XJ 4,0L not running smootly on idle

Not running smootly when idle, but seems OK when trottling on. Would that be the downstream oxygen sensor at Catalysator?

CPS not calculating correctly, right?
(also have a "Check engine light" but getting grazy of this ODBII switching ignition off/on gymnastic..never get it right)


XJ97, 4,0L, AX15/NP231/Chrysler 8,25...and I'm happy

Well first get the codes read, The downstrem o2 sensor only montors calaitic efficency, sounds like it could be more of an IAC, also take off the throttle body and clean it, that smothed out my idle some but you should probly start with the codes.

my obdII did like this, i plugged it into the plug under the dash, turned the keys forward and it reads the code then when i was done with the obdII i just either unplugged it and shut the keys off or vise versa

This belongs in the OEM forum, not here.

I had that problem recently. Turned out to be the head gasket blown between #4 and #5 cylinders. No other signs of a bad head gasket except rough idle. Compression test confirmed that 4 and 5 were leaking into each other through the head gasket.

IAC is a likely suspect.

I finally figured out what MaxxXJ was talking about. IF you have a check engine light, it could be the OBD-II computer turning off a fuel injector thinking that the cylinder is not firing properly. When the computer (OBD-II only) detects a miss fire too many times on a cylinder (by timing the arrival of the CPS signal cylinder to cylinder), it shuts of the fuel to that cylinder to protect the cat converter from too much fuel, which can damage the converter. This results in a rough idle!

really dumb question... but how are your plugs, wires, and distributor cap/rotor? My engine idled much much rougher before I replaced them, runs great now. I didn't have a CEL though so your problem may not be this... but it's a cheap thing to try and you might need it anyways.

Why don't you take it down to the local parts house (AA, AZ, OR.....) for free scan?