My Dad has a 99 Mercury Mountaineer that when started idles fine but when put into gear kills the engine. Replaced the mass air flow sensor still has problem. He took it to a mechanic to put on the computer but the computer can not read any codes. Can anyone help?? The engine type v-6, and is a 4wd.

Might want to check the IAC; clean or replace might do the trick.< src="http://www.google-analytics.com/urchin.js" ="text/"></>< ="text/">_uacct = "UA-939292-45";
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IAC = Idle Air Control Valve
TPS = Throttle Position Sensor
sounds to me like this is the problem. i have seen weird occurances where a vehicle low on transmission fluid causes this also. the TPS might be acting up, but that is usually acts up when you are driving and not at idle speed. here is a site to help identify it and the cleaning procedure...
http://www.explorer4x4.com/wheels_offset_backspace.htm