Engine cranks, but only starts after letting off key? - I'm perplexed

[creich68]

The wiring is old as well. Quite within the realm of possibilities that the harness and the connections are getting less than reliable. Just another thought, sorry.

I haven’t looked yet to see if places like LMC or similar sell harness sections. Also haven’t looked to see if the original is built in sections (I.e. disconnects at certain points) where portions can be replaced vs the entire harness

 

[gmbellew]

Are there any wires/connectors behind the radio area that I could have damaged when I removed/reinstalled the radio? Or is it just a coincidence that it started acting up right after I reinstalled the radio?

. It comes in the truck next to the AC box on the outside, and is routed over the large duct on the inside and goes to the ecm behind the glove box in a single bundle.

 

[1984K30]

Replace the ignition pick up in the HEI distributor.. My C20 350/TH350 had similar issues for one year, the the pick went out 100%. Also reshimmed the distributor shaft while in there..

 

[YakkoWarner]

You are chasing similar problems to what I have been experiencing on my 1989 Suburban - not identical though. If you can trigger the problem by tapping on the computer even after replacement, that starts to sound a lot like dodgy connectors. I used to get frequent code 42's - I solved it by adding an additional ground wire directly from the coil bracket studs on the intake to the firewall to ensure consistant grounding to the distributor. Solved the code 42 for at least a year, but now I am also into a replacement ECU for other problems.

 

[Copymutt]

Not at all familiar w/ your ECU set up. However some get whacky from radio interference. Easy enough to disconnect the hot to the radio or any com device in the truck.