Cadillac Repair: gm 3800, vacuum leaks, vacuum gauge

Question
hey there kevin,im new to this and i hope you can help me, even tho i have a buick not a caddilac.....its a 1996 buick lesabre 3.8 k motor, i had recently bought this car from an old man who told me it had sat for a year....i bought it because i knew it needed an intake and well i replaced the upper and lower, then plugs and wires, and fuel filter, but then the map sensor broke in my hand so i replaced it, then started right up, but since then its been having an irrat idle slightly high and slightly low, mostly in nuetral or park..... but it would actually stall so i pulled the iac and it was filthy so i changed it with an aftermarket just because... everything seems to be in the right places, runs and cuises beautiful the problem mostly occurs when i throw it in park......idle will just jump up and down and almost or even cut right off it will even cut off sometimes in gear when i let off the gas .....note it will start right back up but seems to have trouble regaining a steady constant idle........i dont know what else to look for?

Answer
Bill,
Buick, Cadillac, no problem! ;)
First things first, are there any trouble codes? If not, check the IAC passage and throttle body and make sure they're clean. Both can effect idle quality. Beyond that, verify there's no vacuum leaks, and make sure the fuel pressure is ok. Then, I'd probably do a cylinder balance test, and even throw in a compression test too. Have you installed a vacuum gauge yet? If so, what was the reading at idle? Was it steady? Let me know on this stuff and we can get more in-depth if need be.