Long story short - I got a 1999 Sentra GXE (~140K miles) in poor shape for essentially free. I helped the previous owner out with some IT stuff for his business - and he bought it from a shady dealer in it's current condition... so I don't know how it got to be this way.

I'm having a hard time figuring out this problem though - it is idling pretty rough, and it has some severe hesitation when going up hills. It also seems to "pulse" the headlights and dashlights when idling... it's not bad but noticeable. It idles at ~800 rpm. It's got no check engine lights on - and It showed no codes when I hooked it up to OBDII. I don't *think* there is a misfire... I disconnected each of the plug wires 1-by-1 while the engine was running, but every cylinder sounded the same to me.

I have:
Checked fuel Pressure (Normal for both Engine on/off)
Changed Spark Plugs (The old ones weren't that bad)
Cleaned out air Filter box (it was BAD - looks like it had a bird's nest in there!)

I'm gonna try and clean the MAF and Throttle Body tomorrow. Will also drive to Autozone to have them check the alternator if I can find the time (pulsing lights is weird - what else could it be besides the alternator?). If cleaning the MAF has no effect, I'm at kind of a loss. The MAF or one of the 02 sensors is going bad (would have expected a code...) ? Perhaps a vacuum leak (I don't hear a big one.. and I'm not convinced a small one could cause this much problems...) ?

Thanks, I appreciate any thoughts you can spare!

Check the rubber accordion bellows on the air inlet for cracks inside the pleats. They often go bad on the back/bottom and it's fairly invisible. I've seen them look perfectly fine on top with a crack on the bottom you can stick your finger through. The result is often a small leak at idle that becomes huge when you accelerate and engine torque rocks the engine in the mounts, causing the bellows to stretch.

Thanks! I'll go ahead and take off the whole intake so I can take a close look at all of those creases when I go to clean the MAF. It would be amazing if it really is a simple as a hidden vacuum leak!

Good chance of that, it's very common on older Sentras. The rubber gets hard with old age and the rocking of the engine simply breaks it.

Another great catch by the Wiz!

So - I've got the 'Air Inlet Duct' (16577-1M200) off. Not seeing any holes in or around the pleats, but I do have a pretty large rip where one of the metal houses connect into it. Although, it is quite possible I ripped it when taking it off. Either way - I can only find that duct/hose from one store, and they want ~$100 for it... so I'm going to try sticking it back together with some black silicone sealant. Will hit up the dips in the pleats as well, in case it's got some super small cracks I'm missing and as a preventative measure now that I know you can't buy that duct anymore!

Do yourself a favor and use gray death for the repair, not regular RTV. Permatex Ultra Gray, it's God's gift to ultra-durable adhesives. The parts store probably won't have it but can certainly get it, or there are numerous sources online. Strip the surface with alcohol before application, there's always lots of oxidized crud on old rubber parts.