Later models will often give a CEL if the timing belt has jumped.
In your case you just have to pull the cover off and rotate the engine to see if the timing marks still line up. When the belt gets old and stiff or the tensioner gets weak the belt will bounce around and hit the cover. If one of the ripples makes it over one of the teeth on a gear, the timing is permanently off.
I bought a used van a couple of years ago cheap because it ran like sh!t. Found out they had just put a new timing belt on it at the Dealer, but it was off one tooth. Ha ha, nobody's perfect!

If the plugs and wires are good the idle should be smooth even if there is a vacuum leak, although the RPM's will wander around. If the idle goes up and down till it almost stalls, a vacuum leak is a good bet.