Just changed valve cover gaskets. Car purred before that. Changed spark plus and every sensor but O2 and crankshaft. Coolant fans aren't coming on. Just changed ECL sensor. Still get code 301 misfire and sometimes low valve manifold pressure. Pour seafoam in brake booster and no smoke came out the exhause. I also hear a rattling underneath when I accelerate. I'm guessing CAT is about to go out. Pretty sure no vacuum leak I've checked. I spent $30 in gas today it's burning so fast. Please help!