knocking valves

My 91 with a 3.0 won't burn 87 octane gas without the valves knocking regularly. I can put 89 octane and it runs fine. Anyone got any ideas on how to fix the problem? I've had the injectors cleaned recently and that didn't help.

sounds like your timing is off. If the timing automaticly adjusts, it would be a bad Crank position sensor.