I keep getting a #3 cylender misfire code and the engine is clearly missing. If I disconnect the battery the engine light goes out, and the enginesmooths again. After about 15 mile it starts all over again. Any idea what it might be? ECU?

 

You might want to check/gap your plugs first. .35 Clean the cap/rotor with hot soapy water. Or some just go with all new...there aren't allot of moving parts there though. Careful getting the wires off. Sometimes you can slide the insulator up off the plug so you can get the metal clip off without ripping it loose from the wire. Also look around the engine for loose/rubbing wires. I had some that would only short sometimes. Welcome! By the way.

 

do a search on "heat soak" or "3 misfire" or P0303. Common problem

 

The 00-01 are the most susceptible to "heat soak".

I agree that a search here and on other XJ forums (including NAXJA) would be a good idea.

It is a common thread.