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I agree with Foster. Check the AMM, pressure regulator, and temp sensor (your car has 2 of them, one for the gauge on the dash and the other for the signal to the computer).
Also, check for proper operation of the Idle Air Control Valve (IAC) which ensures that the car has enough airflow to keep it idling when the butterfly/throttle body is closed (when your foot is not on the gas).
Looking at your engine from the right hand side of the car, look at the exhaust manifold. On the left side of it, just below the spot where the 4 tubes join into one, there should be a brass colored protrusion pointing at the firewall. This is your oxygen sensor. There are 3 wires coming from it that plug into the wiring harness along the firewall. The stand-alone wire is the oxygen sensor signal, and the other two wires that share a connector are power for the internal heater (warms up the sensor for quicker response when starting the car).
Take a voltmeter and a thin piece of wire, disconnect the oxygen sensor, and insert your small wire through the small hole in the sensor lead. Re-attach the sensor wire (so that you have a 'tap' off the signal line) and run the wire to the positive lead of your volt meter. (put the negative lead of the voltmeter to the negative battery terminal) You can now watch in real-time the behavior of your oxygen sensor without interupting the signal that's going to the computer.
Oxygen sensor trouble shooting: If the signal is around .3 volts, the sensor is seeing a LEAN mixture. If the sensor is around .9 volts, the sensor is seeing a RICH mixture.
If the sensor is sending a signal of around .1 to .3 volts and you know the mixture is rich, your sensor is probably toast.
If you are sure of a rich mixture, and the sensor shows .9 volts (which would be correct for a rich exhaust), then the problem is elsewhere (like a bad IAC, or a Throttle body with a lot of gunk in it, etc).
If the sensor is not the culprit (it's working just fine and showing a rich mixture), do a walkthrough and inspection of the air management route. Verify that your air filter is clean (replace if unsure), that all the connecting tubes between the intake (by the radiator) to the throttle body are clean and free of debris. Check the Air Mass Meter for proper impedance (see your Haynes or Bentley manual for specs), verify that your IAC is working properly (I had to remove mine from it's bracket, plug the tubes, and look through it with the wires still attached and the car running to verify that it was bad/closed). Clean out the throttle body with some strong carb cleaner. Note: Gumout carb cleaner may not be strong enough to do the job, try using Pyroil Carb cleaner (costs twice as much but it's 10 times stronger, believe me, it works!).
If you're having trouble cleaning the throttle body, you may need to remove it to do it thouroughly (unbolt it from the intake manifold). IPD sells a kit just for this purpose ($18) that has a new gasket, a brush, instructions, and a can of some strong carb cleaner. The part number for the kit is CU2K1124 and it's in their catalog.
Assuming you have a good working Oxygen sensor, you'll know when you've found the problem. Keep cleaning and fixing things until your O2 volt readings start fluctuating around .5 volts or so. It's ok for it to jump around, as long as it's averaging towards the middle. The Bentley manual should have more info for you.
Good luck and God bless,
Fitz Fitzgerald.
P.S. Always set up your voltmeter before hooking it up to the oxygen sensor. If you turn it on and flip past a Current/Amp setting, you can cause serious damage to the sensor.
P.P.S. After you see improvement, set your mixture by folowing the procedures listed in your service manual. The adjustment knob is the black circular knob on the throttle body, but you must follow the instructions since you need to have the IAC in the closed position when making this adjustment. (which is done by grounding a test lead wire in the engine bay)
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'87 Blue 245, NA 214K
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