Yesterday I pulled my Blazer (2000 Blazer 4wd W engine 112,000 miles) into the driveway to replace the rear rotors. Car ran fine pulling in, although it has been running a little rough at idle on occasion lately, but I haven't tackled that issue yet as I'm waiting for my tune-up parts to arrive UPS from AutoPartsGiant first. Rotor replacement was uneventful, and I proceed to try to start the engine to pump up the brake pedal. When I try to start the car, I get a backfire under the hood and the car barely starts and then dies out. After a few attempts, I get it to start but it won't hold idle -- it lopes between 500 rpm and 2500 rpm and really sounds like the engine's end is near. It's also making this horrendous whooshing/sucking sound that is hard to describe but sounded almost like the sound releasing compressed air makes. I take it for a drive and it runs fine from about 2000 rpm all the way up to highway speed. I plugged in my code reader and it displays no codes whatsoever. I search this forum and narrow it down to a vacuum leak or a MAP sensor, although I'm doubting the vacuum leak because I'm thinking, how could it develop such a huge vacuum leak so quickly? The car behaved exactly like what rlith described in his MAP sensor quiz a little while back (except for the odd sound), so I'm thinking maybe I should buy a MAP sensor just in case, but the wife has the other car so I'm stuck at home. While I'm waiting, I start inspecting the vacuum lines again, and everything is looking okay. Then I pull out the PCV valve and notice that the round plastic valve/fixture that is attached to it by the hose (I don't know the name, but it has the PCV and 2 other vaccuum lines attached to it) is sitting above but is not actually inserted in the upper intake where it should be. Apparently this round plastic valve had been inserted in the hole but NOT turned to the lock position. I must have done this accidentally when I changed the PCV valve a while ago, and I suspect it has been leaking vacuum, popping up and getting sucked back down for quite some time. Today, however, it popped out and stayed out, explaining the huge sudden vacuum loss. Simple free fix, the car runs great, and I'm feeling like an idiot who just dodged a bullet. Although I must admit I found the rlith quiz a little bit annoying at the time (because I thought he should have just posted the solution from the start, all of those guesses and the ultimate solution allowed me to immediately rule out a lot of common problems (eg. Fuel pump, egr valve, Ignition system) and focus instead on intake manifold pressure and the ultimate solution. So, thank you rlith and everyone who posted guesses.

An easy thing to check would be plugging the brake booster port.