Trying to go to a different engine size is a real bad idea. Most of the time the project will end up in front of your house with a "For Sale As Is" sign in the window. For sure you're going to need a larger radiator, and stiffer front springs. You already know you have the world's worst front suspension system when it comes to tire wear and alignment problems. A heavier engine is going to make that worse. Your truck has a brake proportioning valve carefully-tailored to its weight distribution and weight transfer during braking. A lot of variables go into designing that part. Engine and radiator weight, and spring stiffness are just two of them. Lawyers and insurance investigators love to find these kinds of modifications so they can shift some of the blame from their client who ran the red light and caused the crash, to you.
The 4.0L was available in '90 but what I can't tell you is if the front cross member and engine mounts will be the same. You also may need a different Engine Computer, and the exhaust pipes will likely be different. You're going to be much happier if you stick with the original engine size or go out and buy the truck you want. By the way, my cousin is currently looking for a replacement truck. He has a '97 Ranger 4.0L, with all its problems, for sale.
Monday, January 13th, 2014 AT 8:39 PM