My 90 honda civic is surging at a idle but not at a higher rpm what could it be

Check your idle air control motor. Also right next to the idle air motor there is a fast idle valve, I think that's what it is called, check that. Both might be bad or the vacuum lines might be clogged. You can clean the fast idle by taking it apart but the idle air motor is better to just replace it. Hope this helps.

my 90 honda civic is surging at a idle but not at a higher rpm what could it be
It probably is your idle air control valve (IACV). The wax valve portion of it tends to deteriorate over time.

If none of that other stuff works try checking your egr valve, I had the same problem but my check engine light was on.