I just put a new 383 stroker in my 88' Blazer and my check engine light keeps coming on. It runs really nice untill the light comes on, and then it runs really slugish and needs alot of gas to get going. Im getting codes 12 and 42 every time I check, but I don't know exactly what they are. Code 12 is no engine speed reference pulse, I can't find anything in my Chiltons manual about a sped sensor. Code 42 says that the ECM has seen an open or grounded EST (Electronic Spark Timing) or bypass circuit. I don't know what this is indicationg either. I don't want to start guessing aout what the prolem(s) is and waste money on tryin to get it fixed. Any ideas or opinions would be greatly appreciated. Thanks a bunch!!!