94 STS Rough Idle

Recently purchased a very nice 94 STS. Has approx 155k. I was very impressed with the interior/exterior condition or I probably wouldn't have purchased one with as many miles. Anyway...The car seems to idle rough when sitting at a stop sign/light when it's in gear. I don't notice it near as much when in park. It doesn't seem to hesitate under acceleration and runs/drives great other than at idle. I replaced the plugs with the Delco OEM style plugs and am still having the issue. No check engine light is on. Any ideas???

Unless you have a maintenance history that would indicate otherwise, the next logical step, at 155K, would be to replace the plug wires. You can try to mist the wires with water in darkness looking for arcing but that will not reveal everything.

I'd also give the throttle body a good cleaning.

The other thing that I would look at is the coil packs near the fire wall. Like with some of the v-6 that use the same DIS units and mounting, if one of the coil packs is loose it can mess with your idle as well( remeber this is still a waste spark system like what they used for 3.1, 3.4, 3.8 and the 4.6L) I have seen MANY idle and rough running issues overlooked by seasoned mechanics that just over look this simple fix.