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This is something I posted in response to someone else's problem a while ago that may help you:
I have a 93 which had the same problem, plus a part throtle miss at cruise. I've developed a kind of check list approach that I'll share. From my investigation, an idle problem can come from four possible sources: (1) vacuum leaks, (2) ignition/electrical problems,(3)fuel delivery problems, and (4) sensors/ECU/AMM problems. I solved my problem through the process of elimination by checking the four areas in the above order, which starts from easiest and/or cheapest to hardest and most expensive. I first checked for vacuum leaks. Pay particular attention to the intake manifold and the molded hose from the flame trap to the breather box, below the intake manifold, and the lines to and from the idle air controller (IAC). While you're at it, check the hard plastic lines going from the intake manifold to the charcoal cannister below the right front fender. Mine were bad, but this didn't solve the problem. I used both the soapy water method and the propane method. Don't use chemicals which could damage the O2 sensor. I then checked the ignition, and did the same thing you did (plugs wires, rotor, cap), plus I cleaned with electrical contact cleaner (CRC from parts store) all the electrical connections and applied dielectrical grease (but NOT on the 02 sensor connections). Also, check for proper grounds, especially on the fuel rail, and check the large-amp fuses on the + battery terminal. Sometimes these fuses can become burned, but still not blow. Next, I checked the various sensors. The first one was the engine coolant temperature sensor (ECT), which is next to the knock sensor under the intake manifold. You can suspend it in a bath of water and check the resistance to see if it is in spec. However, make sure you get the chart that's correct for your car. I believe the one in Bentley is for the earlier, blue-headed ECT. Mine turned out to be out of spec, but this didn't solve the problem. I also checked the throttle switch as per the Bentley procedure and adjusted it, just to be sure. A lot of folks recommend cleaning the IAC and I did this (even swapped in a new one), but that didn't help me. If you suspect a bad one, there's a neat trick that was posted here to test it by swapping a gate valve and adjusting it for a constant idle. I next checked the fuel delivery system by checking the running and residual fuel pressure as per the Bentley procedure. I also removed the
injectors and cleaned them using the Ponte method of using a battery charger, backup light bulb, and compressor, but be careful this is dangerous. I cleaned the throttle body and used a new gasket (also a good time to check the operation of the throttle switch). Lastly, as everything checked out I swapped in a rebuilt AMM and cured the problem. From what I've read, these usually fail in a
no-start mode, but mine must've been the exception. When I think back on it, my problem was intermittant, so maybe this was a give away. My 93 still has a slightly rough idle when warming up, but having a working thermostatically controlled flapper valve seems to help (I differ on this with most
others). Pls post how you eventually find your cure.
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