I have an 88 Coupe DeVille, 99,800 miles on it. I noticed it had started idling at a higher RPM, but just figured it was cold out and that was why, and it ran fine when I drove it. It progressed from that into the engine chugging even when I would drive it. The Service Soon light came on and I looked up the code and it was E30 (ISC RPM Out of Range), so I figured that something was wrong with the ISC Motor (which after looking up the location, I still can't seem to pin down exactly where the damn thing is). When I popped the hood, I noticed that there was a small hose behind the air breather that was disconnected, so I put it back on, fired up the car, and it ran WAY better... for about a day. I think it must have just been a coincidence, because it is back to running kind of rough, and the Service Soon light has come on about half of the last several times I've started it. Also just to clarify, when I say running rough, it's not like it is constantly chugging the whole time I drive it, it's just kind of sporadic.
I put a bottle of injector cleaner when I filled up the tank, also. Maybe it just needs new plugs & plug wires?
From what ive been told those little isc motors dont really go bad but you have to re-tune the system. If you get one of those books there is a procedure in there its not too crazy really. It deals with throttle position sensor. If you readjust it and its still weird. Its possible you would need isc motor, if you do buy one only get a delco dont go to autozone for one, or grab one from another caddy at a junkyard.
About all the ISC does is control the throttle plate angle to prevent engine stalling or overspeeding. A bad one is a pain, but not likely to cause your complaints. I suggest you carefully go over all of the hoses and hose connections under your hood and make sure they are tight. Your symptoms don't really point to wires or plugs either, as these would be constant. Injectors are another matter. Fuel supply. When did you last change your fuel filter? Have you tried "rebooting" your computer? Take off your battery negative cable for about five minutes and hook it back up. This will dump the car's computer, the good, the bad, and the ugly. When you reconnect and re-start, it should be back at factory settings which are resident on the chip. This may fix your symptoms, but will change the car's operation a bit. I don't know whether the 88 models have the "learn" function which teaches the computer as you drive to operate the car as it was intended by the designers. If it doesn't, you should find a procedure in the manual to "reset" the operating parameters. That's a clean hands operation, done from behind the wheel. Finally, if all of this does nothing to help, see if you can find a known good computer (ecm) and swap it in replacing the one you have. Use your existing chip. This will sometimes fix everything if the ecm is at fault, but it won't help if your trouble is loose or disconnected wires and/or hoses. Incidentally, don't go to Cadillac and buy a new ecm. They are expensive. There are enough 88 Cadillacs out there and in the junk yards with good ecm's for peanuts.
I have two 90 Cadillacs with 4.5 engines. 186,000, and 111,000 miles. They both run beautifully, so it's not the engine or the systems, its a glitch.