So on an old school car, when trying to determine if the noise from the engine is a rod knock? You would pull off the spark plug boot from each cylinder one at a time. If the noise changed (compression with spark event, raising cylinder pressure) when removing the spark on a cylinder STOPS. It's likely a bad rod bearing.
Now that we have COP (coil on plug) you disconnect the connector from the coil pack on each cylinder.
The difference being: Now that we have OBD 2 systems, if you remove the connector the DME see's your actions and will usually leave that cylinder turned off. We don't want that, so we must take an extra step:
Now we can do the above test with a scanner hooked up:
Start engine, remove connector from #1. Noise still present?
Key On Engine off (KOEO) clear faults. Reconnecting #1, Start car and remove #2.
Keep repeating the above until you find one cylinder that makes the engine quieter.
Bad rod bearing on THAT cylinder. He got to the offending one with #1.
Most usually, the rod bearings that go bad first, are the two farthest back in the engine. Owing to distance from the oil pump/supply.
The connector in question is the connector for each ignition coil.
The hardest thing a rod bearing has to do is sustain the pressure generated during an ignition event. Aided naturally with a thin coating of oil.
When one cranks an engine; lets assume the plugs are installed but the connectors are removed from the ignition coils. The cylinder pressure when the valves are closed is about 160-185 PSI. The standard compression an engine makes bereft of a ignition event. Very easy for a rod bearing to handle that small load.
Now lets have the ignition coils hooked up, shall we. As each cylinder fires, the compressed air & atomized fuel now receives a spark to light things off. The pressure in each cylinder can be about 800- 1,200 PSI. (depending on C/R, cams etc) If a rod bearing is bad, the much increased pressure will make the bad rod bearing make noise. Owing to the (now) increased clearance of the bad bearing.
The OP was astute and he stopped, diagnosed properly and caught in in time before it destroyed the large end of the connecting rod. (Usually gets oval shaped) and the hardened journal of the crankshaft hadn't run long enough to receive egregious pounding (clearance) and depositing of hot & melted bearing material.
A forum community dedicated to BMW E46, E90, and F30 owners and enthusiasts. Come join the discussion about M performance, turbo kits, engine swaps, builds, modifications, classifieds, troubleshooting, maintenance, and more!