If you are talking about that rectangular flat piece that is now bent into a horse-shoe shape, Yes. It is a shield to keep road dirt and debris from getting up into the air filter. Since all the incoming air comes from underneath, it aids in keeping out most of the material diverted up from the air damm. I have a '90 too and I left mine in. I'm using a K&N filter and it keeps it clean longer. The air filter doesn't get dirty as fast. It won't make much difference in air flow. But if there's nothing to hold it in place, it's your call.!
Just another hint. Since it is spring and the cottonwoods will be everywhere soon. Don't know if you have them in Cal. After they are done(in late May) Be sure to blow out your air filter(s) and the radiator/condenser. These C4's are like vacuum cleaners and suck everything up.
