Alright today I had to jump my car cause one of my lights mysteriously turns on (that's another story. maybe ghosts? just kidding)

I jumped my car and my engine was running pretty hard. I looked at the RPM and it was up at 1500 gettin near 2000. usually when im idling it's around like 1000 or less.

It's happened another time when I had to jump my car too. I got in it and went off and my car revved up to about 3000 while driving down the road. I didn't have my foot on the gas. I just had it in drive and I got up to about 30mph.


If it helps I got a XJ40 1989 with a straight 6.

 

I am familiar with the XK/XJ series, not yours, but in those cars the ECM looses the fuel trim data when the battery voltage drops below a certain level. That is about 10v. When started without fuel trim memory the ECM runs the engine RPM up to about 2k for a few seconds until the O2 sensors and the ECM reestablish proper trim.