'91 4.3 idle rough at stop light
 

After replacing distributor cap, rotor, plugs, wires and PCV (I’ve done this regularly since I purchased the car new with no adverse results) – it now idles rough at operating temp stopped red light. Smog failed HC (130 with 124 being the upper limit). I checked all vacuum hoses, triple checked plug wire routing, checked new wire resistances, and finally reinstalled the original wires – NFG. Took it to shop -no error codes, they recommended replacing the EGR valve. I did this as well with the no change. Is there anyone who has been here before?

RE: '91 4.3 idle rough at stop light
 

cat clogged?

RE: '91 4.3 idle rough at stop light
 

IAC? Air filter?

RE: '91 4.3 idle rough at stop light
 

Plug gap, vacuum leak, if it was a missfire your HC would be really high. Sounds like a minor vacuum leak with the rough idle. Try lightly tapping the EGR when it's idling rough to see if it smooths out.
What's your idle RPM? If you can raise the idle 50-100 RPM, that might cure the HC problem.

What's the CO reading? And what's the allowable CO

Other usefull info would be engine, year, model, fuel type etc..

RE: '91 4.3 idle rough at stop light
 

Plugs were replaced with Champion 4018 “Truck Plug” – gapped of .035. I have sprayed water and the shop I took it to sprayed carb cleaner around vacuum hoses at the TBI. The mechanic did tap on the EGR and thought it made a difference - hence changing EGR.
I do not know the RPM.

Here is the smog report:
15 MPH - HC – MAX 124, MEAS 130….CO – MAX .75, MEAS .17
25 MPH – HC – MAX 104, MEAS 65….CO – MAX .95, MEAS .36

Engine: 4.3 TBI “Z” – 1991 S10 2WD Blazer – unleaded…..