Engine smokes after 30 mins

My 92 Cherokee starts fine and runs great. After about 30 mins the you can see smoke out of the exhaust. If I stay in park with the engine running the same thing happens. It doesent happen all the time. Sometimes it will happen at every stop light and sometimes not at all. The smoke seems to be white but its nat a thick cloud so I can't reall tell.

I have the same yr/ model... does the same thing, mostly at idle speeds. I have been told by several friends that the problem is the valve guide seals. At idle, oil seeps into the cyl and burns off causing white smoke. Auto zone or any knowledgeable parts store should be able to direct you on what it requires and how to do the job if you are a do-it-yourselfer like myself. I have not yet fixed mine but I have looked into a kit that allows you to use compressed air to hold the valves closed to remove the springs, etc without having to remove the head.

Thanks for the info. How would you rate it on the difficulty scale of 1 - 10?

Generally, white smoke would indicate water (or coolant) is getting into the combustion chamber. Blue smoke for oil, black (or gray) for fuel. I'd try a compression test first to check the overall cylinder condition.

Are any fluid levels low?

Interesting kit. Sounds like a device screwed into the spark plug hole that connects to a compressor. How big a compressor do you need?

I have never heard of any type of water in combustion problems unless head is cracked and you would notice low coolant problems along with water dripping in exaust more than normal. Difficuly of repair would depend on experience. If you have no experience, I would personally suggest to take the head off but you could hold that for last case scenario (only if you drop a valve into the cylinder)
As far as the kit goes, yes, it does screw into the spark plug hole and it does not have to be a big compressor, just something to keep pressure in the cylinder to hold the valves closed. the valves must also be closed on the cylinder that you are working on. it is easy to tell with the valve cover off and you can move them easily by engaging the starter only for a moment. the compression check would be a good idea but it does not guarantee whether or not it would be the valves or rings. If you don't have any difficulty when starting, would not be too concerned with it. mine has bad compression and starts hard but I have already ruled it to be the valves, and the seals are also bad so I have decided to take the head off myself and get it worked (usually costs around 60-80 to get someone else to do the head) but I am going to put it back together myself.