Freeze plugs

I do not see a 3.1l engine in a 1991 Isuzu Trooper. Are you sure it is a 3.1l, because I have either the 2.6l four cylinder or the 2.8l six cylinder. Look at the eighth digit of your VIN number, if it is an E, then it is that 2.6l and if it is an R, then it is that 2.8l, but if the letter is something else, let me know.
Or perhaps you have the year wrong, what is the tenth digit of the VIN number?
Wikipedia says in 1991 for the 1992 model year, Isuzu came out with a 3.1l engine. Although looking that up I see a 3.2l with either a V or W for the eighth digit of the VIN number, unless of course this vehicle was not sold or exported to the USA.
The 3.1l that I do see is an inline four cylinder diesel.
If the tenth digit of the VIN number, is an M then it is a 1991 model year. N would be the 1992.