96 concord lxi 3.5 engine miss and rpm high on idle

i have a 96 concord 3.5 also. my problem for the past 2 years, yea 2 years.

1. runs real ruff
2. rpm when warm is up at 1800 on idle
3. loss of power
4. when i remove pass side rear plug engine never changes
5. no starting to backfire when air conditioning is on
6. some of the spark plug tubes start to take on oil. i pull a plug every now and then and it is like a dipstick when i pull the socket out. now for this problem i pulled the intake and drivers side valve cover off. the spark plug tubes look fine. no holes or anything. i tried to remove them without any luck. thinking there is some type of gasket on the bottom of the tube.

i have changed plugs, wires,coil pack, throttle position sensor, idle control sensor on back of manifold, cleaned both throttle bodies, cleaned pvc valve, changed pvc hoses, probaly other stuff.

havent changed oxygen sensor, wonder if that would help. is that located under the hood on the drivers side?
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It is usually better to test and not guess. Get new valve cover gaskets. Check for vacuum leaks. It sounds like you have one. Pull any trouble codes and fix them. Use a scan tool and read the live data before you change the O2 sensors.

The PCM will compensate for some small vacuum leaks, but it will not for the larger ones, I tend to agree that you may have a vacuum leak problem. Common Vacuum leak sources for the 3.5 are:

Leaky Intake Manifold Gaskets (The 3.5 engines are known for this problem. Spraying small amounts of carb/choke cleaner along the manifold surfaces will find them as a running engine will react to the flammable source getting sucked in by the leak)

Leaky Fuel Injector Gaskets (Can also be detected when checking for an intake manifold leak seeing as both gaskets are in the same close area, you may think manifold gasket leak when it's actually the injector gasket leaking or even possibly both are leaking)

Either one being present, intake gasket sets for these engines are cheap to get (usually $10 or less) at the parts stores and will come with replacement EGR Tube Gaskets, Fuel Injector Gaskets, Lower Intake Manifold Gaskets, and Upper Intake Plenum Gasket. So if you detect a leak in the lower intake area it would be wise to use all of the gaskets in the set to make certain you get all possibilities fixed the first time in this area as it will take some time removing the upper half to get at the gasket areas to replace them.

And finally:
Disconnected or broken vacuum lines There are 7 connections on the car, 2 small ones on the passenger side of the intake plenum for the Fuel Pressure Regulator and EGR, one larger connection on each Throttle Body, one for the MAP sensor Mounted to the Intake Plenum on the Drivers side), one for the Cruise Control and Manifold Tuning Valve (Connects to the Intake Plenum on the drivers side. This line runs from the engine to the drivers side strut tower and routes along the drivers fender to a vacuum canister mounted under the battery, then a second line returns from the canister to "T" connection the cruise control servo and continues behind the engine to the MTV solenoid located on the passenger side strut tower), and finally one large connection to the brake booster (Drivers side Intake Plenum). Make sure all those lines are not broken or disconnected and fix what you find.