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I have a 6.5 diesel with the electric injector pump. It has been a good engine. Lately it starts hard. High idle works after I tap the accelerator after it has started. What gives the engine high idle at start-up? Except for this issue it runs great.
Welcome to the forums!
Two things come to mind. A failed/failing engine temp sensor, or an injection pump that's trying to leave you. The timing and engine speed are PCM controlled, but are dependent on sensor data, and an injection pump that responds to input. I suggest testing (or simply replace) the senders, first. Pump diagnostic and repair is a different ball of wax, and will quickly dig deep into your wallet, so save that for last.
Is the "Service Engine Soon" light on?
Do you have access to a scan tool?
A scan tool can tell you what the sensors are reading and sending to the PCM.
A quick test of the ECT sensor is to simply unplug it. This is the sensor on the cross over/thermostat housing, not the one on the driver side head. With this sensor disconnected, the PCM should read -40F engine coolant temperature and provide maximum glow time, maximum cold high idle and other parameters for cold starting. If it starts better with the sensor disconnected, replace the sensor.