Got a '93 Deville 4.9 with a run rough problem. Sometimes it idles smooth, other times very rough. If you accelerate at 20-30 mph, it acts like its dying. If you let off the gas, then accelerate again, it runs fine.
So far the shop has replaced the battery, the alternator, the distributor, the plugs and wires, the air filter, etc, etc.
No change.
I checked the codes myself yesterday, and it shows an E98,the ISC motor.
Before I replace yet another part, could the ISC cause these kinds of problems?
Is there any way to check the ISC or adjust it? This has been a good car, but now it's turning into a pretty deep money pit.

The ISC motor only controls idle. Once on the throttle the ISC motor is out of the equation. It usually ratchets when shut down or hunt for the correct idle when going bad. Have you rodded out the EGR tubes? Look down the TB bore. If the two tubes looking back at you are all carboned up, clean them out.

The only adjustment you can do to the ISC motor is to change how far the motor presses on the throttle. Otherwise, it's not rebuildable or adjustable. There's all kinds of information on here about ISC motors. I just replaced mine last weekend. Not too tough to do. Like Ranger said though, I don't think that will contribute to your acceleration problem.

My 91 deville has no vacumn leaks, new plugs, cap, wires and still has an unsteady idle. Runs great otherwise.

So I just want to get this straight. ISC should not ratchet AT ALL after shut down? I'm thinking of replacing it.

If you hear it from inside the car when you shut it off then its going bad. Its $50 through rockauto.com

Just out of curiosity, since there's so much trouble with the ISC, why wouldn't you just unplug it?

Well, I had to do this as a temporary fix and it just wont work. Starting the car without the ISC connected is a pain because you basically have to mimic the ISC's functions which would be to give the car throttle to start the car and maintain an idle where the car wont die while parked or stopped. Its just a pain in the ass and well worth the $50 if you uy new or $3 if you head out to a j-yard and get one.

Went through this problem on one car '93 Deville, spent a fortune on it -- sold it and then sold a '91 STS that developed a similar problem for the subsequent owner. It turned out to be a worn distributor gear. An oscilloscope will probably catch it. Stop spending money and have it checked. A missing tooth could cause it to skip at random.

removed the battery ? Did you or the mechanic re-program the computer ? Procedure is in the owners manual. Mine does run allot better after I run through this 25 minute easy procedure. According to the manual, you need to do this every time you remove the battery. As for the code, I have no clue.