: 2000+ Northstar Plenum Replacement steps

06-04-09, 02:47 PM
First I want to say Thanks for all the help and tips from this Great Board !!!!

This is how I did this procedure...

1) Removed gas cap

2) Pop hood remove Top cover and bleed off pressure from fuel rail pressure relief valve

3) Take off top of air box and air tube closest to tb, unplug maf plug

4) Just loosen the 10mm nut that holds fuel lines down and undo lines from the black clip on top

5) Unclamp the top line on the coolant jug (a little might come out) Lay it to the side

6) In front just unhook one vacume line and unplug the plug going into coil pack

7) In back just unhook the vacume lines that are in the way (I think it was 4 or 5 of them) and unplug the plug going to that coil pack

8) Take the 4 10mm bolts out that hold the fuel rails down

9) Carefully pull up on the fuel rails evenly, they will all pop out together, pull them straight up together you'll have to wigle it around a little but you can get enuff room to work

10) Take the 10 10mm bolts out of the manifold

11) Loosen the plenum bolt (The strap clamp) there are also 3 10mm bolts that hols the plenum in. You'll see them from driver side, pretty easy to get to

12) Hold the fuel rails up out of the way and start from passanger side and lift the manifold up and out

13) Turn manifold over replace all 8 Blue gaskets (most local parts stores have these $15-$20 for a set of 8 )

14) Take off old plenum ( I used a big screwdriver and pryed up around the sides)

15) Put your $30 new plenum on (the same way the old one came off)

16) I took my shop vac and vacumed out everything so nothing would fall in engine, then just wiped it down with old shirt rag

17) Hold fuel rails back up and drop the manifold back into its home

18) Reasemble everything you just did. On the manifold bolts just start in the middle and work your way around, don't tighten till there all hand tight.

Thats pretty much it. Of course inspect everything as you go.

When i did this job all 10 of my manifold bolts were loose like all of them were just hand tight. I thought that was crazy..

The whole job took me about an hour and a half. because i have never done this before. Stealership probably can do it under an hour. Started it up and it runs perfect. :highfive:

Hope this helps Thanks for Everything

06-04-09, 04:07 PM
Good post - the factory torque on those manifold bolts is light: 89 in/lb., or 5.5 ft/lb.

06-05-09, 01:16 PM
Just curious - can't you leave the injectors and fuel rail attached to the manifold and R and R it as an assembly? You can on a '97 - maybe 2000+ is different?

06-20-09, 01:31 PM
guys, whats the part number for the plenum?

06-22-09, 10:20 AM
Plenum: 12555840

06-25-09, 10:53 PM
I'm halfway done my job at the moment - taking my time, and being careful not to break any plastic..:histeric:

One thing I noticed - my fuel lines are stainless steel, and they're held on by a small bracket just to the right and down off the TB. The rails would not come up out of the way until I managed to get the rail bracket loose, which of course involved removing a clp around a big ol' cable next to the fuel lines, then moving the cable out of the way so I could get a deep-dish socket (13mm?) on the nut holding the rails. Once the rails were loose, I could move them up and out of the way from the other side of the car, and then lifted the manifold out completely.

The plenum was indeed cracked.

Have attached another pic of the fuel rails that I had to jimmy out of the way. Anyone with a newer NS will probably have these rails to work with.

Tomorrow after work I'll be picking up the intake manifold gasket set ( I forgot to order one when I ordered the plenum. Did remember the TB gasket though, which oddly enough cost more than the plenum did.) I'll be putting things back together and hope to finish tomorrow night or Saturday morning if the weather holds.

As others have noted, this is not a difficult job. Getting the steel fuel lines loose took me longer than I thought, because I tried to think my way through removing the manifold without doing it - but there is no way to move the front and rear fuel lines if that bracket isn't loose. And I didn't want to kink any thing...

09-05-09, 05:05 PM
2000 Seville SLS Plenum boot & intake seal replacement.
Getting 0171 & 0174 lean codes and 0101 MAF code.

Stealership wanted $560 & said it was a 4 hr job. Took me 2-1/2 going slow & $73 for parts (8) 12557497 & (1) 12555840

I have to thank all that have contributed to this thread & the regulars that help us weekend warriors all the time. I followed MRVSPITBULLS directions almost to a tee & had zero problems with this repair. And I'm a carpenter with just enough tools to get me in trouble. Working in a stone driveway & not a garage it took me total of 2 1/2 hrs. That was going slow so not to screw up this marvelous piece of engineering called the NORTHSTAR.

