1991 3.1, idle problem

I have a 1991 Corsica 3.1, 106K that still looks like it came off the show room floor. The idle is (and has been) rough for some time, but otherwise runs great . It shows no codes and I have the idle adjusted to keep it running as well as possible. When you listen to the idle at the exhaust, it will kind of pop back from time to time. I'm assuming that this might be a bad valve or valve guide. I would hate to get rid of the car, but am wondering if the cost of having the heads re-done would be worth it. Or could I keep running it as is?

I have a 91 with 168K on it, have had it since new. Common problem on these was idle air control, which sits toward the front of the engine. usually gets clogged up with carbon and has to be cleaned every couple of years. Comes out easily with two screws and unhook the wires. Clean it of all carbon, then clean the area in manifold as well with injector cleaner spray. If this is carboned up it will idle poorly. Also be sure that the rubber air intake (from air filter to manifold) has NO holes in it. Will hurt performance as well. I recently replaced mine because of a hole in it.

With that many miles I would suspect a bad coil pack or ignition module.
Or maybe it just needs a tuneup.

Yes, another good possibility.Best thing to do is to be sure you have new plugs, wires, air filter. Then check the IAC control as I suggested, as well as the intake (rubber) assy to be sure it is not ripped. The coil packs do indeed go bad, and there are three of them (2 cyl ea.), which in turn are over the ignition module. If the tune up does not work, have it check for failed coil packs, which can be replaced individually for around 50 bucks or so. Some people think you should have all three replaced if one is bad, but we had one fail at 95K and the other two continue to work AOK. I do recall the ECM being replaced at about 80K or so, but this was because of false errors. Bottom line is, the car is well worth putting some money into. Be sure to have transmission flushed if dirty or you are unsure how long it has been since changed. Don't just have fluid changed, have it flushed and a new filter installed. These cars are prone to having the TCC solenoid fail, usually due to age, and sometimes junk in the fluid.

Try the above things mentioned in the earlier posts first... If none of the above solve your problem, you may have a vacuum leak, and 3.1's are notorious for cracked intake manifold gaskets.