1988 cherokee 4.0L hgih idle/engine tick

A while back I had a problem where I would start the Jeep and it would idle extremely high and sometimes it would lower within 20-30 seconds and other times it would not come down for 20 minutes. I brought it to a mechanic and they replaced the 'idle speed stepper motor' as well as the air filter + cleaned the throttle body after that it was good for about 2 month's or so. SLowly the problem came back and after getting sick of taking it back to the shope every two weeks, I started removing the 'idle speed stepper motor' and spraying it with WD40. This needs to be done EVERY 3 weeks or so because if not it gets worse and worse. How can I solve this problem for good?
After driving the jeep for 15 minutes, the engine starts to knock and tick when it's idle (at a red light or something) but as you get to the higher RPM's it goes away. Can anyone give me some feedback.advice on what to check or look at for both of these. I am sure people with the same year jeep have had these issues. Thanks.

1) Check for a vacuum leak, or some blow-by, which is making those parts dirty faster than they should.

2) How is your oil pressure? If you don't have a pressure gauge (just a warning light) then you might need to have it checked. I'm sure you're using the right oil and changing it regularly....also a tick could be an exhaust leak.

How many miles?

The tick you hear is a very common thing in the older 4.0 how many miles are on it and does it do it all the time or just occasionally i have three cherokee's and all but one have the tick at idle its probably just a little lifter tap as the oil pressure drops at idle. And i had the same problem with the high idle in my 92 it turned out to be a vacuum leak and a throttle position sensor.

The older 4.0 engines were famous for excessive skirt wear on the pistons. What you hear may well be piston slap. Chrysler cured this problem with the 96 model, if I remember right. It's an annoying problem, but seldom fatal. (to the engine, not you! :iceslolan)

Sorry, forgot the stepper motor problem.....................

Best advice, quit spraying that WD on your equipment. It's great for removing road tar, but leave it off your engine. When you need to clean linkages or something like that, use carb and choke cleaner. The WD attracts dust and dirt, and adds to the problem. Choke cleaner will leave it free of dust attracting stickiness.
Hope this helps.

Great I will switch from WD40 and see how much longer that last for, but how do I check for a vacuum leak? or is that something I need to bring it to a shop for?

Good call on the WD40 attracting dirt, I missed that.

For the vacuum leak check, just visually inspect all the vacuum tubing you have under the hood. Roll it between your fingers to see if it is soft as it should be, or if it's turned into that glazed plastic kind of rubber and developed cracks. Check especially close to where the vacuum lines are attached, and at any bends in the lines as these will develop cracks sooner.

If you have one on that year, check your PCV valve as if it's clogged then the venting it is supposed to do will occur elsewhere.

Is the vacuum tube the large rubber one connectiing the throttle body to the air box?

And about the wd40 thing, it's weird because whenever I remove the idle speed stepper motor (the 2 torx screws and has the small plunger on the end) to spray it with wd40, it's never dirty. For some reason lubricant seems to stop the issue, maybe it's inside the throttle body? I don't think using carb cleaner on that part will do much because it was replaced recently and looks brand new.

Vacuum hoses are usually small, like 1/4" in diameter and made of rubber. They snake into different areas. The one you're talking about is the intake...if that has openings along its length, replace it; there should only be one opening at each end.

First off, are we talking about actual vacuum lines, or could you be confusing some of the emissions tubing for vacuum lines? Typically those plastic emissions tubings are a real PITA on older vehicles. They get brittle and start leaking the signal they are intended to send from various sensors and temperature switches, to the ECM. Best advice for your year is to go through and replace every inch of that junk. Be careful to replace it size for size, or you could create problems that will keep you awake nights. And whatever else you do, DON'T get them mixed up. Tag them so that you KNOW where each one came from.
An excellent way to locate leaks in real live vacuum lines is to get a can (or more) of aerosol choke cleaner, and insert the little tube they supply with it. With your engine running at idle, liberally spray the choke cleaner on every place where vacuum hoses connect to various fittings, most especially around the larger vacuum lines like the one going to your brake booster unit. Each time you pass the choke cleaner spray over a leaking area, your engine idle will briefly smooth out. Go slowly and be observant to what you're seeing. You might find you have only one leak, and when you find/fix that one, your engine will run better. Or, you might find that you have a number of smaller leaks that contribute to a fairly large vacuum loss. When you start spraying the choke cleaner, you might want to start around your carb base, or throttle body depending on year. Pay close attention to the throttle bore shaft, and the base gasket. Those are common leak areas. Trouble shooting a vacuum leak can be frustrating sometimes, but once you understand the idea of chasing down leaks, you can make rapid progress, and when you hit the leak and your engine smoothes out, the big grin is well worth the effort.
Good luck.