There's all kinds of things it could be. Almost allof them are not to serious really. The thing about a truck that you don't know the history of, is you really don't know what you have. There is a tendency to start hit & miss tinkering w/ it. $25 here $15 there another $50 for something else. A dry & wet compression test will tell you right from the start if you might be beating a lame horse and how much time & money you should or could invest (or may be throwing away) The tests will tell you the basic health of the engine, weather the compression is balanced throughout the cylinders, conditions of the rings & valves. Alot of the problems after those are ruled out are just makeing up for previous poor maintanence. You'll be able to decide to do the minimum to get a few more miles out of it or this, which greatly improve performance & MPG at the same time (about $140):
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