Flash PCM?

Just exactly what does 'flashing a PCM do?

it updates the firmware/software that actually runs the whole thing. It's basically a full "embedded computer" that watches the sensors and figures out how much fuel to inject and when to fire the spark plugs, and chrysler (back when the XJ was still in production, and for a while after) would periodically issue updates and fixes to problems they discovered with the software after selling the particular jeep you have. Putting the latest firmware on the ECU generally will improve performance, safety, fuel economy, diagnostic systems, reliability, or some combination of the above.

Alternatively, it means programming the ECU for your mileage and vin #. It can also mean deleting the S.K.I.M. programming from the ECU, assuming it's from a donor vehicle that had the sentry key immobilizer system. Note: deleting the S.K.I.M. programming won't do a bit of good if your vehicle also has the system...you'll also have to remove the S.K.I.M. module from the vehicle.

Thanks for your replies.

You can also have it flashed by an aftermarket company for a performance tune.

Just make sure you do it in the garage with the door closed. Indecent exposure will land you in jail.