96 blazer hard start cold, runs bad till reg operation temperature

Hi there, I have a 96 blazer.. I don't have much history on the vehicle because i just bought it. Anyway.. It doesn's want to start when it's cold. After a few times it will start.. Runs weak and keeps cutting out until it's at normal operating temperature, after it warms up it doesnt cut out bad but still doesnt have alot of power.. it does fine at very low RPM's. Changed fuel filter. Checked Fuel pressure and it is fine. But i only checked it when it was warmed up. Didnt check it while driving. I guess that is my next step tomorrow morning. Only check engine code it's spitting out is something about radiator coolant circuit high output. The catalyctic converter is cut off and they did a poor job putting flex pipe in it's place, and the exhaust leaks, but that shouldn't effect performance right? Guy said he just gave it a tune up. And that the owner before him changed the fuel pump, but i thought he mentioned something about it sitting for a long time. Anyway, i would appreciate any help. Thanks, Tim

check the plugs is one side black if so Fuel Pessure Regulator is bad the diaphram haas ruptured

What engine.
Removed cat is illegal and affects engine running.

Wha is the code no?
Are you sure the air intake and engine tempt sensor are reading good?

Hard to start cold always check the cold start cranking fuel pressure.
If not around 64-65 lbs or a few lbs low the injectors will not squirt on a cold start.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold and fuel pressure testing guide line.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.