[Mr. G]

From what I have read, this is a common problem of the Northstar engine as the plenum is made of plastic and is only heldb down by 6 small bolts. I problem started as a high pitch whistle coming from the passenger side of the engine. Upon inspection I noticed the bolts attaching the plenum to the engine were figure tight but when I attempted to tighten them the hole they go through all began to crack crumble and split. I couldn't even get the bolts anywhere near the torque specs as the plenum was just giving it up with each turn of the bolt. I beleive the whistling is coming from the engine pressure releif valve located behind the power steering pump as I see that other people have had that problem. So I guess I need a pressure releif valve and a new plenum. Does anyone make an aluminum plenum for the caddy? Is there anywhere to get the plenum other than the dealer? Thanks!

Mr. G

[Larry P]

Mr G Im pretty sure I have a plenum for your Northstar. Its a used one from a engine that I swapped out. It is complete with the throttle body on it. I have worked on a ton of Caddys with the Northstar and have only seen a few of the plenums fail. Most of the time it was from the engine being short cranked and it backfired into the plenum.
Let me know if I can help.
Larry

[Mr. G]

That is very interesting as I have had the short start and backfire happen a few times. Is the backfire due to the failed engine pressure releif valve (aka anti backfire valve) ? Is the backfire responsible for the loose plenum bolts? The Caddy forum I looked at had quite a few people discussing the cracked plenum problem and proceedure for repair so I had the impression that it is a common failed part. The one on my car isn't cracked other than where the bolts go through and the more I tighten the bolts, the bigger the splits get. Seems like plastic fatigue, but perhaps the backfire issue is a cause for the weakened state of the plastic plenum. I appreciate your insight!

[RS72LEMANS]

BAH I hate those northstar engines. Ive seen a few cracked plenums one from another shop who did the leaky oil pan gasket and wasn't so gentle lifting the engine. another I never did figure the exact cause Ill have to look into the backfire theory and check next time i run into one. Ive seen alot of the boots torn or ripped at the plenum causing leaks. and my fair share of water pump head gasket failures. Got to give the engines credit they are smooth when they run right and any engine that can take the abuse of being ran 4-5qts low on oil between oil changes and survive cant be all that bad.

[saltman 1965]

Mr.G my 99 Deville Does the same backfire thing occasionally now I know what to look forward to. I like the car it has 176k on it.So I cant complain. But I really think Cadillac must be the French word for leak. Have repaired refgerant, p/s, atf,and I just live with the oil leaks.

[Mr. G]

Funny, I had my engine out for the leaking case halfs, and the whistling started soon there after. I have had this car for 9 years and have put less than 20k on it and there is always something going wrong with it. I don't drive it much as I am afraid of it breaking.

[68gtoMN]

Beware of time serts. Uhoh...

[Mr. G]

??? I must be missing something here.

[67rstbkt]

I had an '01 Deville that developed the plenum leak at around 100,000 miles. I never had any backfiring or anything like that. Mine was cracked on the bottom, where you could not see it until it was removed. I bought a new rubber plenum from the dealer, along with new intake gaskets and went at it. Just taking my time in my garage, it was about a 1-1.5 hour job. Reading through the Cadillac forums, it did seem to me like it's a common problem. My local dealer had one in stock and IIRC, it was less than $100 for the part.

[Mr. G]

68rstbkt, was that at Scott Motors on Virginia street that you found the plenum? Also, is the plenum actually rubber or are you refering to the boot? I too am in Reno so this is most helpful as I saw the plenum available on the internet for $21.00 but there was no way to order them.

[67rstbkt]

Originally Posted by Mr. G

68rstbkt, was that at Scott Motors on Virginia street that you found the plenum? Also, is the plenum actually rubber or are you refering to the boot? I too am in Reno so this is most helpful as I saw the plenum available on the internet for $21.00 but there was no way to order them.

Wow, small world. Yes, Scott motors was where I picked it up. The plenum on my '01 was all rubber and it connects the intake manifold to the throtthle body. It's all one piece. I think the 98 is the same, but I could be wrong. Do you have the plastic intake? Do you have any of the other symptoms of a cracked plenum? Reduced mileage, check engine light, slight stumble?

[Mr. G]

Yes, it is the plastic intake. The car runs OK, but has had this high pitch whistle that you may have read about on the caddy forums and is associated with a failed engine pressure relief valve located under the plenum behind the power steering pump I recently learned. It was this whistle that I was trying to figure out when I noticed the plenum bolts were quite loose. When I attempted to tighten them the hole bosses in the plenum began to crumble and split from the top down where the head of the bolt presses on the plenum. The more I tightened it the more it split. The car still runs but I can't get any torque pressure on the bolts . I figure the plenum has to come off the replace the engine pressure relief valve so I may as well do the plenum while I am at it. Did you need to use a fuel line disconnect tool as described on the caddy forum? Thanks.

[67rstbkt]

Originally Posted by Mr. G

Yes, it is the plastic intake. The car runs OK, but has had this high pitch whistle that you may have read about on the caddy forums and is associated with a failed engine pressure relief valve located under the plenum behind the power steering pump I recently learned. It was this whistle that I was trying to figure out when I noticed the plenum bolts were quite loose. When I attempted to tighten them the hole bosses in the plenum began to crumble and split from the top down where the head of the bolt presses on the plenum. The more I tightened it the more it split. The car still runs but I can't get any torque pressure on the bolts . I figure the plenum has to come off the replace the engine pressure relief valve so I may as well do the plenum while I am at it. Did you need to use a fuel line disconnect tool as described on the caddy forum? Thanks.

Yea, you might as well replace the plenum if you're taking it apart anyway. I did not have to use any specialty tools. I remember removing the intake manifold to do the job. but can not recall how I disconnected the line. I know that it was not that difficult.

[Mr. G]

Great! Thanks for your help. I will make a list and head off to Scott Motors.

[67rstbkt]

Originally Posted by Mr. G

Great! Thanks for your help. I will make a list and head off to Scott Motors.

You're welcome. Let me know if you have any other questions.