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I just had a similar problem after cleaning out my TB. It turned out that the TB plate was not shutting completely and air was leaking past the plate in idle mode. Test this condition by completely closing the idle adustment screw and start the vehicle. If the TB plate is completely closed and the idle adjustment screw is completely closed the vehicle should start, studder and die out. If not, your TB plate may be misadjusted. Take off the TB; looses the lock nut on the TB plate adjustment screw; and adjust (back off) the screw to allow the TB plate to close completely. Also check to verify all the TB mechanical parts are moving freely. Forward adjust the screw until it just contacts the TB plate contact and turn an additional 1/4 to 1/2 turn. Tighten lock nut carefully while not moving the adjustment screw. Check your TPS to ensure it clicks when moving the TB plate open and close. Reinstall, keep the idle adjustment screw completely closed (no gas pedal) and start the vehicle. It should now studder and die out. Open the idle adjustment screw to obtain the specification idle rpm (approximately 720 to 750 rpm).
If not, you may have air entering the intake manifold via a stuck idle air control valve; the intake manifold gasket, TB gasket or possibly through leaky vacuum hoses. You can plug the intake hose from the IAC valve by removing the this hose from the air intake hose connection and placing a sheet of plastic over the connection and reconnecting the IAC hose. Restart the engine & check results. The IAC valve should be fully closed at idle. While your in there, take off the intake air supply hose and check for cracks by bending the flexible hose. Replace if cracked. You're allowing excess air into the engine that is not accounted for by the AMM, resulting in poor engine performance. Lastly, check the thermistor in the air cleaner housing & replace if defective, and filter if necessary. Otherwise you will eventually fry your AMM. Good luck.
Rustydog
1986 240DL
148k
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