I have a 1980 242DL with the B21F engine. At highway speeds (60-70 mph), and seemingly only at that speed range, I am getting what sounds like a spark knock or pinging when the engine is placed under a light to moderate load. The pinging becomes apparent when, while cruising, you accelerate slightly, such as going up an incline. If you accelerate more aggressively, the knocking/pinging ceases. Seems to be most prevalent around 3500 rpm. It does not happen at lower speeds in situations such as starting up from a standstill and even hauling heavy loads uphill at lower speeds.

In the recent past I have replaced injectors, fuel pressure regulator, filters (air & fuel), plugs, cap, rotor, O2 sensor, timing belt, properly adjusted timing, and properly adjusted CO and base idle. Also, throttle body and intake manifold have been thoroughly cleaned.

what else could be the cause ? Could it possibly be something like a weak fuel pump, or even a flaw in the O2 sensor circuit, so that even with the new sensor correct information is not getting through ?

Thanks, as always.

Mike