Markolc

My '86 Pontiac Bonneville (LG4 motor) is throwing codes 23/45/and occaisional 54. All this relates to the MCS. With the key in the on position, the MCS does not click. It used to, but for some reason, doesn't anymore. The ony way my car will run is to unplug the wiring harness off the MCS but then I get absolutely horrible gas mileage. The MCS only clicks when I plug the wiring harness in and out of the MCS manually very rapidly, so the pink wire has 12 volts no problem. The MCS will NOT click howevcer when pligged in and I try to ground the bright green dwell connector. Can there be another electrical part on the car causing this MCS not to work anymore (it seems like it grounds, and stays grounded completely flooding out the motor when connected), or is the ECU now screwed? I tried running a brand new Lt blue wire form the ECU to the MCS, but with absolutely no luck. Any advice? Money is tight and $50 for a used ECU right now isn't looking too great, and getting MPG in the single digits isn't great either. PLEASE HELP!!!

Vader

Mark,

All is not lost. The MC operates based on the ECM. The ECM controls the MC based on several inputs, maintly the TPS, MAP/BARO, and O² sensors. The ECM will try to satisfy the O² sensor above all else, and if the sensor is old and weak, the ECM will just keep adjusting the mmixture richer and richer until the MC is open all the time (no duty-cycling).

There are a few things you need to verify:

1. The TPS is set a 0.48VDC at warm idle;

2. The MAP/BARO sensor(s) are signalling correctly;

3. The O² sensor is generating the appropriate signal;

4. The main metering well plugs are not leaking, causing a rich mixture for which the ECM cannot compensate.

For instructions on setting the TPS and MC solenoid, check this file:

E4ME Adjustment.pdf

But before you do, test your input sensors as follows:

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MAP/BARO Test

For testing the MAP/BARO sensor, you'll need a vacuum pump, or an accurate vacuum gauge and a good vacuum source (a running engine). You'll also need an accurate voltmeter. The sensor should output a 0-5VDC signal at the 'B' terminal. The output voltage is based on absolute pressure at the hose connection. The sensor should output full voltage (5.0VDC) when there is one BAR (barometer, or atmosphere) of pressure applied. This means there is basically no vacuum (0" Hg) and the Absolute Pressure is at 14.7 PSIA.

When the Absolute Pressure decreases (due to vacuum) the voltage output on the 'B' terminal should decrease as well. Test the sensor down to about 20" Hg of vacuum, or 0.33 BAR (4.9 PSIA), where the output voltage should be aroud 0.8-1.2 VDC. The voltage should vary in direct proportion to the Absolute Pressure, or amount of vacuum applied. If the MAP output "jumps" or "sticks" at a given point, the sensor bridge circuit or the mechanical diaphragm is failing. Since there is no good way to repair a MAP sensor, replacement would be indicated.

Of course, when installed in the car, the hose connecting the sensor to the intake manifold needs to be intact, free from cracks, leaks, splits, or kinks. The electrical connector needs to be clean and tightly seated.

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Oxygen Sensor Test

If you still have the DMM at your disposal, you can test the oxygen sensor fairly easily. Remove the sensor and connect the voltmeter on a 2.0VDC scale. Hold the sensing element end in the flame of a propane torch while measuring voltage. The sensor should produce a signal of at least 0.75VDC within 60 seconds. Removing the flame should drop the sensor output to less than 0.10VDC. This does not test the quality of the sensor, but will tell you if it is completely "dead" or not. A propane torch flame is nearly devoid of all oxygen and should produce a fairly high signal as a result, just like a very rich mixture. If you don't get the voltage, your sensor may be toast, or contaminated with coolant, carbon, incompatible RTV curing agents, or oil soot.

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If the above steps reveal no sensor problems, you may have a fuel leak from the bottom of the carburetor body. The two main metering wells have plugs staked into the drilled passages on the underside of the body. These plugs can eventually leak, causing uncontrollable rich mixture. To check this, you'll need to remove and invert the carburetor. Inspect the plugs for signs of fuel leakage (wet spots, fuel additive contamination, excessively clean compared to the surrounding area). Stake the plugs in lightly but firmly with a small hammer and sharp punch or chisel. Clean the area suppounding the plugs, and seal over them with any metal- or polyester-filled epoxy made to adhere to metal, such as JB Weld. Once the epoxy has cured and you're satisfied there are no leaks, reinstall the carburetor and proceed with the adjustment. If you do have to remove the carb, it would be advisable to remove the mixture screw covers while it's off, just in case the MC solenoid adjustment range is inadequate.

I hope that gets you closer to a solution. Please keep us posted on your progress.

SkyWarrior

check for mcs travel. stick a small lolly pop stick, or the like, into one of the small holes next to the idle air bleed valve. the hole should be just to the right and tward the front of the car from the iabv(its the big screw looking thing in the top of the airhorn, should be brass in color, anyway gentley put this stick in and you should hit the mcs plunger,(you can use a flash light to look inside, you should see a little metal plate lookong thing) and you should be able to push it down. If you can't push it down, then your rich stop it too low and needs to be adjusted, if you can push it down, you need to relace the selenoid.

Markolc

Thanks guys. It was the fault of the ECM. I replaced the ECM and the car worked perfect again. Thanks for the suggestions!