Whenever I start it up, it idles just fine at 1K. But after the engine (4.0) warms up and I park/sit in neutral, (AX15) the idle gets unstable and starts fluctuating anywhere from 600-800. Causing a fairly shaky/rough idle. This only happens once the engine is warmed up.

 

o2 sensor

 

600-800rpm sounds about normal to me.

 

Shaky and rough ain't normal!

 

No it's not. But it's also not related to a low idle.

 

Where's that located?

 

Anywhere below 1k its shaky. 1k-1.5k, not as bad. 1.5k+ is normal. However, it only does this when its warmed up. From a cold start I have no issues whatsoever.

 

Just as a reference point: mine idles (warm) at ~700...

The o2 sensor, if it is that, is probably the pre cat sensor. Bank 1, sensor 2 essentially. Does it ever backfire at the manifold or start sucking a lot of air when it's struggling? If so these are def signs of fuel delivery issue, stemming from a bad signal from o2 sensor/s. Outside of that, get a code reader on it if ya can. Function of IAC, TPS and overall condition of ignition and distribution is important. Clean, replace and check it all..

 

Agree. Start with a FULL tuneup. Champion plugs gapped to .035, new plug wires, distributor cap, rotor, fuel filter. If you are still having issues after that, clean both throttle body and idle air control. Next up, test your throttle position sensor.

 

I'm having the same problem. Idles perfect on startup, around 900 rpm. But after warm up, in drive at a stop, idle feels "low and rumbling". The entire jeeps has a low vibration. 2000 XJ with 101k stock.

What I've done so far. New plugs and air filter 5k miles ago. Sea foam in gas twice in last 5k miles. Cleaned the shi_ out of the throttle body and IAC. Brand new BWD TPS. Nothing helps so far

My next step. New IAC and/or upstream O2 sensor. No codes being thrown and drives perfectly.

 

Cleaned the IAC, throttle body, and also seafoamed it. It's still on the unstable side. Now I'll work on a tune up and checking the o2 sensor.