Searched all over but couldn't find an answer.

I swapped out the spark plugs on my XJ 2000 sport, taking care to use the right copper plugs gapped to .035 and torqued correctly. I also double checked that all cables and bolts were reattached just like I found them.

Now, when I go to start, the engine won't turn over, and instead i get a loud fast clicking sound ( I do have power).

Final twist: when I use a mobile battery to jump the car, the engine does start, but then only idles rough for about a minute before shutting off.

Did I do something wrong with the spark plugs? Is there another issue? Any help is greatly appreciated.

 

Battery cables corrode under the insulation. Usually it manifests itself as not starting, you twist on the battery terminals to get them tight, moving the cable and regaining connection in the process.

You may have done the reverse...

The fast clicking sound indicates a battery cable is loose or the battery is severly drained.

I believe the ground cable on the later models still connects to the motor on the passenger's side, at the back, at or near the dip stick tube, close to where you were working...

 

Thank you! I will triple check all the connections and everywhere I was working tonight

 

And XJs do not like weak/marginal batteries. I learned that the hard way after I left the dome light on in my 99. Happened on my 98 too but with the headlights left on, but I had learned by then. I have always been able to put a battery tender on a battery and get it back to normal. On the XJs, one that battery gets run down it will just not idle. New battery fixes the problem. If you do a search, you'll find a lot of the same story.