Mitsubishi Repair: 91 plymouth laser, mass airflow sensor, coolant lines

Question
i got a 91 plymouth laser rs 2.0 non turbo fwd and its got a idleing problem. it goes up and down and is between 1200-2000rmp. i have changed the mass airflow sensor checked the idle air control with the ohm meter it got 28-30 oms for all 4 coils. i have changed the egr valve, cleaned the throattle body, new fuel filter, checked and replaced the vacumm lines. could u help?

Answer
Check the throttle switch located on the throttlebody. It needs to be adjusted properly so that the PCM knows it is at idle. You can hear the switch click if it's set right, then check to see that it works electically.  Verify that there is a slight amount of free play in the throttle cable and then clean the throttle plate if you didn't do that when cleaning the throttle body. Readjust the idle using the manufacturers recommended procedure, as the PCM can only adjust the idle motor so much, and many people just screw the bypass air screw located on the throttlebody without care of the proper setting. Ideally, use a scan-tool to ensure that the idle motor count is sitting around 15 or so. I typically see these set at 0, which is where the computer has no more provision for rpm control, resulting in the idle going up until it hits over 1800rpm and then kills the injectors to drop the idle. Then repeat over and over again. This problem can also come from the mechanical cold idle valve that sits at the bottom of the throttlebody (has the 2 small coolant lines running to it). When this fails open, then the idle motor cannot control due to the high amount of bypass air the cold idle valve lets by. The PCM can also be defective, but you don't chase this until all the above has been verified and/or repaired.