Chrysler Repair: 96 Chry. 3.5L V-6: rough idle, lean mixture, split pipe

Question
I have a '96 Concorde 3.5 that idles very rough, but smoothes out off idle. Compression is between 185 & 204 PSI on all cyl. 2000 miles on new spark plugs & they look to be burning normally. I sprayed carb. cleaner around intake and all vac. lines, no indication of a leak. I have some oil in the bottom of a couple of spark plug holes, but none leaking into cyl. I get 26 MPG on the hwy. I just discovered, in the owners manual that I should be using 89 Octane gas & I've been using 87 Octane, could that cause the propblem?

Answer
Hi Greg,
I doubt the octane number would affect idle. It is mainly related to misfire (ping) under higher levels of engine load not idle conditions. I would suggest that you take a look at the egr valve that is located below the throttle body. It is mounted horizontally in a pipe that comes from the right exhaust manifold and passes gases to both intake manifolds by a split pipe. Notice the flange area between the round vacuum operated top of the valve. Inside the flange you will find a rod which is the valve stem. It has a circumferential slot. Use the tip of a screwdriver inserted in the slot to move it back and forth, against spring action in one direction which attempts to close it. Make sure that the spring will close it to a dead stop in the closed position as otherwise it will leak when it should be closed, such as at idle and when accelerating. That causes a too lean mixture and thus a rough idle. Spray some WD-40 on the valve stem where it enters the valve body and move it some more to be sure it will close tightly on its own. That is one of the more common reasons for rough idle without any fault codes. You also could try to get a fault code readout for free at an Autozone parts store or for around $40 at an independent shop. Let me know the code numbers and we can compare that to what you were told they were and what to do about it and the cost.
Roland