94 K1500 4.3 running rich

I replaced long block with used. Old motor had 10# compression on #6. When I swapped the motor it ran great. Was driven for 2 days and all was well. 3rd day it started spittin and sputterin and backfiring when I asked for more throttle. If I'm light on the throttle it will accelerate, but if too heavy it will backfire. It actually blew the seam out of the muffler. Plugs are black, no codes tripped. R/R the egr solenoid and valve due to inoperative. Fuel pressure 13#. R/R 1 fuel injector (tbi) due to ohm out too high. The pickup coil ohms out ok but fails the wiggle test. Pulled connector to MAP, no change. Compression is 140 on all cyl. Now it will actually die at an idle and sometimes harder to start. I think due to being rich. I need help isolating the problem to a particular system. I need to know whether ignition, fuel, etc.:runaround:

What is engine vacuum reading at idle and at 2K rpm?

Good hot spark to all plugs?

Dist and dist shaft good?

Pickup module and dist pickup coil good?
cap and rotor and wires good?

Give the pcm the old tap/wiggle/twist test.

...1 fuel injector (tbi) due to ohm out too high...

I am not sure with TBI systems, but if the injectors are batch fired, electricity will take the path of least resistance. So if one injector has lower resistance than the other one, all power will go to the single injector instead of both. You may want to try swapping the injector you took out for the other old one. Did you ohm check the new one to see what the spec should be?

Also make sure to check for wire harnesses contacting exhaust manifolds, especially the O2 sensor.

Vacuum is 16# at idle. It drops on acceleration and comes back up which is normal. One thing I did notice is the needle shaking when reading vacuum. I know this means something but I can't remember what. I put new head gaskets on while motor was out. I swapped the distributor with the one that came with the used motor. I drove it afterwards and it drove ok but same problem just not nearly as bad. It still has a little sputter when load applied. There is a possibility that it was better because I drove it at midnight in a lot cooler climate. 85 and humid in day and cool 68 at night. It has new wires and plugs. It had new ac platinum in it. The stock plug is ac 43, I had a new set of ac 45 on the shelf and tried them because I think they are a little hotter, these plugs make the problem less noticeable. The injecters both ohm out at 1.3. I set base timing at 0. I put the timing light on #1 while running and noticed that the light would stop before the engine died. I don't think this means anything though because the only reason it dies is because I'm not giving it any throttle. I can keep it from dying if given throttle. Thanks for all your help. I am very appreciative for any help you've given. :banghead:

With the ac 45 plugs the problem is less dramatic. I put the plat. plugs back in and the problem is back to normal. I checked the vacuum again and it is 16-17# at idle and 20# at 2000 rpm. The needle shakes at idle. I remember this meaning something, maybe leaking intake or something. It has new head and intake gaskets. If intake is leaking wouldn't I be losing antifreeze through cylinder or externally. :banghead: