04-08-08, 12:45 AM
At 50,000 miles, my 2000 STS began to develop a very mild case of rough idle. Every few seconds the engine would shudder or vibrate a tiny amount, this could be seen when looking at the engine and can be felt when sitting inside the car at idle. Now, at 115,000 miles, the idle is somewhat worse, with the engine doing a slightly rougher shudder every second or so at idle. I changed the spark plugs about 1,000 miles ago, the old plugs didn't look too bad and the new plugs didn't help (and didn't worsen) the condition.
I know there was a recall that involved new camshafts and other modifications. Is this the same condition I'm experencing? It's annoying but not a dealbreaker. Could it be something else?
2000+ Northstars have a rubber coupler AKA plenum between the TB and the manifold. They are known to split and cause a vacuum leak which seems to cause symptoms like yours. Spray some Brakleen around there and see if you get an RPM increase. If so, replace the "plenum".
04-13-08, 03:56 PM
A cracked plenum is indeed the problem! Thank you!!!
07-02-08, 12:52 AM
I am having the same problem, only thing is I have a 1998 STS. Do 98 Caddies even have a plenum. If they do not have plenums,what else vacuum wise could be the problem. Does anyone have a picture of what a plenum looks like.Below is a list of what I have done so far. Thanks in advance.
No, the "plenum" is on the 2000+ Northstars. Check all the vacuum lines including the PCV fitting. Don't overlook the intake mounts and do not over torque them.
07-02-08, 01:03 PM
While you're in the front of the intake manifold listening for a leaking PCV fitting, stick the stethoscope further back in there and listen for a vacuum leak around the overpressure blowoff valve in the center end of the manifold.
Intake manifold bolt torque is only 9 ft/lb. A 2000+ "plenum" is a very short bellows arrangement with a flange plate/bolt clamp on the TB end and a hose clamp on the other (manifold) end. Plenty of pictures in here a few months ago.