1991 Chevy S-10 Idles/runs good until warm

The parts stores I went to told me they don't test the computer -- the thin black plastic box mounted near the driver's side wheel well. They do test the ignition control module, but not for trouble codes, just if it's good or bad. They also have for sale a little black plastic (about 2"x2") device that supposedly plugs in somewhere under the dashboard and will give trouble codes that can be interpreted using a code pamphlet that comes with the device.

I think the fact that it starts running lousy and emitting black exhaust once warm is some kind of clue that should narrow it down. Can any of those possibilities you mentioned in your first reply to me be ruled out by this?

I just started it up again, crawled under and found that most of the exhaust is exiting through rusted, corroded out holes in the muffler. Could this be the cause of running well until warm?

The ignition control module checks out OK, and I replaced the distributor cap and rotor, which were both toasted looking. However, it still did the same thing after idling smoothly for around 20 minutes.

I guess my next step is to maybe buy the trouble code tester that plugs in under the dashboard, or just give in and take it to a shop.

Thanks very much.
Ken

Hi,
I bought a code reader, used as directed, and got two ECM codes -- 15 and 33.

For 15 the booklet says, "Coolant temperature sensor or circuit fault -- signal voltage high."

For 33 it says, "Mass air flow (MAF) sensor -- signal voltage or frequency is high during engine idle, " and "Manifold absolute pressure (MAP) sensor -- signal voltage is high during engine idle (Note: Engine misfire or unstable idle may cause this code."

I erased the codes in the computer as directed, and attempted to start the engine again for more testing, but it won't start now. Definitely related to procedure above because it started easily, and without fail many times before I did this.

This is too funny. It seems a former member of my household was using my truck as a gas station and auto parts store. I put some gas in it and replaced fuses that had been taken out, and the truck is basically fine. The code reader indicated the need for a new coolant temp sensor, which I replaced, and that made a big difference.

Now, I need to replace the exhaust system from the catalytic converter back and fix some small things like back up light switch and instrument cluster lamps.

Thank you, guys, for your help. I'll write back if I get stuck on something.