91 240 SE wagon engine light on and no codes.

Recently my vo has developed a sluggishness on hot days. It will spit and sputter until it moves a little bit and then wakes up. Now it has a rough idle that waivers and poor fuel economy. I run a straight pipe from the cat back and i had to put my mid muffler back on for a state inspection. After reinstalling the muffler the check engine light is on. I tested the code box and the light just stays on no flashes. I was told it may be the check valve that runs from the top of the intake manifold to a hose that goes into the firewall. I have no idea what this is called or what it is for but i do not think that this little vacuum check valve should cause all these problems.