re: 81 fleetwood brougham

    There were also PROMs available to disable the 8/6/4.

  11. #9
    sevillejussi is offline Cadillac Owners Member
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    Re: 81 fleetwood brougham

    Quote Originally Posted by The Ape Man View Post
    There were also PROMs available to disable the 8/6/4.
    Hi, The Ape Man
    What do you mean, could you tell us more?
    ----------

  12. #10
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: 81 fleetwood brougham

    He means that the dealership disable the cylinder deactivation feature (when customers complained that 6 cyl mode was vibrating) two ways. One (like my car), they cut the orange wire that sends the high gear signal to the computer that the trans is in 3rd; or, by actually swapping in a different computer that does away with the deactivation. Check under the car first.

  13. #11
    The Ape Man's Avatar
    The Ape Man is offline Cadillac Owners Connoisseur
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    Re: 81 fleetwood brougham

    Not the whole computer just the PROM chip. Somewhere I've seen a list of GM PROMS and some were listed as 8/6/4 disabling.

  14. #12
    uso
    uso is offline Cadillac Owners Member
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    Thread Starter
    I'm gona have to checc on that...both the wire and the PROMS.

  15. #13
    carnut is offline Cadillac Owners Master
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    Re: 81 fleetwood brougham

    If the wire is cut, run the wire thru a toggle switch. Mount the switch at the bottom of the dash. Switch off until you reach freeway speed then flip the switch to engage 4 cyl. mode. The only thing problematic is the plastic valve covers tended to crack if the owner or mechanic was not careful replacing the gaskets.

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