Rough idle

EYEU81

MEMBER
2003 CADILLAC DEVILLE
V8
FWD
AUTOMATIC
134,000 MILES

Runs rough when warm at idle, engine shakes, almost stalls. Engine light had coded a bunch of codes( general misfire, MAP sensor, etc), but I had cleared and last time it came on I had it checked right after it came on; it showed lean mixture in bank 1 and 4. I have replaced EGR, idle flow sensor, throttle position senor, MAP sensor, repaired two vacuum leaks, run Techron fuel cleaner. Some people say it has PCV valve other say just orifice which I think I found near ignition coils front and back and cleaned them. Have not checked O2 sensors. Has lots of vacuum in line from right side of manifold to back "PCV" orifice little to none on front "PCV" orifice, and little in small lines going to sensors. I have sprayed lines with carburetor cleaner and all are open. Appreciate your help.

1 Reply

FACTORYJACK

MECHANIC
4,159 POSTS

The codes I would assume, were P0171/174 banks 1 and 2 lean. You state a couple vacuum leaks were repaired, where were the leaks? It does not have a PCV valve, it is an orifice in the rear bank cam cover. The PCV line will have vacuum as it is off of the intake, the other line is a breather so it will pull crankcase vapors as opposed to pulling a vacuum on the crankcase. The small vacuum lines are for the air management system. If you are getting lean codes for both banks, it is a central issue, and possibly result of unmetered air leaking after the mass airflow sensor. In most cases, what I have found to cause this, is the plenum boot between the intake and the throttle housing. This boot splits on the lower portion, and is invisible in situ. This can also be intermittent, as the split can change in result to manifold vacuum changes. If you were able to monitor fuel trim data, and it was indicating lean, you could use a propane wand, or spray around the lower portion of that boot and observe the change. I would caution on using flammable liquids, as it can get hot enough in the engine valley to flash them, and that is not safe. The symptom that also can be associated with this, is a higher than normal idle speed, in addition to the codes. Since there are not many vacuum sources on the manifold, this is one of the more common possibilities, and could be worth researching.