Here is a list of items that affected the idle of my 88 XJ, 4.0 AW4, all seemed to contribute, every one seemed to have a helpful benefit. Listed in the order I found and fixed them.
Plugs, changed from Bosch to Champs
Distributor cap
number 2 spark plug wire high resitance
Replaced all the cables, because the boots were hard and cracked
Air filter, went to a oil and cloth filter, mainly because if the paper filter gets covered in oil (happens) it passes very little air. Not because of flow benefits or HP gains. Really helped
Fuel injector cleaner
Worn drive gears in the distributor
Worn spacer between gears and shround
Cleaned all grounds, really helped
Another load of injector cleaner, really helped
Plugged the vacume line where it connects to, the EGR valve (gonna get a new EGR sometime) The springs were weak and the EGR was opening too soon, also may have been a slight diaphram leak, seemed to help the idle some. Made the motor stumble some until it got above 1000-1500 or so.
Replaced cat and muffler, the old cat was still good (found after removal), the muffler was partialy (mostly) plugged
Adjusted TPS, really helped
Unpluged and plugged, many and varied connectors, the rubbing of a little in and out action, often helps with contact.
Still to do list
fix the broken stud on my intake
New intake gaskets
intake and throttle body cleaning
wiring to the 02 sensor, temp. sensor and knock sensor is fairly cooked, found a junk yard harness.
Along with a smoother idle, my gas mileage has been going up, up up :loveu: along with power.