What's going on everyone, I have a 2000 xj 4.0l as stock as they come in reference to the powertrain. I recently did a big head job on the motor, resurfaced, valve job, pressure tested. The 9's. Put it all back together and it drove fine until the following morning it drove for about 45 mins and it started jumping on the rpms at idle. Pulled the code and as the title says I got cps. I replaced the oil driver and the cam sensor with oem parts straight from the factory. Didn't solve the problem. Adjusted the oil driver while it ran to see if retarding or advancing the timing would solve it. It didn't. Replaced the crankshaft position sensor and it didn't solve it. Im lost as to what to try next. Read some threads about replacing the pcm but I rather stay away from that as long as possible. If any xj wizards see this your recommendations are much appreciated.

 

So what do you mean with "jumping on the rpms at idle"?

High idle, up and down rpms?

 

What Roler asked.
How long before the problem popped up?
And you said you used factory parts for the cam sensor and oil driver but did not specify what you used for the CPS.

On a side note.
When you say oil driver you mean #4 at the illustration, 5083881AA, at this link?

https://www.moparpartsgiant.com/part...agram=00i66982

Curious as to where you found a factory one.

 

Might be a long shot be do a compression test and leak down test. My XJ is currently doing the same at cold starts. A compression test has shown me that I have a cylinder that has very low compression.

 

The intermittent symptom sounds like it could be related to the crankshaft position sensor (located on bell housing). A fault in that circuit can cause RPM to drop out and have no OBDII codes displayed.

* Verify that when reinstalling the head you put a good condition ground strap back on that goes from the left rear head bolt to the firewall
* When putting the head on there is a chance the wire to the crank position sensor was damaged. I recall it routes very close to the left rear corner of the head and I have heard of cases where the wire was pinched or otherwise damaged in the process of getting the head back on.
* Verify that the both ends of the connector and pins are OK and connector is fully seated. Look for a bent or pushed in pin
* Verify that the crank position sensor is installed correctly and securely bolted in place.
* If you have a heavy key ring with lots of weight hanging on the ignition key stop doing that.