Ok, so, my 4.0 was idling high when it warmed up, and I read up on it a bit, and decided it was probably a loose exhaust manifold. I'd also heard a low 'hissing' sound occasionally at high throttle, so it seemed quite possible.
Well, I looked, and some of the bolts on upper (intake) side of the manifold were loose enough that the washer was able to move freely. I tightened all the ones I could reach down.
It apparently fixed the high idle--and replaced it with a low idle, somehow. While previously, it would run at 1100-1300rpm when sitting either in gear or neutral, after the engine got up to temperature, now it sputters down around 500-600.
It also seems to have trouble starting now, too; it turns over, and fires, but then falls down 200-400rpm and usually dies if I don't give it any gas. If I do goose the pedal a little, and get it up 2000rpm for a second or two, it stays at its happy 800 after that (until it warms up). If I let it die, and start it again, the ECU revs it up to 1500 on the second start attempt, and it settles down to 800 like when I do the same thing 'manually' on the first start.
Oh, also, the oil pressure is reading a little lower than usual (20-30 instead of 40 when idling), but my level is fine, and there aren't any signs I'm leaking or burning any fluids.
So, in light of the loose bolts, and the weird temperature-based idle problems, and the hissing, should I just spend the $8 on a new manifold gasket, and spend an afternoon putting it in and torquing all the intake/exhaust manifold bolts down to spec? Will that fix my problems?