I have an 85' 300 diesel, non-turbo. It has 204,745mi on it as of today. I change my oil (have always used Mobil 1 full synthetic)every 3000mi with filter, and air filter every 6000. The fuel filters are <1000mi old.

My question is: Lately the engine will begin to lope (idle roughly) after extended driving (over 10mi) when the engine is at full operating temperature. Please note that idle speed does not affect the problem, as in this car there is a knob on the dash that controls idle speed. This problem is troublesome as it causes the car to shake while idling in gear. So far it has never stalled. What would cause this problem? I am confused because there is no problem when the engine is cool regardless of engine speed.

Could it be the injectors, the fuel pump, the fuel pump start-of-delivery adjustment, or transmission related? Also, I thought it could be something to do with the timing chain, as it could expand as the engine warms up. Please let me know if any of these could be the cause, or if I neglected some possibility.

TIA
Dr. Adam T. L. Delecki