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Cadillac Tech Tips - How to fix it Discussion, 2000+ Northstar Plenum Replacement steps in Item Specific Cadillac Discussion; 2004 Seville SLS Update: R&R Intake Plenum Gasket, TB gasket, and Intake Manifold Gaskets. Also, cleaned TB. Notes: No tear ...
  1. #31
    Closet Boy's Avatar
    Closet Boy is online now Cadillac Owners Fanatic
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    Re: 2000+ Northstar Plenum Replacement steps

    2004 Seville SLS

    Update: R&R Intake Plenum Gasket, TB gasket, and Intake Manifold Gaskets. Also, cleaned TB.

    Notes: No tear in plenum gasket; however, was oddly worn. See pic below.

    2004-plenum-compare_50.jpg

  2. #32
    Chris_2011 is offline Cadillac Owners Member
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    Re: 2000+ Northstar Plenum Replacement steps

    Thanks very much for the tutorial! I replaced the intake plenum on my 02 SLS today. My car wasn't setting any codes but had a rough idle once the engine was warm.

    Quote Originally Posted by redfishsc View Post
    I ordered the Dorman plenum/gaskets from RockAuto, and the blue gaskets were NOT useable. They were the right "size/shape" but they would not seat firmly into the recess. Even if you managed to put them in and keep them there, they'd start falling out when you shimmied the intake back into place.

    I tried ONE of them, and since it didn't seat, I didn't touch the old gaskets. I left them in place. Cleaned them with WD40, cleaned the aluminum head where the intake bolts on with carb cleaner so that it was shiny again, and seems to be fine.
    I had the exact issue that you had with the Dorman gaskets - I also left on the original gaskets.

  3. #33
    notdel is offline Cadillac Owners Member
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    Re: 2000+ Northstar Plenum Replacement steps

    Quote Originally Posted by Chris_2011 View Post
    I had the exact issue that you had with the Dorman gaskets - I also left on the original gaskets.
    +1 on this tutorial. I replaced my plenum today and also had to replace the 90* elbow on pass side of intake manifold. As for the Dorman set from RockAuto, I had only one blue gasket not seat properly so used a little silicone RTV to hold in place. All is well. Runs like a Cadillac again!!

  4. #34
    B1G-AL's Avatar
    B1G-AL is offline Cadillac Owners Member
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    Re: 2000+ Northstar Plenum Replacement steps

    I just did this and my Deville is running much better...however after doing the job my gas pedal is now sticky and does not smoothly press down like it did before. What could have caused this?

  5. #35
    MoistCabbage's Avatar
    MoistCabbage is online now Cadillac Owners 10000+ Posts
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    Try cleaning the TB and make sure the throttle cable isn't binding.

  6. #36
    GetOffMyLawn is offline Cadillac Owners Fanatic
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    Re: 2000+ Northstar Plenum Replacement steps

    Just got done with this procedure on my 2002.5 STS, idles so smooth again I had almost forgotten, and my PCM codes all went away. Here's a complete parts list with RockAuto links and my notes - everything is cheap, you might as well replace it all while you're in there. Tools were standard screwdriver set, ratchet set, and ratcheting wrench sets (you'll definitely want a 10mm ratcheting wrench for the 4 fuel rail bolts).

    Parts List:
    Useful Procedure Notes:
    • I'm not the only one who's found the Dorman gaskets that come with the plenum to be imperfect fits - might as well spend the extra $10 for the Fel-Pro and not worry about it
    • If you're careful and patient, you can leave the steel fuel rail connected - after disengaging a plastic clip that held the lines to the throttle body bracket and the nut for the rail itself, the assembly swung out of the way 90 degrees with no resistance or kinks. Total dis-assembly was much less intensive than the factory instructions - I was able to do it just disconnecting air/vacuum-type lines and electrical plugs, nothing liquid or mechanical.
    • Clean the throttle body from both sides, especially the plate edges and interior hinges - once you've got the intake off, you just have to unplug the MAP and another pigtail connector, and you can move the TB to the side of the engine bay (carefully) without having to unhook the throttle cables. I used a toothbrush to get it mostly spotless.
    • Plenum was oddly worn like Closet Boy's above, and some of the intake manifold gaskets had worn themselves completely flush/flat, as had the TB gasket - there was ample carbon buildup around each of those seals that deserved thorough cleaning - hand-cleaning the seal surfaces was by far the most time-consuming part of this procedure.
    • I took off the Overpressure Relief Valve on the intake manifold to clean it, as there was a very significant carbon buildup on that as well
    • I failed to find a reasonably-priced device (under $100) to accurately measure the super-low torque values specified by the factory instructions, so I very carefully winged it when reassembling - YMMV.

