Bad cylinder?
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500, 472, 425, 368 Discussion, Bad cylinder? in Cadillac Engine Technical Discussion; On my 425, the plug on cylinder #7 gets caked with carbon deposits, while the others look fine. I just ...
  1. #1
    ferrisworld is offline Cadillac Owners Fanatic
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    Bad cylinder?

    On my 425, the plug on cylinder #7 gets caked with carbon deposits, while the others look fine. I just put a new one in, and it's already black on one side after a week of driving. I haven't checked the compression. The motor has about 160k miles. What do you guys think is the most likely cause? Bad rings? I don't get any smoke. Stuck or burnt valave? When I did the rocker cover gaskets recently I noticed the rockers are covered in sludge.

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    deVille33 is offline Cadillac Owners Connoisseur
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    Re: Bad cylinder?

    If it fuzzy black, that cylinder is showing rich mixture. If it is hard, baked on deposits, you most likely have a worn valve stem and/or guide. The problem may go back to a worn rocker that isn't getting oil to keep it lubricated or as simple as a valve seal that cracked and broke off ages ago.

  4. #3
    ferrisworld is offline Cadillac Owners Fanatic
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    Re: Bad cylinder?

    Yeah the old plug was just caked with baked on stuff. So let's so I redo the heads... The engine has at least 160,000 miles on it (speedo has read slow for who knows how long), should I just rebuild the whole thing, or would just doing the heads be fine? I figured I might as well go to a new cam, maybe an mts #3 or 5 and find a better intake and manifolds as well while I'm at it.

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    deVille33 is offline Cadillac Owners Connoisseur
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    Re: Bad cylinder?

    That depends on what caused the problem. The 425 cams are not impressive as a performance profile cam would be. Not a lot of lift and short duration. From an working everyday driver, the design is fine. These cams are not subject to the wear a performance cam would have. Check that al cam is operating as it should. pull the plugs and turn the engine over with the valve covers removed to see that the valve train is operating as it should. At 160 K If you want ot do both heads, it wouldn't hurt. I have seen thse engines go over 200 k befor needing work.

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    ferrisworld is offline Cadillac Owners Fanatic
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    Re: Bad cylinder?

    So if I put in an mts #5, how long will it last?

    Is it a better deal to just get a pre ported intake from mts, or get my own from pick-n-pull?

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    deVille33 is offline Cadillac Owners Connoisseur
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    Re: Bad cylinder?

    Without looking at the specs, I cannot tell you how the mts #5 will compare with the stock cam. Cam wear depends on the resistance the valve train offers against the profile. As the lift and duration increases, the valve spring tension will increase. In a true performance application special springs are installed to prevent harmonic valve bounce, or float as it is called, which occurs at higher engine speeds.
    How are you planing on using this engine?
    If it is going to be a daily driver, you do not need a ported intake. A ported intake will require the you port match your heads intake ports to enhance induction, if that is your objective. If you are trying to shed a little weight, you can get an Edelbrock intake for the 472, which will fit and match fairly well with little effort.

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    ferrisworld is offline Cadillac Owners Fanatic
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    Re: Bad cylinder?

    Yeah it's just a daily driver in my 79 deville. I live in Cali though, so I need to keep the EGR. I doubt a smog tech will notice if I use a 500 intake. They say the #5 cam is close to the stock 1973 cam, which is like the best stock cam there is for performance (they say). But from the description, even their #10 cam would work I think, I just don't know if I would still be able to get good mileage with it. They say you don't need to worry about the stock valve train until you go to the #15 cam.

    So I guess then, just clean up my heads and put in new valves and springs, no porting, 500 intake and exhaust manifolds, #5 cam, and I'm set.

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    drmenard is offline Cadillac Owners Enthusiast
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    Re: Bad cylinder?

    If it were me I would just get a good cloyes timing set and use the keyway to advance the cam timing 4 degrees and fix any bad valves and maybe then change the intake ... I would do a little cutting on the intake to open it up....

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    ferrisworld is offline Cadillac Owners Fanatic
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    Re: Bad cylinder?

    I'd like to get all that sludge off of there though, so I figure if I have to take off the heads, I might as well put in all new valves and springs. You don't think any of the exhaust stuff is necessary?

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    deVille33 is offline Cadillac Owners Connoisseur
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    Re: Bad cylinder?

    The stock right exhaust manifold is restrictive on the #1 exhaust port, by design to clear the right upper control arm. If you want to use the 500 exhaust manifold. you can go in there with a die grinder and trim out some clearance. Is it nessasary? No, if you use the # 5 cam grind, as described. Most likely, yes, with the # 10. If you get the # 10, you should concider changing the needles in the carb to provide more fuel to the mix. As drmenard mentioned, a new timing set and a tune up, will do wonders to restore performance. Check and replace any dried, cracked or split vacuum lines (this includes the lines inside the passenger compartment) and ensure your dashpots all hold vacuum, as described in you SM.

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    The Ape Man's Avatar
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    Re: Bad cylinder?

    I'm late to the party but how about making sure the ignition is OK?

    Wires can get screwed up easily if the boot is not carefully and expertly removed. They take about 1/2 turn to break the seal to the spark plug insulator before pulling.

    Whacked wires can work under light electrical load and fail when required firing voltage rises. The plug gets loaded. Even new wires only removed once can fail.

    Cadillac engines do NOT wear the same way other brands do. I have yet to see a Cadillac valve guide get bad enough to oil stain a spark plug.

    The camshafts are just loafing in there as said. Bottom end will explode long before the cam even has a gray hair.

  13. #12
    ferrisworld is offline Cadillac Owners Fanatic
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    Re: Bad cylinder?

    That could be Ape Man, especially since #7 is the hardest to get to, so it's harder to tell if the boot is really on. I'll continue to check the new plug from time to time so see how it's holding up.

  14. #13
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    Re: Bad cylinder?

    Quote Originally Posted by ferrisworld View Post
    That could be Ape Man, especially since #7 is the hardest to get to, so it's harder to tell if the boot is really on. I'll continue to check the new plug from time to time so see how it's holding up.
    You might try swapping just that one ignition wire. They can misfire even with the boot on.

  15. #14
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: Bad cylinder?

    Ape Man is right on that. Check the resistance of the plug wires. There is an outline in the SM on the resistance of the different wire lengths resistance ratings. I usually take the readings of the shortest wires and determine a rate per inch then apply that to the longer wires. If a wire goes over the rating, I replace it. The longer a wire the more susceptable it will be to spark leak. A little nick, crack, or rub wear in the outer covering can allow spark to escape the insulating sheathing and cover. The High Intesity of the spark charge will make it more apt to escape in this manner. Like water it will follow the path of least resistance. That is why they went to 8 mm wires for the HEI spark system. Also check under the distributor cap. These large caps are susceptable to moisture affecting the receptor surfaces. If they appear black along the spark line they may be at fault.

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