P0174 Code Driving me crazy

JoeSmooth · #1 (**permalink**) 07-12-07, 06:38 PM

2001 DHS and I keep getting a PCM 0174 code (Fuel Trim System Lean Bank 2). I did a little searching and found this post: http://www.cadillacforums.com/forums...171-174-a.html (Po171 & 174)
Seems like we're having the same problems but what is the Phelum or plenum?

I replaced the spark plugs and MAF and want to replace this and see if it fixes my problem too. Thanks.

-JoeSmooth

carnut · #2 (**permalink**) 07-12-07, 06:50 PM

a plenum is like an intake manifold on an older engine except with fuel injection it only distributes air to cylinders. The history post speaks to a potential crack or warp allowing too much air in, leaning out cylinders. Have your local Cad parts counter man show you the picture of the part and the seals required to replace it.

ewill3rd · #3 (**permalink**) 07-12-07, 09:31 PM

Actually for some reason on the newer N* the "plenum" is a rubber piece between the intake and the throttle body.
It has a tendency to crack at the bottom and cause a vacuum leak and lean exhaust codes.
Usually when this happens you'll get codes for both banks not just one, but the only way to really tell is by testing or looking at the fuel trim data.
The plenum failure is common on these as they get more miles on them.
I have some pictures of the parts at work.
Do you have some brake cleaner or choke spray?
I can tell you an easy way to check.

**Ranger** · #4 (**permalink**) 07-12-07, 10:35 PM

Well I'll be dipped. I always thought that the plenum was the manifold and the rubber piece was a coupler.

JoeSmooth · #5 (**permalink**) 07-13-07, 12:12 AM

Quote:

Originally Posted by ewill3rd

Actually for some reason on the newer N* the "plenum" is a rubber piece between the intake and the throttle body.
It has a tendency to crack at the bottom and cause a vacuum leak and lean exhaust codes.
Usually when this happens you'll get codes for both banks not just one, but the only way to really tell is by testing or looking at the fuel trim data.
The plenum failure is common on these as they get more miles on them.
I have some pictures of the parts at work.
Do you have some brake cleaner or choke spray?
I can tell you an easy way to check.

I have roughly 115K on the car now. If you could post up a picture that would be great.

And yes I do have brake cleaner. At one time I was getting 2 of my banks running lean (I believe I was gettting a P0171 and a P0174 code) and after I replaced the spark plugs I've only been getting that one bank.

-Joe

ewill3rd · #6 (**permalink**) 07-13-07, 08:53 AM

Ranger, you'd think that... on any other car if you look up a "plenum" you get the composite intake piece, but on the N* the catalog shows the plenum is just this piece and they just call the composite piece the intake.

Joe,
Get your brake clean, remove the beauty cover, start the car and spray at the base of the plenum in short bursts. The plenum is the rubber coupler with the big hose clamp right between the intake and the throttle body.
It usually develops a slit at the bottom that allows for air ingestion.
You should be able to immediately tell a difference in the idle speed and you might even hear the suction noise when the liquid gets drawn into the hole in the plenum.

I'll find a picture of one I recently did for another forum member and post it.

Here we go.
One is a close up of the slit in the boot, the other is a picture of pretty much everything you need to do the job.
You need to replace the 8 blue intake seals and the plenum.
The rest is just taking stuff apart and putting it back together.
The car will be very happy if you fix this.

I think they changed to this setup for 2000 up until the new DTS version.

JoeSmooth · #7 (**permalink**) 07-17-07, 08:30 PM

Ok so I used the brake cleaner like you said and the idle got irregular like you said. Now the only thing I need to know is what is the easiest way to replace this part. I got a few pics as well. I think I'm looking at the right thing.

-Joe Smooth

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JoeSmooth · #8 (**permalink**) 07-23-07, 09:33 PM

Ok I got everything off and replaced all the parts and she's purring again. No more codes and idle is back to normal.

Thanks for all the help.

-JoeSmooth

ewill3rd · #9 (**permalink**) 07-24-07, 06:44 AM

Wow, sorry I missed your post earlier.
Glad you got it fixed.
Did you take any pictures of the procedure?

sherfield2002 · #10 (**permalink**) 07-24-07, 08:45 PM

Do either of you have a part number for the plenum and intake gaskets? And about how much did it cost you? Can any flamable spray be used to check for the vacuum leak or is just brake cleaner/choke spray recommended?

**Ranger** · #11 (**permalink**) 07-24-07, 09:36 PM

Brake and choke cleaner ARE flammable. Guess that answers that question.

mtyler7807 · #12 (**permalink**) 07-25-07, 12:03 AM

I have the same problem. If you have pictures could you post them. If you don't have any pictures could you post a step by step on what you did. I hope that you are a shade tree mechanic!!!

ewill3rd · #13 (**permalink**) 07-25-07, 07:00 AM

I wouldn't use "anything flammable", I usually use brake clean or choke spray because they are on hand.
As far as part numbers go, I am not sure they are universal. I always recommend using the last 8 of the VIN and calling a GM parts department for pricing. They will give you part numbers and prices on the phone, then you can "shop around" for the best prices on those part numbers.
They might be the same, but not necessarily from year to year.
I have to get some of these for my Mom's Aurora, I can post the part numbers later but I don't know if they will be the same for your N*.

pugel · #14 (**permalink**) 07-25-07, 12:51 PM

Quote:

Originally Posted by JoeSmooth

Ok I got everything off and replaced all the parts and she's purring again. No more codes and idle is back to normal.

Thanks for all the help.

-JoeSmooth

I have the same problem too. (search forum for p1071) My parts should arrive today from Luke and I'll be putting them in soon. Looks pretty straight forward but nothing better than talking to someone who just did it. Any tips or gotcha's or order dependent steps to watch out for? I just replaced the injectors in my vette so I know what to look out for there.

ewill3rd · #15 (**permalink**) 07-25-07, 01:31 PM

It is really not too hard, it just takes a while.
You just need to follow steps to remove the fuel rail, then undo the 3 bolts that hold the plenum to the throttle body... sometimes that requires removing the throttle body first.
After that you just remove the bolts that hold the intake to the engine and work it out of there.
Replace all 8 seals and the plenum and put it all back together.
Torquing the bolts is good.

You will probably notice an idle improvement right away.
Clear the codes and drive happy.