1991 p/u truck , 4.3 v6 TBI -starts but at warmup floods out. Changed the plugs, truck starts fine, just as soon as operating temp is reached, starts choking out, lots of black smoke, finally dies, fouling out the plugs. I assume whatever tells the ecu to switch to a leaner mix is not working. I found two temp sensors, one on the manifold, one on the head. Are they both linked to the ecu? How can I test them if that is the problem? Engine shows no codes except 12. Something else I should be checking?