I've been noticing a very high pitched whistling noise coming from under the hood of my 98 Eldorado (N* with 75K) for the past few months. I'm not to sure how long ago this started but its very noticable when its cold outside and the engine is cold. At first I was kinda thinking it was just a normal whistling noise but a few weeks ago I popped the hood just after start-up to see if I could pinpoint the location of the noise. It was very evident this was not a normal hissing or light whistle. This noise was very pronounced and loud with the hood opened. It seemed as though it was coming from the passenger side of the engine compartment.

Fast forward to just the other day. I bring the Caddy to a local shop that does my oil change (cant go wrong for $25). I was getting a low AC message in the display so I had them recharge and check the HVAC system. While they were working on the car I asked them to check the origin of the whistling. The tech could clearly hear the whistle I was refering to and quickly found the culprit. They referenced the culprit as a crack in the "intake plenum". I had the tech describe the cracked piece a bit further as he just walked me out to my car and showed me to clear up any miscommunications. The crack is very small and is located on the rear (toward the firewall) passenger side of the black plastic intake piece directly under the cosmetic engine cover. He kept refering to this piece as the intake plenum. According to this post I searched for http://www.cadillacforums.com/forums/cadillac-deville-1985-2005-including-1985/129135-weird-squealing-noise.html this is not the plenum but the actual intake manifold. Irregardless, the piece has a small crack on the passenger side rear portion which is causing the whistling noise. I have not received any CEL's to date but have had a little issue evey now and then with hard starting when the engine is warm and the air is humid/dense. Seems like the hard starting when the engine is warm and the un-metered air entering the system in addition to the loss of vacuum would be linked. Also, since diagnosing the problem, I've been keeping a close eye on my MPG and have noticed very poor numbers! The last week I put on close to 300 miles, 90% of which were all easy highway driving and have returned an average of 20MPG. This is far below my usual 25 mpg I've seen with extended highway driving!! Again, this seems to indicates the cracked intake plenum/manifold! As an added kicker, the manifold or plenum, whatever its called, has some hand written number on the top of it indicating its a junkyard replacement piece. This might not be the first time its been replaced!! Maybe it was over-torqued originally??

My question is this; what the hell should I do? Can I repair the originally piece with some sort of plastic weld product (JB weld, etc..), the crack is very small! OR, should I all out replace the damned thing (probably going this route). A brand new one from the dealer is $400, a replacement from the junkyard is $200 :cookoo: Is there a better/cheaper source to replace this part?

BTW, this all came about because my radiator failed on my way home the other day. I'm replacing the radiator now and just want to do it all at the same time. With the plenum/manifold off, I'm putting some new plugs/wires on as well. Seems a bit easier to get to the back plugs with the plenum/manifold removed!!

Your research is correct.....the manifold was plenum years ago and plenum is driver's side rubber connector now.

At some point the manifold crack will begin to expand and the leak will begin to set a fuel distribution code, so you should fix it. Your mileage may be down because fuel trim is now trying to compensate for a slightly lean condition by changing injector pulse timing, but not enough to set codes.

I, personally, would feel antsy about trying to epoxy the crack. Try the junkyards for a while and cross your fingers. Heck, try eBay or search for the part on a Google used parts entry.

I think there's a new style of intake gasket set which supercedes your old style. Dealer, and not expensive.

itll get worse in time
you can seal it to prevent any further cracking of it but it will only be a temp fix
replacement is best before you start seeing the CEL and the car will run like an untuned 4 cyl

....My question is this; what the hell should I do? Can I repair the originally piece with some sort of plastic weld product (JB weld, etc..), the crack is very small! I would be tempted to at least try a JB Weld product.

Preparation would be important. I would drill a stop hole in the material at each end of the crack. Then cut a groove or trough from hole to hole the length of the crack to provide increased surface area for the JB product to adhere to.

Double or quadruple the curing time.

Thanks for all the help and insight guys. For now I'm seeming to have a hard time locating the crack. As the tech showed me the Manifold/plenum the other day, I was content with just seeing the plenum as I haven't heard of a manifold being refered this way before. The shop I have do all my more major work can seem to relocate the leak and they cleaned the piss outta the plenum/manifold trying to find the crack so now the damned thing looks brand new and I cant see the small crack! It just so happens we cant duplicate the whistling sound either cause the garage is warm and I'm not gonna leave the caddy outside to cool off (tough neighborhood)!! I thought I saw the crack the tech was refering to when the he pointed at it but this was a few weeks ago and I had a million things going on in my head and wasn't paying much attention (idiot). I stopped by my shop tonight after the gym as my buddy welcomed me to show him this crack! I spend the better part of half an hour poking around trying to find the crack, "WTF" I told him, the damned thing looks brand new and I cant see any cracks!! Typical, now we're gonna have all the plug wires off to replace them, so now is the time to do the manifold/plenum!! I'm thinking of having the plenum/manifold pulled off anyway and taking a closer look! The shop that diagnosed the problem is pretty good and I think would never try and rip me off! If they are, what the hell could this whistling be. The hard starting and poor MPG could be attributed to the spark plugs. We pulled them out and they looked like crap. The gap is double what it should be. So much for getting 100K outta the originals! Mine had 75K and by the looks of them should been replaced some time ago. Or, maybe the shop is right about the leak and the lean condition (for several months) is causing the plugs to prematurely foul out!! So many variables. I'm gonna talk to the original shop tomorrow and see if the tech can give me a better idea of where the crack was. I feel like such a idiot, I was like yup, yup, yup when the guy was giving me his $.02 as I was thinking I'm no idiot and can find a F'ing crack on a manifold . This is no big deal right now as the radiator is getting pulled tomorrow so I have a day to ponder!!

