This sounds like either an ignition related issue or the O2 sensor is bad.
What could be happening, when your engine is running rough, it's misfiring and allowing raw fuel to enter the exhaust which would coat your sensor with fuel and make your computer trip a code.
Or, the sensor fails intermittently and your computer can no longer calibrate that bank of the engine correctly. Then when the sensor kicks back on, the computer gets the information it needs and the idle smooths out.
At idle, while your engine is running rough, perform a cylinder drop test by pulling the plug one cylinder at a time. If the r.p.m.s drop noticeably, then that cylinder was firing correctly. If not, then that cylinder was dead. (An ignition related issue)
If all of your cylinders were firing, then check that O2 sensor for failure.
Finally, this could possibly be a fuel related issue. But usually a fuel related issue would not allow your engine to run better as r.p.m.s increase, but just the opposite.
Jan 20, 2010 at 9:02 AM