  7. #37
    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: 2000+ Northstar Plenum Replacement steps

    alex309, Your Northstar question has been moved to its own thread in Engines, Northstar.

  8. #38
    TexasRay is offline Cadillac Owners Member
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    Re: 2000+ Northstar Plenum Replacement steps

    Joined the group to thank everyone who contributed to this thread. My Dad left me his 2003 Deville "Daddy Caddy" five years ago and this is the latest in a series of repairs I've done do it. Oil pressure sensor, AC actuator and oil filter adapter gasket are other were the others I can think of. The actuator was a trip. I did not take out the seat. I wish I had noticed that thread before doing the job.

    I do wonder about something regarding the plenum etc. When I had the oil leak I took it in to the Cadillac dealer for a diagnosis. They called and said the leak was the main seal and said i would be better off just adding oil. I agreed with that and told them to tune it up and clean the injectors while they had it. Drove OK, but no big improvement. A few weeks later it threw the P0171 and P0174 codes and started running rough. I kept driving it for a while until it was annoyingly rough. A little frustrated with it I started driving and old '98 F150 my grandson returned to us after getting an upgrade. Eventually I decided to fix it up and sell it. When I was changing the oil is when I noticed the leak was NOT the main seal, but the oil filter adapter. Hmmm, maybe it's worth keeping. I made all the repairs but while doing so, like others in this thread, I noticed my rubber elbow on the vacuum line from the rear of the intake was either already dried, cracked and broken or I did it.

    My question is, could the dealer messed up the connection when they did the injectors? They would have had to remove that line to pull up the rail. I suspect they cracked it a little and it kept getting worse. If that connection was broken would it have thrown the same P0171 and P0174 codes ?

    I've attached an image from further up this post with the elbow that was damaged circled in red. Again, thanks to all who contributed to this thread and the forum in general. It's made keeping this car in good shape a lot easier.Capture.JPG

  9. #39
    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: 2000+ Northstar Plenum Replacement steps

    FWIW, Due to age and heat many of those formed plastic vacuum lines with rubber connector boots crack and break - and so do the boots. Unless you are truly anal over "original", any of them - connectors and all - may be replaced by a suitable length of proper sized vacuum hose from the Help! racks at parts stores. In the case of the PCV dirty air hose (full engine vacuum) use a length of reinforced rubber fuel line. One piece - discard the rubber connector boots.

    Otherwise, use the NalleyGMC or GMPartsGiant websites for original pieces (if available).

  10. #40
    TexasRay is offline Cadillac Owners Member
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    Re: 2000+ Northstar Plenum Replacement steps

    I went to a parts store and found a 90 degree hose that was the right size and cut it to fit and put the clamp back on. I'm sure it will work. I was just curious if it could have caused the codes to come up and not the plenum. Plenum also had a crack in it but I'm not sure if it was there or I did it prying it off the manifold.

  11. #41
    Submariner409's Avatar
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    Re: 2000+ Northstar Plenum Replacement steps

    Any vacuum leak can set mixture codes - cracked connectors/elbows, plastic pipes or the plenum. Hard to go back and isolate what might have been the problem.

  12. #42
    clothe36589 is offline Cadillac Owners Member
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    Re: 2000+ Northstar Plenum Duct Replacement steps

    Much thanks for this procedure.
    2002 Seville SLS has been fixed!
    Had to favor drivers side to remove manifold to avoid PVC tube snag.
    Other than that , all went as you described.
    Total time about 3 1/2 hrs, cautious and real slow during dis-assembly, re-assembly went quicker, included clean-up time.
    Local Cadillac dealer for in-stock plenum (MSRP) , local auto parts for gaskets, $16.00.
    Thanks again.

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