Over the past few months I've noticed an increased tendency for engine knock, especially at highway speeds. Yesterday with ambient temp around 100F the knocking was so bad I had to get off the highway and take local roads, fill the tank with 91 octane (normally run 89). When I got home I checked the O2 sensor output and was shocked to see it going negative! So that guy is getting replaced shortly. I have not had time to check for any codes yet, but that's next on the agenda.

When running well, I could easily make the car knock by punching the throttle in high gear. The knock would last about a second, then go away. Currently the knock is not abating, and occurs even at part-throttle.

The subject is a '92 with M47 and LH 3.0.

What concerns me a bit is that I thought an oxygen sensor failure would result in an over-rich mixture, and with all the engine knock going on that's surely not the case. A knock sensor failure should result in timing retarded enough to notice a decrease in performance, and that hasn't happened. A coolant sensor failure should result in an overall too-tich mixture at ambient temps over 60F or so.

So I'm not sure what else is going on, and what else I should be looking at. Any suggestions or comments?