97 Altima idle

[LVG]

Hello. I have a 1997 Altima, 120K miles, All original except as noted below. Location is Atlanta, GA. Car has had a rough idle along with a spitting/sputtering sound through the exhaust. Engine runs perfectly at any other speed. There were trouble codes for lean condition and Cyl#4 misfire. Distributor cap and rotor replacment didn't help. Plugs are fairly new and have a very small amount of white blistering. Ignition wires are pretty new and resistances are OK. I'm pretty sure all the injectors are clicking. Fuel pressure is fine and the regulator seems to work. MAF sensor output is fine. O2 sensor output is VERY LOW at idle speed(about .02-.05 V), but normal at any other speed. TPS sensor output seemed a tad low at closed throttle(.4V). I've created a large vacuum leak and also added some propane gas into the manifold, neither of these smoothed out the idle. I have not checked compression yet, and I don't know how to check for any manifold leaks. I'm running out of ideas and have a month until I must pass an emmissions test. Many thanks for any advice!

[NISTECH]

your #4 intake port has sucked the intake gasket in from the bottom. If you place the propane hose near where the intake manifold meets the head below the number 4 runner your idle will jump up. you need to replace the intake gasket. This is not a fun job for a novice. but word of advice,do not seperate the lower intake from the manifold. just unbolt the manifold from the head. the bolts on bottom are hell to get to. this job is very time consuming and tedious.