I considered the possibility of a vacuum leak since the '93 manifold was moved over to the '98 engine. I could not find any leaks, but to be sure, I removed the manifold, soaked it in a 5 gal. can of carb cleaner and removed ALL the gunk. I reinstalled it with new lower and upper gaskets and torqued it to factory specs using the factory torque sequence. Once reinstalled, I still have a 1200rpm idle.
Wiring: It is not a wiring problem. I went to the trouble of replacing the engine wiring harness when I removed the manifold(yep. the entire engine wire harness---it's not as hard as it looks)with a different harness I had in the shop. Different wiring and I still have a 1200rpm idle.
I have triple checked all vac lines and replaced the power brake vac lines, including a new rubber grommet in the power brake booster. I have no vac leaks.
I can, however, remove the PCV tube at the throttle body and create a massive vac leak which would lean out the air-fuel mixture. With the extra air supply coming in through the open PCV tube and leaning out the air/fuel mix, the idle goes down.
It's a puzzle.