3 out of 3: Intake Plenum
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Northstar Engines and System Technical Discussion Discussion, 3 out of 3: Intake Plenum in Cadillac Engine Technical Discussion; For anyone out there that is wondering what percentage of 00-05 Northstars have a torn intake plenum, for me at ...
  1. #1
    bigtone's Avatar
    bigtone is offline Cadillac Owners Fanatic
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    3 out of 3: Intake Plenum

    For anyone out there that is wondering what percentage of 00-05 Northstars have a torn intake plenum, for me at least it's 100 percent. We have 3 Devilles in my household, 2 2001's and 1 2002. Last year I bought the second 2001 with a bad plenum. Within 2 weeks of changing it, the other 2001 lit up the SES light with the P0171 and 174 codes. Another torn plenum. I bought the 2002 for my wife this past September with 47k miles on it. I thought I would be off the hook with a plenum for a while. Now the 2002 has never been as good on gas as the 2001's, and had a slight miss at idle. The other day the SES light came on, good old 171 & 174. Just got done installing the plenum tonight, took 1 hr 15 minutes. Had a nice big hole in the bottom. Runs as it should. Hopefully the mileage will be come up a bit now. I'm still 2 out of 3 with the mode actuators, I'm waiting for it to go on the other 2001.

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    OffThaHorseCEO is offline Cadillac Owners Connoisseur
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    Re: 3 out of 3: Intake Plenum

    that reminds me, i have an extra plenum i can sell. brand new in the box, didnt arrive until i got rid of my DTS

  4. #3
    Ranger's Avatar
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    Re: 3 out of 3: Intake Plenum

    At 7 yrs/85K mine is still good, but I fully expect it to go some day soon. How much do you want for it?

  5. #4
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    Re: 3 out of 3: Intake Plenum

    Why are these intakes going bad?

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    jimsbox is offline Cadillac Owners Enthusiast
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    Re: 3 out of 3: Intake Plenum

    Yes, how common is this failure?

  7. #6
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    Re: 3 out of 3: Intake Plenum

    Quote Originally Posted by jimsbox View Post
    Yes, how common is this failure?
    Synthetic rubber, heat, vibration, different rates of expansion between the plastic composite intake manifold and the aluminum throttlebody/water crossover casting, time, and oil and fuel vapor attack.

    They all seem to split in the lower part of the bulge (bellows) so my guess is that's the place most subject to collecting moisture and acids from the PCV function - when the engine cools it pools there and the disintegration of the synthetic rubber begins.

    For the model 2000,1,2,3,4 Northstar I'll bet it's a 100% failure, BUT time/mileage dependent.

    It's probably a good thing that you can't see what's sitting in the bottom of the runner log in the intake manifold........................see that long flat part in this shot of an upside-down manifold ? That's where all the air begins its run up and around the tuned runner for each cylinder.

    FWIW, This would be a perfect time to do a thorough TB cleaning - but the inside of the manifold is impossible, so don't even try.

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    Re: 3 out of 3: Intake Plenum

    Here's the plenum slipped on the manifold snout............

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    Re: 3 out of 3: Intake Plenum

    Good explanation.

    So the clamped TB coupling is the part that fails?

  10. #9
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    Re: 3 out of 3: Intake Plenum

    Quote Originally Posted by johnny kannapo View Post
    Good explanation. So the clamped TB coupling is the part that fails?
    Yep - here you go................

  11. #10
    jimsbox is offline Cadillac Owners Enthusiast
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    Re: 3 out of 3: Intake Plenum

    Sub, my DHS has 280,000 miles and the plenum has not failed yet. I am wondering, your opinion, replace now before I install the intake manifold or wait and see. Also, while I am at it, any tricks in mating the engine to the trans in the cradle, it was a pain in the nether regions separating the old ones. I am thinking of greasing the torque converter nub and the locator dowels but I am installing it alone with a shop crane.

  12. #11
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    Re: 3 out of 3: Intake Plenum

    Jim,
    Not to answer for Sub, but it is amazing that you made it 10 yrs and 280K without replacing the plenum duct. If I had to guess, I'd have to say it's days are numbered. Not expensive or difficult, but since you already have the manifold off, I'd replace it now.

  13. #12
    jimsbox is offline Cadillac Owners Enthusiast
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    Re: 3 out of 3: Intake Plenum

    Thanks Ranger. That is pretty much the way I am leaning.

  14. #13
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    Re: 3 out of 3: Intake Plenum

    Yepper, yepper..............

  15. #14
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    Re: 3 out of 3: Intake Plenum

    i would advise the same as Ranger, since it takes a bit of labor to get in there to replace the plenum (and the plenum is only about $35, list price) there's no reason to not do it, really

    interestingly, i was just talking to one of my techs who was working on an '04 SRX w/ the N* engine and was asking for a plenum
    i questioned him as to why those crack and he didn't give nearly as good of an answer as Sub just did
    what was weird to me was that we've never done one of those plenums before for the longitudinally mounted N*'s before
    i guess they may have improved on the pooling of 'stuff' inside there
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  16. #15
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    Re: 3 out of 3: Intake Plenum

    Quote Originally Posted by RippyPartsDept View Post
    i would advise the same as Ranger, since it takes a bit of labor to get in there to replace the plenum (and the plenum is only about $35, list price) there's no reason to not do it, really

    interestingly, i was just talking to one of my techs who was working on an '04 SRX w/ the N* engine and was asking for a plenum
    i questioned him as to why those crack and he didn't give nearly as good of an answer as Sub just did
    what was weird to me was that we've never done one of those plenums before for the longitudinally mounted N*'s before
    i guess they may have improved on the pooling of 'stuff' inside there
    When the Cross-Over is done it can damage the Plenum Boot. Also if you don't loosen the clamp and just run the throttle body bolts back down after changing the cross-over gsk's you can twist the boot and it will fail.

    The next time you see a customer with a bad boot look at the history and 75% or more of the time the Corss-Over was changed with in the last 12 months. Hmmmmmmmmmmmmmmmmmm

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