I'm with Jim. I'd give JB Weld a shot (nothing to loose), but I would not cut any grooves. I'd be afraid of weakening it. I WOULD drill stop holes and scuff it up real good with a coarse sand paper for good adhesion. I'd bet it would last forever.

I had the whistling noise for a year and it did not effect the performance of the car.
Searching this forum is full of info.
The problem is a engine pressure release valve and located on the intake behind the power steering pump.
It took 1.5 hrs to replace the valve and the intake manifold gasket.
The valve is also called a backfire valve on this forum.
Hope this helps.

Did you read post #1? They found the problem. It wasn't the over pressurization relief valve.
The tech could clearly hear the whistle I was refering to and quickly found the culprit. They referenced the culprit as a crack in the "intake plenum". I had the tech describe the cracked piece a bit further as he just walked me out to my car and showed me to clear up any miscommunications. The crack is very small and is located on the rear (toward the firewall) passenger side of the black plastic intake piece directly under the cosmetic engine cover.

Thanks for all the help and insight guys. For now I'm seeming to have a hard time locating the crack.

I hate it when that happens!! :canttalk: Your doctor might have some alternatives, but I prefer a good length of hose. Vacuum hose. As long as you have the whistle, use a length of vacuum hose to your ear as a stethoscope and move the other end around the area you suspect. It should get louder near the crack. Your neighbors might think you've lost it if you're standing in your driveway with a piece of vacuum hose to your ear, but it does work.

I hate it when that happens!! :canttalk: Your doctor might have some alternatives, but I prefer a good length of hose. Vacuum hose. As long as you have the whistle, use a length of vacuum hose to your ear as a stethoscope and move the other end around the area you suspect. It should get louder near the crack. Your neighbors might think you've lost it if you're standing in your driveway with a piece of vacuum hose to your ear, but it does work.
Neighbors already think I'm nuts! The car is actually already at the shop getting the radiator replaced as well as a tune up. I'm just gonna go back to the original shop and see if I can speak with the tech again. I don't know how I'm missing the damned crack!! The whistling noise is the most prevelent when its below freezing outside and the car is cold. In the garage its too warm to get the full boar sound but I can still hear it, its just a lot quieter. I'm gonna be sure to keep this thread updated. I'll snap a picture of the crack once I find the damned thing!!

Go to your local Walgreen's or Eckerd drug store and buy a $4.95 stethoscope. Great troubleshooting item. Don't go listening to your engine in a big parking lot or someone will call 911 and they'll haul you off in a white jacket.

Follow Subs advice. You will be amazed at what you can hear with a stethoscope.

I am having the exact same problem with my 98. The high piched wistling, lower than normal fuel milage, and hard to start when hot. I will have mine off this weekend and let you know. And I will JB fix it.

Bill

Cracked intakes can be extremely hard to find. The vac hose and stethescope are excellent ideas. Pro's use smoke generators.

Obviously you have to locate it to fix it! Try some starting spray carefully sprayed on the intake while running. The engine will rev when you are close to the crack.

I've got the same problem going on my 99 SLS. Shitty millage and rough idle. Did wires and plugs 25,xxx mi ago and just did the fuel filter. I live in cold MN, so right now she whistles all the time. I didn't notice any cracks in my manifold either. After replacing the manifold gaskets and still getting the whistle :banghead: I took a stethoscope with a tube and i'm positive that it is the release valve behind the power steering like p.vinson said. If you can't find the crack, I would start there with the smoke or stethoscope. I couldn't seem to find this part or the procedure to change it out though. Does anyone know if it only comes with the whole intake manifold? How does it get removed? Please help, I'm spending too much time on such a little problem and it is is driving me :cookoo:

Just wanted to update everyone and share what I learned. It was the backfire valve. The seal around the flapper was bad. I found this by taking a stethoscope and tube to find the origin of the whistle. The backfire valve unscrews counter clockwise. I didn't have to remove the fuel rail or throtle body from the intake manifold because it just needs to be unbolted and slightly lifted. Minus the difficulty I had getting the new valve in (had to remove the O-ring and place that in first) it only took about 45 min. Total cost for the job from begining to end: $45 for new intake manifold gaskets and $32 for the backfire valve. Total time (if gaskets and valve would have been done at the same time): Approx. 1.5 - 2 hours depending on how thurough of a cleaning job is done. Now the rough idle is gone, not sure about the gas millage yet, but sure this will help. :cool2::cloud9:

Checked for the noise today. Could not locate it by hearing or feeling. So I got the starting fluid and sprayed around in different places, and found one place that the engine speed lowered instead of raising, which I thought it would raise. So I FOGGED IT GOOD and killed the engine!!!!! Then I noticed that ALL the bolts that hold the plenum on were FINGER tight or less. I tightened all the bolts, did not stop the noise, did not change the idle speed, BUT when I fogged it again with starting fluid, it did not change the engine speed at all!!!! So I think I found part of it. I have not checked the backfire valve yet but will.

Bill

I have had a very similar problem recently. Hard starting when the engine has ran in the past couple hours. A loud whistling noise too. My performance has still been quite good though, no rough idle or low mileage. I'm probably at the start of the problem though from what I am hearing on here. The local mechanics claimed that the noise was because the intake manifold had warped and wasn't sealing right. They also said it could only be torqued to about 2/3 of the reccomended amount. I am going to examine the backfire valve first though. Has anyone actually heard of the intake warping and causing the seal to leak?

ryan, For your '99 series there have been a couple of posts lately concerning people discovering that their intake manifold (plenum) bolts have loosened over time. Use your FSM to find the recommended torque and sequence and give them a gentle once-over. I believe that even the '99 has a plastic/fiberglass composition manifold, so it should resist warping fairly well, and if slightly warped, should seal with the proper torque.