The only changes or deviations I made were the following:

1. I blew off the whole area with the air hose prior to any disassembly, hoping to avoid dirt getting where it shouldn't.
2. Move step #5 up to step #2.5. I fought to remove airbox & tube (I'm a carpenter) w/o removing Radiator overflow.
3. Per Jim Hare - loosening the bolt that holds the metal fuel lines near TB helps give you more movement in fuel rails.
4. I found an extra plenum bolt sleeve precariously sitting right next to an intake port when I removed the plenum. I checked to see if I had dropped it. But, I had 10 bolts & 10 sleeves in my hold cup. Sleeve must have come from the factory. Had it fallen into the port while I removed the plenum - I would have never known it & probably would have blown the head apart.
5. Per SUBMARINER409 - oil on injector "O" rings helped reassembly.
6. Not trying to make me seem like a dumby, but was I surprised to find a starter buried under all of this mess!!!!! Should I have replaced it while I was in there? I didn't. Car has 83K on it & never (touch wood) had a starter issue.
7. A good cleaning of all mating surfaces & vacuuming out of all ports before reassembly.

That's about it - pretty uneventful procedure - thank God. Runs smooth & strong again.
And thanks again for all the help this forum provides.

05-13-10, 08:30 AM
Here's another heads-up: the "plenum"-to-throttlebody mounting plate has 3 nuts which are welded to the plate. You must remove the 3 long bolts which go through the throttlebody casting in order to remove the entire plenum - a while back someone in CF tried to loosen the nuts and had a hell of a time.

05-14-10, 04:06 PM
Fantastic write up... Ended up having to replace mine last night and did so easily after a single glance of your breakdown. The whole process took about an hour - including all 8 upper intake seals and a vacuum line (preventative measure) and would have been done even quicker if I weren't so pedantic about cleaning all the various bits. ;-)

Two small notes from my own experience... First, I found no need to disconnect the coolant line from the tank - it just wasn't necessary. Second, with the three plenum bolts pulled back a ways through the TB, I found the manifold was much easier to lift out from the driver's side rather than the passengers'.

Nonetheless... This was most helpful :thumbsup:

02-28-11, 01:01 PM
Some manifold and plenum pictures for 2000+ reference. Click my username, open the profile. 2 albums, 6 pages of 2000+ Northstar parts - many of the entire intake system.

02-28-11, 02:43 PM
Here's my take on the plenum gasket replacement. Keep in mind I don't know all the technical names for certain items, but I've tried to supply enough pics to make this easy for the common car guy doing his own repairs....my thanks to mrvspitbulls for his tutorial, which I used and modified a little for this tutorial:

1)Remove the gas cap.

2)Remove top cover and bleed off pressure from fuel rail pressure relief valve.

If needed, this is the time to spray and clean the top of the engine to keep debris from falling inside the block/intake/injector holes.

3)Loosen clamp on overflow tubing and remove hose. Loosen nuts on clamp studs, pull tubing up and fold tube/hose to front of engine.



4)Unplug MAF sensor plug. Remove clamp at the throttle body (TB) throat. ‘Snap’ out the air box and air tube assembly, and lay aside.





5) You should have unobstructed view of the TB.

More to come....


03-13-11, 02:07 PM

6) Remove front valve cover vent hose and rear PCV valve and hose; unplug the coil packs (beware the safety clips—I broke mine).




Remove the four 10mm studs holding the fuel rails to the intake manifold (the same ones also hold the overflow tube in the front)


More to come....(this thread limits me to 15 pics per post)....

03-20-11, 12:29 PM
7) OPTIONAL STEPS: I chose to remove the TB to clean it, because mine was filthy…you may not have to do this, and can skip to Step 8. But if you choose to do so, it is a good time to do it while everything is disconnected. I unplugged the orange doohickey….


…the throttle cable….

…the vacuum line….


…the top hose…

….and the electric wire cable from the TB body and bracket, to get extra work room and to avoid breaking anything important.


I removed the 3 bolts holding the TB to the engine…



You can see the welded nuts on the plenum gasket here which do not move.

I removed the long bolts, and pulled the TB away from the block.

You can clean or replace the rubber TB o-ring at this point (I just cleaned mine)

I cleaned the TB inside and out with carb spray and a rag, as well as the block mounting area.

More to come....

03-21-11, 11:13 AM
8) Loosen the 13mm bolted clamp that holds fuel lines down and undo lines from the black clip on top. I accidentally broke the plastic clip (no big deal), and removed the bolt holding the fuel lines secure.


This was the hardest part of the job for me, because that bolt was directly under the taut wiring harness, and it was a pain to get the wrench in that space. It has to be loosened, though, or you won’t be able to raise the fuel rails high enough to lift the intake free.




9) Be sure to unplug the vacuum line from the rear of the intake; I forgot to, and broke the nipple off…after a little repair work, I fixed it, but it would have been avoidable had I thought to unplug the hose.



10) Carefully pull straight up on the fuel rails evenly, and they will all pop out together; you may have to wiggle it around a little, but you can get enough slack in the rails to work the intake out from under them.



