I agree with Richards's comments about the Chevy 4.3L engine. I have owned my Chevy Astro since new (150HP non vortec version) and the only engine related problems was that I replaced the o-rings for the injectors. I belive that the fuel injector cleaner I used ate away at the original o-rings. The only other minor problem is that my rear transmission seal is starting to leak.
My only complant with this engine is that the valve seals tend to leak which causes a puff of smoke when starting the engine. Since access to the valve seals is very difficult on a Astro van I just live with it.
I do firmly belive that the quality of an engine is directly related on how well it has been maintained.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles