Expected Cylinder Compression Values
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RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, Expected Cylinder Compression Values in Past Cadillac Vehicle Discussion; Does anyone know what the expected cylinder compression values are for an '84 deville (4.1L)? I can't seem to find ...
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    nebulosity's Avatar
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    Unhappy Expected Cylinder Compression Values

    Does anyone know what the expected cylinder compression values are for an '84 deville (4.1L)? I can't seem to find it in my shop manual.

    Right now, one of the biggest problems with my Caddy is the serious lack of power; I can't seem to get it to go faster than 30 or 35 mph, and it's acceleration is really slow. I've been doing a bit of research on possible causes, and I know the compression in one of my cylinders was really not good, but I sort of expected that even still, I should be able to get it running well enough so that I can go have it emissions tested and licensed to give me time to plan more expensive and time consuming repairs.

    So I got to thinking... what if the compression in my engine really wasn't satisfactory and is the major contributor to lack of power? Here are the compression test results we got last fall.

    Cyl #1: 115psi
    Cyl #3: 105psi
    Cyl #5: 60psi
    Cyl #7: 100psi
    Cyl #2: 120psi
    Cyl #4: 120psi
    Cyl #6: 80psi
    Cyl #8: 115psi

    Relative to the highest values, Cylinder 5 is really crappy, cylinder 6 isn't much better, and I'm not sure what they're ACTUALLY supposed to be. I know I'm going to need an engine rebuild, but I really NEED to be able to get it licensed to give me more time to work on it (save money, make a plan, find a shop or do it myself, etc).

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    csbuckn is offline Cadillac Owners Connoisseur
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    Its not really the numbers, it's the difference between them. There shouldn't be more then like 20% difference or something like that. Unfortunately, yours doesn't look too good with a 50% difference in one cylinder.
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    outsider is offline Cadillac Owners Connoisseur
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    Re: Expected Cylinder Compression Values

    According to this post: http://www.cadillacforums.com/forums...test-pics.html

    Should be between 140 and 160.

    I (thankfully) do not have any ht4100 engines so I can't tell you for sure but that sounds more inline with what I would expect.
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    Re: Expected Cylinder Compression Values

    Quote Originally Posted by outsider View Post
    According to this post: http://www.cadillacforums.com/forums...test-pics.html

    Should be between 140 and 160.

    I (thankfully) do not have any ht4100 engines so I can't tell you for sure but that sounds more inline with what I would expect.
    Thanks for helping me find that post! I forgot about the ht4100 forum. I need to visit there more.

    Kinda sounds like I really need new rings......

    ----------

    Quote Originally Posted by csbuckn View Post
    Its not really the numbers, it's the difference between them. There shouldn't be more then like 20% difference or something like that. Unfortunately, yours doesn't look too good with a 50% difference in one cylinder.
    Yeah. I wonder how much my power is affected by the differences in compression in the cylinders.

    You know, I'll have to do another test when I get the Caddy back (unless the guys at the shop decide to do it). It was my first compression test -- I'm assuming you can't really do them wrong, right? o__O

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    cadillac kevin's Avatar
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    Sounds like you have a 6 cylinder ht4100....not good.

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    Re: Expected Cylinder Compression Values

    Quote Originally Posted by cadillac kevin View Post
    Sounds like you have a 6 cylinder ht4100....not good.
    Yeahh. i just don't have a driveway to be able to fix it. we'll see what Larry's (the shop) says.

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    Aztec ETC ECS's Avatar
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    Re: Expected Cylinder Compression Values

    Quote Originally Posted by nebulosity View Post
    Kinda sounds like I really need new rings......
    More likely valve leaks. Put compressed air into the plug hole with the valves closed.
    Take the air cleaner & oil cap off and listen for air. TB or exhaust- valves. Oil fill- rings.

    I've seen engines w/broken rings have more than 80lbs.
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    Re: Expected Cylinder Compression Values

    Quote Originally Posted by Aztec ETC ECS View Post
    I've seen engines w/broken rings have more than 80lbs.
    Interesting... so if the valves are good, how does a mechanic determine whether the rings are a problem? By the fact that the compression is low and the valves are good?

    Quote Originally Posted by Aztec ETC ECS View Post
    More likely valve leaks. Put compressed air into the plug hole with the valves closed.
    Take the air cleaner & oil cap off and listen for air. TB or exhaust- valves. Oil fill- rings.
    Okay, I'll see about trying that (if the shop doesn't get to it first). Do weak lifters cause the valves not to seal properly? Or does a valve train that needs adjustment cause valves not to close properly? Or do valves warp over time? I'll do some research using google in the mean time.

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    j2rossit is offline Cadillac Owners Enthusiast
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    Re: Expected Cylinder Compression Values

    The old school trick to confirm bad rings is to squirt some heavy oil into the the cylinder and redo the compression test. If the number on that cylinder jumps up, then the rings are bad.
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    Re: Expected Cylinder Compression Values

    Quote Originally Posted by j2rossit View Post
    The old school trick to confirm bad rings is to squirt some heavy oil into the the cylinder and redo the compression test. If the number on that cylinder jumps up, then the rings are bad.
    Just like the lifter test we did.

    WAIT....... do you think valves not sealing as they should would be the reason why I had wayyyyyy too much pressure in my valve cover???

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    Re: Expected Cylinder Compression Values

    No, lifters wont do it. Either the valve or seat is N/G.
    This exhaust valve came out of my 455:


    It's not impossible for worn rings to do it though. The air test will tell you.

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    Re: Expected Cylinder Compression Values

    Quote Originally Posted by Aztec ETC ECS View Post
    No, lifters wont do it. Either the valve or seat is N/G.
    This exhaust valve came out of my 455:
    OUCH. How did that happen?

    Quote Originally Posted by Aztec ETC ECS View Post
    It's not impossible for worn rings to do it though. The air test will tell you.
    Okay, thanks.

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    Re: Expected Cylinder Compression Values

    Quote Originally Posted by nebulosity View Post
    WAIT....... do you think valves not sealing as they should would be the reason why I had wayyyyyy too much pressure in my valve cover???
    Another clue. That would be blow-by past the rings.
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    Re: Expected Cylinder Compression Values

    Quote Originally Posted by Aztec ETC ECS View Post
    Another clue. That would be blow-by past the rings.
    Is piston ring failure a typical occurrence of engines at 89k miles? o.O

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    Re: Expected Cylinder Compression Values

    Quote Originally Posted by nebulosity View Post
    OUCH. How did that happen?
    Hard to tell for sure. The seat probably leaked first, causing the valve to burn

    ----------

    Quote Originally Posted by nebulosity View Post
    Is piston ring failure a typical occurrence of engines at 89k miles? o.O
    No, but the 4100's had their share of problems.

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