1991 D250 Runs poorly after 10 minutes running.

this motor was replaced with a multiport fuel injected motor, (1995-2001) not sure where to find plugin to scan codes,not looking like there is one below dash anywhere, a mechanic told me some kind of sensor in the distributer could be bad,after you shut off motor, but leave key in on position, you can hear fuel still shooting to injectors and the fuel press regulator on fuel rail

plug in should be under the dash or on the firewall under the hood