11) Remove the ten 10mm bolts from the manifold.

You should now have loose fuel rails, and an unbolted plenum gasket and intake manifold.

These are the bolts you should have (right-to-left): 1 fuel rail clamp bolt, 3 TB bolts, 10 intake bolts, and 4 fuel rail clamp bolts.

12) Hold the fuel rails up out of the way, and starting from the passenger side, break the intake seal bonds and lift the manifold up and out from the front of the engine.

13) Now is the time to check the starter connections and that doohickey plug; also, cleaning the intake valley (with a shop vac) and the intake port seal surfaces (with a rag) is a good idea.

More to come....

03-25-11, 01:45 PM
14) Turn intake manifold over, remove the 8 old seals, clean the mounting surface, and press-in all the new blue rubber gaskets; be sure to align the small tabs on the seals in their appropriate intake manifold slots.




15) Remove the old plenum by loosening the clamp, then use a screwdriver to pry the old plenum gasket off of the intake…it can be really stuck tight (use carb spray).


Clean the intake throat, and inside the chamber now (if needed).

16) Here’s the source of the P-codes and crappy performance…a huge, 3” tear in the bottom of the old plenum gasket (which I understand is quite common in these engines), sucking in extra air.



17) Install your new plenum to the intake (it goes on the same way the old one came off); leave the clamp loose for adjustment to the TB/block during re-installation.




18) Hold fuel rails up again, and push the manifold back into its home; install the manifold bolts by starting in the middle, and working your way around and outwards. Don't tighten them snug until they are all hand-tight.


19) The rest of the re-assembly is the opposite of removal….just be sure to reconnect everything you’ve taken loose or removed, check and double-check all your bolts and connections.

20) Clear the PCM codes and start the engine. Your idle should be back to normal (750-800 rpm), and there should be no surging or skipping. Congratulations….you’ve just repaired the faulty plenum gasket on your Northstar!

Here’s a shot of the parts I used for this repair: The plenum gasket and the Felpro intake seals.



And there there you have it, my shade-tree tutorial with photos....I hope this helps others with the same issues! :yup:


04-02-11, 07:32 AM
Before you start this procedure you NEED to study this entire thread !!! Both pages............from the beginning.

Some of the fittings and vacuum lines - EVAP and PCV are two - are different from 2000 to 2001 to 2002. Close, but different. Back about 20 pictures the orange plug "doohickey" is the MAP sensor - it sends an inverse vacuum signal (negative pressure) to the PCM. This picture series has a rattle-type PCV valve. Later engines do not - the dirty air pipe slips over a permanent fitting with a fixed orifice inside. Remember that the 3 long bolts through the throttlebody are what secures the plenum metal flange - they must be removed before the manifold will come off the engine. The manifold bolts (10) get torqued to 89 in/lb - only about 7.5 ft/lb, so be careful. Break a manifold bolt boss and you'll be looking for another manifold. Smear a bit of engine oil around each injector O-ring before you drop the fuel rail back in. It makes life a lot easier. Double check all the injector electrical connectors.

12-11-12, 02:49 AM
Is the tear a common problem for newer than 2002 northstar engines? I have a terrible idle, (shakes the car) and misfires. Noticed that the intake bolts were loose the other day, one was almost all the way backed out.

02-21-13, 09:19 PM
wow... thanks for all the time and knowledge...just bought my caddy and thinking I will have to do this....the pics were awesome... I'm also a shadetree mech... but kind of got lost about all those connections and hoses...
I just want to replace my plenum gasket... I have cleaned the TB, mass flow, and map sensors... also installed a new cam sensor... the caddy idles a little rough but not bad... I did replace the plugs.. it had bosch in there.. and that made a difference... I have to install a fuel pump and fuel sensor when weather permits...just got the pump today...anyway just wanted to say thanks for the upload... I might be taking pics of replacing fuel pump and filter....this seems to be a common problem also...
thanks again,

03-21-13, 11:04 PM
I did this repair last weekend after procrastinating for ages. Many thanks to those who contributed to this thread, as it was a huge help to have the description and images throughout the process. A couple of notes from my experience (2002 DTS, built in January 2002)

1. I found it easier to get the fuel rail up and out of the way if I first removed all of the injector cabling. Pull up on the slider and pinch to release the connector.
2. I had a few extra vacuum lines running around, and a handful of differences from the photos posted earlier in the thread. If you've got an '02 DTS, watch out for these slight differences, but don't let them throw you off.
3. I suggest looking for hard vacuum lines and removing them early on, especially if it's cold out. I was working in fairly cold weather which apparently causes the structural integrity of the vacuum lines to resemble that of a potato chip. I snapped two of them without even realizing it. Fortunately a trip to the local Autozone yielded some flexible vacuum line that fit the same connectors. Don't bother trying to fix the old stuff. I tried super glue and welding but had no luck.
4. If you are the type of person who hates re-installing a filthy part, plan some extra time for cleaning the manifold, throttle body and runners. I didn't have time for this, and it's going to bug me FOREVER, knowing how much black crud is caked in there.


