: Running lean all of a sudden

03-19-10, 06:54 PM
My 02 deville just started fumbling at idle and downshifting weird. The only two current codes I got were PCM 0171 and PCM 0174, which are left and right bank lean codes. Those codes don't tell me what component I should replace.

03-19-10, 07:57 PM
Check the plenum for a rip. Rather common failure. Do a search for plenum. It's been discussed many, many times.

03-19-10, 10:15 PM
Thanks Ranger,
I saw all of those posts after I posted. Is the plenum the same thing as the intake manifold? I looked on EBAy and could barely find any that were listed as the "plenum", but quite a few that were listed as the intake manifold. Are they one in the same, or a completely different part completely. I noticed the intake looks a little different than from my 99.

03-20-10, 11:49 AM
Your intake manifold is one piece - there was a change at 2000 with the new engine design.

The "plenum" is the flex rubber connector between the throttlebody casting and the manifold.

You squirt a small shot of choke cleaner down in the area under the plenum - with the engine running - to check for a vacuum leak. Any idle speed change and the plenum is cracked. Not uncommon. Any loose intake duct connection (vacuum leak) at or after the MAF will also set mixture and O2 sensor codes.

A plenum is a dealer only item - $40 or so.

Click to enlarge. The #2 in the intake manifold diagram is the plenum clamp, which comes on the new unit.

03-20-10, 12:34 PM
Yeah, generally plenum means the intake. Why GM chose to call this rubber coupler a plenum is beyond me. It just confuses everyone.

03-21-10, 10:29 AM
Thanks gentlemen, worked like a charm, although the guy at the parts dept. at my local caddy dealership didn't know what I was talking about, luckily a manager happened to overhear, and he knew exactly what it was. It seems to have worked, although upon examining the old one, I couldn't see where the rip had occurred, but oh well, problem seems to be rectified.

03-21-10, 02:31 PM
.................$230 ???.............or did you DIY for $45 +/- ??

The bottom of the "bulge" is the culprit. My guess is that oil vapor and PCV acids pool and literally rot the rubber over time.

03-21-10, 04:40 PM
That has always been my suspicion as well Sub. I can't come up with any other explanation.