Try
www.planetisuzoo.com as it's a great source of info. As for the '88 Trooper, they were all fuel injected 2.6L fours and quite reliable. There is a Weber carburetor kit available for less than $300 if you want to convert, so take a close look to see if maybe that had been done by the prior owner. Typically the Isuzu injection setup is nearly bulletproof. The fuel pump is accessible from the top of the fuel tank, which is fairly easy to service but you'll first need to remove the armored cover before dropping the tank. Do try changing the fuel filter first as it's cheap and easy to do. One of the greatest things about working on Troopers is that you can crawl right under them and get to just about everything, especially on the 2.6 models.