Hi, my 2004 xj8 n/a recently developed a rough idle. It would start shortly after driving. It is very pronounced at a stop, but it also gets bad when driving up an incline. What's really got me stumped is that the CEL came on this morning, and the idle smoothed out. The codes were random cylinder misfires and something about excessive misfires affecting emissions. Why would it smooth out when the CEL comes on? I'm getting no restricted performance warning. Thank you!

 

When you first start the car, before the engine warms up the ECM is running in "open loop" which means it is not yet referencing some of the sensor inputs to manage engine fueling.
Instead the ECM is just referring to base fuel mapping tables to run in a general rich state until the engine starts to warm.
Once the engine has reached a predetermined running time and/or temperature the "loop closes" and the ECM is then adjusting fueling according to sensors such as O2 etc.
This may explain why you are getting a cold engine behaving normally but warm/hot engine not so much.

You could have an air leak into the intake manifold from a broken plastic pipe etc., a bad O2 sensor or an issue with a Cat.
Post the codes you are seeing so the "collective brain" can see if there are any further clues rather than just your general description.

 

Currently it is a cylinder 3 misfire p0303 and misfire causing excessive emissions p1316. A scan the other day also showed cylinders one and five misfiring.

 

When I got home, I unplugged the MAF sensor and restarted the car. In restricted performance, but idling fine. Plugged it back in, problem is back. I tried cleaning it s couple days ago, with no luck. Could it need replacing?

 

Don't mess with the MAF.

Either swap the coil for another to see if the fault moves or just replace it.

Don't leave it - damage is being done to the cat as the codes have told you.

 

If it was consistently one cylinder misfiring,I would have done that, but it's been random. I've been leaning towards air or fuel as the issue.

 

Electrical power / blocked cat.

MAF would affect all cyls & usually go rich.

 

You said it gets bad when driving up an incline... Maybe an ignition coil is starting to go bad. It will throw a cylinder misfire code soon and the car will shake like a mad man. What are the codes you get now?

 

I've had the same issue many times and it's been a bad coil pack every time. I bought two extra and when one would go bad, I'd make the swamp. If it giving you a specific cylinder misfire I'd lean that way(back yard mechanic analysis).. The MAF effects the entire engine and they do go bad also..

 

I keep two spare coil packs in my trunk at all times with tool box. When a coil goes, you can't and shouldn't drive the car. Also, lots of times there are signs and codes that get thrown, when a coil is about to go south, that could easily make a person go down different rabbit holes. <grin>