04-23-13, 07:20 PM
Well....my 02 DeVille is now in need of a plenum gasket replacement. The car has been acting up and not idling it's normal rock solid smoothness. I also had noticed that the trans was clunking when I went from drive to park, something it never did in the past. I guess that was due to the higher than normal idle speed. The car was idling so poorly today after driving home I popped the hood while the engine was running and clearly heard the vacuum leak around the plenum.
I've been putting off replacing the water pump, so I guess that will be on the list while I'm working under the hood. The serpentine pulley's are making some noise too, and I'm really not looking forward to that job due to the tight space.

Any additional tips not already covered will be appreciated. such as any parts needed that may not be obvious.


05-31-13, 05:26 PM
Great tutorial! Can the split in the boot cause a higher than normal idle speed without setting any codes?

06-01-13, 10:39 AM
Maybe ................ as can happen with any other small vacuum leak - begin by checking all the vacuum lines to/from the intake manifold and throttlebody. (Kicker is: The plenum is usually not a small vacuum leak)

Vacuum diagram for your engine (If not exact, very close) in my albums - Community tab, Albums, the red Seville. 2 albums, 6 pages, 113 pictures/diagrams.

09-17-13, 03:16 PM
How do I findout if my Plenum is bad and does need replaced? 59k on 03. The car was serviced (oil changes and all) at certified Cadillac Dealer. Car came with 9 pages of updates... so they did everything they could. I don't hear any sucking noise, but I might not know what I am looking for.... THanks

09-21-13, 11:11 AM
A ripped plenum duct will set a P0171 & P0174 code.

10-13-13, 08:54 PM
Amazing thread... I did this on my own car in my drive way.. Thanks Ghostsoldier.... those pics were perfect...

10-13-13, 09:17 PM
No problem, Monty...I'm glad to be able to give back here on this amazing forum. :)

10-18-13, 07:36 PM
Did this swap tonight, car now runs and idles great!

On my 2003 DHS, the fuel rails were held in place by a bolt sitting sideways, on the front fuel rails between the throttle body and front radiator shroud (10mm nut.... the bolt is a fixed part of a bracket), you just pop the nut off and wiggle the fixed bolt loose. Very easy to access.

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.

Kelvin Averbuch
12-18-13, 03:40 PM
What a great article! My 2003 Seville had high idle, felt like cruise control was on. 1. Removed and cleaned IAC valve: no difference. 2. Removed and replaced TPS valve: no difference. 3. Replaced plenum connector (with 2" tear at the bottom) per this article, and replaced torn vacuum connector at other end of the intake. Now the idle is perfect and gas mileage around town increased from 16.5 mpg to about 22 mpg. Thank you very much.

03-15-14, 09:01 PM
I want to thank ghostsoldier and others. I compelted the job on my 04 deville today. I used the Dorman part from rockauto 911010 (911-010) for $15.40. It came with the blue gaskets and they did seat fine in the channels for install. On an 04 deville the fuel rail clamp is a #10 nut and not under a wire harness. Also on the passenger end of the manifold the hose that goes to the pcv(?) valve was disintegrated at the 90 degree elbow so i needed to replace that too.

Closet Boy
03-23-14, 07:50 AM
Excellent thread. Within a few days, I'm about to take on this challenge. I too bought the Dorman from RockAuto.com.

Closet Boy
03-27-14, 06:22 PM
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.


03-30-14, 02:21 AM
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.

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.

04-05-14, 05:46 PM
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!!

05-17-14, 09:54 PM
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?

05-18-14, 12:32 AM
Try cleaning the TB and make sure the throttle cable isn't binding.

10-09-14, 06:12 PM
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:

Dorman Intake Plenum 911010 ("Intake Coupling") (http://www.rockauto.com/catalog/moreinfo.php?pk=2785042)
Fel-Pro Intake Manifold Gaskets 93313 (http://www.rockauto.com/catalog/moreinfo.php?pk=447533)
Fel-Pro Throttle Body Gasket 61125 (http://www.rockauto.com/catalog/moreinfo.php?pk=446912)
ACDelco Air Filter A1208C (http://www.rockauto.com/catalog/moreinfo.php?pk=91421)
Bosch Fuel Injector O-Rings and Pintle Caps (http://www.cadillacforums.com/forums/cadillac-tech-tips-how-fix/592737-fuel-injector-pintle-caps.html) (click for thread - O-Rings are easy to find, caps aren't)

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.