Code 42 on 93 Astro

Just bought a 93 Astro, thought I got a great deal.
I changed the fuel pump and filter. Turns out on the TBI sprayers was bad too, changed it.
Engine starts quick, hot or cold.
Idles rough.
If I rev it up, it smooths out?
Now I am getting Code 42, Where do I start?

CODE 42 = Electronic Spark Timing (EST)
ECM has seen an open or grounded EST or Bypass circuit.

Thanks for the reply.
I worked in an office for many years, and now I am going to give this a try.
Could it be that one of the sparkplug wires is grounding, or better yet what would be your next step (s)?

Well if you are new to this I would go to www.factoryautomanuals.com and buy a used manual for the van.

I'd remove the doghouse and start looking for a brown wire with a black stripe. There should a weatherpack connector in it located somewhere near the distributor I think. Make sure this connector is attached good.

With this wire unhooked, the computer control of the timing is disabled. Usually this is done to set the base timing with a timing light and then it must be hooked back up for the engine to run properly.