Rough idle 88 cherokee

I'm new to new group. A question on my son's 88 cherokee. It has a rough idle, new plugs, cap&rotor, o2 sensor. Checked for vacuum leaks to no avail. Is there a way to read the trouble codes without a code reader. It's the 4 liter six

Thanks, Jack

I had the same thing on my 88. I did the cap, rotor, plugs, wires. Then I bought some seafoam. Seafoam worked great, now getting around 20 mpg on good days. Try a search on seafoam.