Don't listen to Gildo, he's can't decide if the diesel engine blows or sucks
. If you do a search here you'll find pretty quicly that leaky heads shows up a lot. Can't comment fully as my XJ choice, initially to be a diesel, changed and I got the petrol.
Since you're new here, a g'day and welcome and some Aussie hospitality: we never got the 2.1L (Renault made) diesel, I believe that was a US and Europe offering only. Our first diesel XJ was the 2.5L which appeared in 1987 in the US and as we didn't get the XJs until 1995 anything we can tell you is about the wrong motor anyway if it really is the 2.1L motor. Head over to
http://www.naxja.org, they might be able to help you out a bit more on that particular motor.