I have an 89 blazer with a 4.3 Intermittently it will not start or will all the sudden start running rough and quit running. it acts as if it has a bad miss. If I try a bit later it will run a bit rough with the check engine light on then the light will go off and it runs fine or it will start right up and run fine. black smoke out the tail pipe. the codes are 15 - 21 - 44 any help would be great

 

Look at the little plug in by your knees under the steering wheel. Take a paperclip and make a "U" out of it and stick it into the top 2 right holes. Now turn on your key and watch the engine light. It will blink 21 three times, then start flashing out your code - 3 times for each code. Eg: blink blink blink pause blink would be 31. Write those down and post it up on here for us. You might have to do it when its got the engine light on.