fuel

I have A 1988 gmc R3500 it has the 7.4L fuel injected it is running extremely rich and I can not figure out why there is no check engine light and I have already replaced the temperature sending unit that is right by the thermostat housing I cant afford to just start replacing sensors without knowing whether they are bad like the mapp,throttle positioning, or the o2 sensors is there A way to test these without replacing or will this pull A code on a scan tool even though there is no check engine light on? thanks