my 1990 4.0 sometimes idles at or just below 500 RPM's sometimes when its in drive and im not moving. is that what its supposed to be at? most of the time its around 600. do i need a new TPS?

How is your throttle response otherwise? Does it idle smoothly?

my throttle response is great, and it idles fairly smooth, nothing out of the ordinary.

I would think that as long as your oil pressure is above about 20 at idle and there aren't any driveability problems, you should be ok. My 4.0 Grand Cher idles at about 6-700 with AC on.

but isnt 6-700 normal? i would think under 500 is weird.

Dipping under 500 for a second might be ok, but if it stays there awhile you're right, that would be a little odd.

Has it been tuned up lately?( Plugs, cap, rotor, wires) that could be contributing.

Otherwise, I'm not so sure about what sensors, etc could be involved...

ill try the basic tune up, because it doesnt just dip under 500, itll stay until i start moving again.