i have a 4.3 vortec that shutters and misfires intermittently really badly. it had an egr code so we replaced the egr valve and the problem was not fixed, so we did new spark plugs and wires, and using an inline spark tester the spark looks fine even while its misfiring. it has 50-60 psi fuel pressure, and on the fuel pressure gauge the needle bounces back and forth from 60-50 psi rapidly while the misfire is happening. my guesses include either MAP sensor or something to do with a spark timing issue.

 

I have what seems to be the same issue, but when I pull the codes on my 1994 S10 Blazer W I get 37. 37 is not listed. Any ideas???