Crass

Hey im a newbee here. recently bought a 95 gmc sonoma sls 4.3 auto (same as an s-10) parked it last week and ran fine. got up the next morning and went to start it and it would not start unless i held the throttle basically down. Ran like a 350 small block with a big cam, not burning fuel and smoking. I did add some of that seafoam injection cleaner in it last week. When i start it it smokes bad and leaves a black spot on the driveway which i just took as burning the carbon off. Once it warms up and im on highway or freeway speed it runs great and normal. So the main problem is its a 95 with an obd1 computer but has an obd2 port. scan tool i bought wont work on it. I tried the jumper thing with the paper clip and it looks like im getting a 12 code something about no rpm sensed i think. not sure what to try next but dont want to waste a bunch of money on replacing **** thats not bad. Any help would be appreciated. thanks