My dad has a 93 ford explorer v6 and in recent years it has lost alot of power. It tops out at 80mph now and there is a horrible rattling in the engine when it gets under any sort of load. The car has 140000 miles and would like to get more out of it if possible. Could this rattling simply be from weak valve springs or something more serious, because it does sound like the valves. Any ideas would be appreciated.