I washed the engine bay late today and cause a limp home condition after driving it for a few miles. This has happened once before years ago when I was running a 3G MAS and now I am running speed density.
After limping it home the last 4 miles and racking my brain to figure out what still could be the common thing that is causing this issue I came to a few ideas.
The very first time this happened it was a torrential rain and after a huge puddle it started acting up. I concluded that it was the MAS that got wet so I dried it out over a few hours and it was fine. So I figured that was it. This time I don't have any MAS and was very perplexed as to what it could be since all the new wiring harnesses for the speed density setup was done with GM seal-tight connectors.
I found that the large stock engine harness plug that sits on the passenger side frame rail was the only common thing between the old stock setup and the new SD setup and sure enough it was soaked. Inside and out. I blew it out with 120psi air and it runs great again.
So, if your car will only idle and rev to 1500 rpms and fall on it's face, pull that plug apart and dry it the best you can.