Jr. I’ve read your post. Sounds like your Dad left you with a challenge, an opportunity. I grew up with GM TBI in the 80s and 90s and accordingly grafted a 91 GM TBI system onto a 1969 327 in my 48 3100. It runs like a dream. Guys: don’t shoot the messenger….we’re trying to help out Curtis here. There is nothing in that ECM (that I’m aware of) that would shut off your fuel or timing at 25mph, but let’s entertain a few ideas. 1) You said in your post that the computer is always seeing that you’re in “PARK”, but you have a manual trans in your truck. With your EBay scanner is that the case as you’re going down the road? How accessible is the ECM? Terminal B10 is the “PARK” input: make sure that wire is disconnected. 2) Did your Dad hook up a knock sensor up on that 70’s engine? Can you confirm if there is a knock sensor and an accommodating ESC Module (Electronic Spark Control)? 3)Does your Ebay scanner show the ECM setting any codes, like a Code 43 for ESC?
