My problem is exactly this: While the car is parked, whether I rev the engine from the throttle body under the hood or I rev it from the gas pedal inside the car the RPMs go to where they should if I hit the gas in neutral but they drop back down EXTREMELY sluggishly. I've been through all of the vacuum lines very recently and replaced some of them that needed it.

So yea if I rev the engine while in neutral just sitting in my driveway it the RPMs do not drop immediately, or as quickly, as they should. It takes a good 5-10 seconds to get back down to 900-1000 rpms.

Also when I do this the tailpipe emits greyish smoke. This is an engine I bought from a junkyard over the summer and have been re-installing and hooking up everything when I have the time. Everything is hooked up, I'm pretty sure I'm not missing anything since this is the 2nd time I've had to do this. I doubt it's a headgasket causing the smoke to be emitted because the car hasn't even been driven yet. On my old engine I had a headgasket blow and I know the signs, there's no bubbles in the coolant at all, I watched it for 20 minutes while it ran warm, and it doesn't even overheat.

I'm not sure if the greyish smoke and the sluggish drop in RPMs are related but any input or suggestions on things to test would be great.
Thanks!

Have you checked the gasket between the throttle body and the intake manifold? Since you swaped this engine in, was it from a car with cruise control? Maybe some of the holes are there, and if your car doesnt have cruise, there is a leak. Check the IAC valve(Idle Air Control valve) It mighyt be full of grime, or stuck partially open. A blown HG doesnt necessarily follow standard symptoms. Mine went the beginning of summer, I had no {major} symptoms except loss of power, my comp, was 180 on all four, no mixed fluids and no "white smoke" but the HG had broken.

Thats why I had mine completely rebuilt instead of putting on an engine that I have no concrete idea how its internals are, except from the word of the guy who sold it.

Do a quick search, I had the same issue, and made this same post a couple years back when I first got my car, and other people have it too.

P.S. mine ended up being a grimmey IAC and broken gasket between the TB and IM.

Thanks for the speedy reply man. I'm gonna go outside and check all that out. Hopefully its something pretty simple that is either cheap to replace or easy to clean.
-As for the headgasket I'm praying that's not what it is. I'm probably gonna buy some of that snake oil bullshit they sell to seal the headgasket through the cooling system. I tried it on the last blown headgasket but it didn't work, most likely because I ended up having a warped head, I doubt this is the case this time.

After all the hours I've put into this car I'm starting to hate it. Once everything is running fine I'm selling the bitch. I might look into buying a 2g turbo awd model sometime in the near future depending on funds. For now my 89' Integra gets me around just fine, but I don't look as good in it.