My '72 280SE with 2.8 engine has a perplexing noise upon startup. The car is a barn find that sat for 6 years before I got it. The car ran surprisingly well but would starve for fuel after a few miles. Repairs that I have made within the last few months are, refurb the gas tank, new sender, new elect fuel pump, changed fuel hoses, changed fuel filter, change oil & filter, adjusted the valves, replaced cap, rotor, plug wires & conveted to Pertronix. After doing that work the car ran better with plenty of power.
The problem is that the engine will make a knocking noise and will run rough upon startup after the car had sat for 30 to 45 minutes, after a spirited drive. In contrast the engine will start and run smooth and would not make the noise if started cold or if restarted within 10 minutes or so when warm. On every start the oil pressure goes immediately to 40lbs + hot or cold and the pressure is usually 40+ before the engine lights off.
At first I suspected a leaky injector was loading up a cylinder. I pulled the plugs and sure enough 1 was black while the remaining 5 had a nice color. I pulled all of the injectors and had them cleaned & pop tested. The pump shop confirmed that 1 injector did leak down over time but the cleaning resolved that problem. They further confirmed that the pop pressure was below 200lbs. The cleaned injectors were reinstalled. The engine ran much better but the starting rough running & noise problem persisted but all of the plugs are now the same color. I then eleminated the cold start injector by removing and plugging the fuel line. The problem persisted.
I finally broke down and installed 6 new injectors shice I wasn't pleased with the report about the original injectors. WOW what a difference in the way the car runs now. It's smoother, quieter and has a bunch more power. All of the plugs now have a light tan color. Unfortunately the noise at start up is still there.
I have considered the possibility of leaky valve stem seals but the car does not display the usual signs such ad blue smoke out the exhaust upon deceleration. The car does not use or leak oil and there is no evidence of oil on the plugs. I have also considered carbon buildup in the combustion chamber or the top of the pistons.
Any input would be greatly appreciated.
