Question: Why is my 1993 Chevy Blazer S 10 4.3 v6 hard starting when cool/cold? I have changed the oxygen sensor, coolant temp. sensor, cleaned injector, checked fuel pump psi.
Answer: You said you checked it, but did not tell us what the fuel pressure was. Make sure it is at least 55 PSI. Even 1 or 2 lbs. less, then the truck will be hard to start cold. This 4.3L engine in your Chevy is like like that.
Assuming that is fine, check that the throttle is not full of carbon. Buildup on the blade, bore or IAC valve will make it hard to start.
Another possible problem is that the fuel pressure regulator is leaking. Check that by putting your fuel gauge on it, turn the key on for 5 seconds to let the system prime, then watch the gauge for about a half hour. If the pressure falls, you could have a leaking regulator. Also, smell the oil to see if you smell gas.
I would also take a fuel sample. Put some in a clear jar and let set for a few minutes. If it shows any sign of separating, your gas is contaminated.