I have a 1994 S10 blazer w/ a vortec 4.3. It has a new low mileage motor in it, but it has a stumbling issue. It only does it at idle, and it is a random misfire. So far we have changed the cap n rotor, plugs, plug wires, fuel filter. It has also been checked for fuel leaks, and has had the fuel return lines replaced.

What are some places for me to check for a random missfire at idle?

 

did u check inside the plenum at all for fuel leaks? this could be causing the stumbling and partly cause the misfire. the plastic fuel lines (nut kit), spider injector assembly, or the Fuel Pressure Regulator (FPR) could be bad and causing leaks or improper amounts of fuel inside the plenum.

 

im having similair problems but mine is an 87 2.8L. try your 02 sensor idle motor control. vacuum lines, EGR valve. pretty much anything in the emissions control system. your best bet is to go to an autoparts store and use their computer scanner. i been trying that with mine but the computer scanners i can get access to dont find any codes.
hope some of this helps.
-Wes-

 

The original motor was replaced because fuel got into the oil and ruined the bearings. It has been getting goo gas mileage, and doesn't stumble when driving, only at idle. We checked it for leaks before we put the plenum back on. When it had a fuel leak prior, it would stumble through out the RPM range. It shouldn't be leaking.