If all of the cylinders show the same or close to the same pressures, then that kinda rules out a head gasket failure. I would dig into the lower intake manifold.

Oh and this is kinda funny... Looks as though the folks over at S10Forum just copied and pasted the info from TroubleCodes.net --> TroubleCodes.net GM OBD1 Although it looks like they copied the info off of AutomotiveForums.com (judging by the picture url) who probably copied it from TroubleCodes.net.

 

great detective work [sm=smiley20.gif]

 

LOL. I just know I have seen that picture before and I know I saw it long before the thread was created on S10Forum, going back to around 2000 or 2001 when I first was introduced to TroubleCodes.net.