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Thread: my 4.1 compression numbers

  1. #1
    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    my 4.1 compression numbers

    I had my mechanic use a good MAC Tools leakdown compression tester that I picked up on eBay. Here are the numbers with 100 lbs of input pressure

    1 -- 90 lb
    2 -- 62 lb
    3 -- 87 lb
    4 -- 86 lb
    5 -- 91 lb
    6 -- 83 lb
    7 -- 96 lb
    8 -- 91 lb

    #2 is leaking through the exhaust valve. #3, #4 and #6 are below acceptable levels and appear to be worn rings or cylinder walls.

    The engine purrs like a kitten and has surprising amounts of power, but with numbers like these it is either rebuild or replace so I am doing the latter.


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  3. #2
    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: my 4.1 compression numbers

    If you're an optimistic type of guy then you could hope the exhaust valve is just carboned up; possibly from too much octane. I would put some Seafoam in it and take it for a nice long ride and flog it a few times. Other than that just drive it. Peace.

  4. #3
    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    Re: my 4.1 compression numbers

    I've used GM Top-End Cleaner, Seafoam, Auto-RX, and a couple of other things on it over the last year to get all the gunk out. It's as clean as it's going to get.

    Excluding the really bad cylinder, the high-low spread is 83-96, which is way outside the acceptable range.

  5. #4
    Edahall's Avatar
    Edahall is offline Cadillac Owners Enthusiast
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    Re: my 4.1 compression numbers

    Yes, it's way out of range for a new engine. But your engine has lots of miles on it. Are you experiencing any symptoms? If the valves are leaking too much, it won't idle smoothly.

  6. #5
    awadecki is offline Cadillac Owners Enthusiast
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    Re: my 4.1 compression numbers

    If the motor is running fine, why bother?

  7. #6
    ehall's Avatar
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    Re: my 4.1 compression numbers

    The motor runs fine but it does have some minor secondary problems. Anyway being a 4.1 w/ 140k miles it is at the end of its expected life--even if the cam is still round and the intake is solid, it still has the two dozen other design flaws that are going to contribute to hard failure, and the exhaust valve on #2 obviously needs to be dealt with.

    I'm not replacing it right away (my Jeep engine w/ 336k miles is in really bad shape), and I am looking forward to being to take money out of this 4.1 instead of dumping money into it.

  8. #7
    Edahall's Avatar
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    Re: my 4.1 compression numbers

    My parents got 260k on their 1987 Deville with a 4.1L. It still ran fine with that amount of mileage on it. My 1988 Eldorado with a 4.5L has a couple of leaky valves which causes a rough idle but it has only degraded a small amount after 40k miles. It now has 240k on the odometer and if I do anything to it, I'll put in a lower mileage 4.9L.


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