'84 300D with 375M. The engine just started pinging (pre-ignition) a week or 2 ago. I haven't changed fuel sources. Doesn't use oil or have a water leak. Pings mostly off idle when engine is cold for first 5 minutes (cold in So. Cal is 55 degrees). I use a diesel Bio-cide from Racor to prevent algae (had a clogged tank screen 6 months ago). Absolutley no change in power at freeway speeds when warm. That's OK. To me Off idle is 800 to 1500 RPM. Injection Pump timing has never been touched since I have owned the car (bought w/81M). Any Ideas/