I have an '89 S-10 with the 4 cylinder motor. I just bought this truck a few months ago. The owner told me that it would run rough and bog, like it is towing a heavy load, then all of a sudden it would straiten out and run good. He said he was sure that the cat was plugged up. I noticed that it runs rough every time you take off from a stop, but then the "check engine light" comes on. Once the light comes on it runs fine. Occasionally at highway speeds it will start to act up- at which point the light goes out. As long as the light is on, it runs good. Well, the first thing I did (and I know it's wrong) was replace the cat with a strait-pipe. The situation did not change. After talking to a few people, the general opinion was that the O2 sensor was the culprit. When I ckecked the sensor, the wire was completely seperated from the sensor. I thought "good, I found the problem." So, I went out and got a new sensor. The situation still remains unchanged. Well, now after a month of dealing with it, it is running rougher than ever. It won't idle on it's own and the RPM's are very low when it is trying to idle. While driving, it is very rough, and it spits and sputters like it's running out of gas. When I come to a stop, it dies. It does start back up easily, but it is hard to try to keep it from dying on ice-covered roads with a manual tranny. Any ideas on what to look at next?