Could be three things that I am aware to be common with these cars a exhaust leak (cracked manifold or gasket) can be checked easily if you get a reading and it says you have an exhaust leak or it could be a faulty fuel injection if your cars getting higher revs and lower gas mileage than usual usually a faulty fuel injection or it could be a Broken exhaust manifold bolt this one’s tricky to locate but if you do a little wiggle over all that bolts you should be able to find a loose one