I have an 87 Dakota that uses the Holley 6280 feedback carburetor.
Recently the warm idle speed has been inconsistent. It switches back and forth between 2 distinct idle speeds. There is no in between, either too fast or too slow. When slowing down or stopping, the engine will stall if it is idling in the "too slow" mode. If in the "too fast" mode, it will jump off the line when I release the brake.
After researching the symptoms, I found that the duty-cycle solenoid in this carburetor alternately leans and enriches the idle mixture, depending on a signal that originates with the oxygen sensor. I disconnected the solenoid and the problem vanished.
Since the problem does not occur until after the engine warms up, likely culprits that I can think of include the engine temperature sensor that informs the Spark Control Module (SCM) that the engine has warmed up, the oxygen sensor or the SCM itself.
I checked the engine temperature sensor and the resistance varied between about
1000 ohms when fully warmed up to about 6000 ohms when cold. The Haynes manual does not spec the range of values for this component so I don't know if it is operating within the correct range, but it does change with the temperature.I have not been able to discern a pattern to the change in idle speed. It appears to be completely erratic.
Does anyone here have any experience with this condition and, if so, what suggestions do you have for isolating the source of this problem?
Thanks, Jack