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87 Idle Intermittance In Closed Loop

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Hey guys,


I've got an 87 4.0 with the C101 connector. I'm having an issue where once the truck gets into closed loop, the idle will randomly act up.


It runs "Totally" fine when in open, or if I unplug an engine management sensor. However my mileage is horrendous when I do that.


There is no evidence of a vaccum leak, it NEVER idles incorrectly in open loop.


I'm kind of at a loss, the idle varies from 1000-1400rpm once the problem starts. Sometimes it drops sometimes it creeps higher the longer it sits.


I've adjusted the brand new TPS(which I'm about to return), and yes I have verified I'm getting a good ground at the dipstick tube.


I've got a new O2 sensor, and the IAC seems to be in good working order due to it not having this problem outside of closed loop.


My next plan of action is maybe the IAC isn't grounding properly? But I have no way of determining if I can ground that to the block or if it goes through the computer.


Normally in the summer I just unplug everything and run it like nothing happened. But it's been extremely cold here in jersey and I'm not having any luck.



Any suggestions?

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