Rough Idle Help

Idle problems, such as surging or stumbling, are pretty common with 87-93 fuel injected 5.0s. This could be caused by a few things. Exhaust gas recirculation problems and bad throttle position sensors are two possible causes. The idle air bypass valve may also be the problem, and is easy to check. If you have rough and stumbling idle problems I strongly reccomend cleaning your idle air bypass valve first to see if this solves your idle issues. You may also want to try and readjust your idle.

Cleaning The Idle Air Bypass Valve

First locate the idle air bypass and disconnect it from the wiring harness. If you aren't sure what the idle air bypass looks like, use the picture below this paragraph. Now that the wires are disconnected, remove the two bolts that are holding the IAB on the throttle body. Remove the two screws that attach the solenoid (the part that the wire connects to) to the valve. Use carburetor cleaner and thoroughly clean the bypass valve. Make sure the rod inside isn't stuck because of deposits. Once you have the valve clean, reattach the solenoid with the two screws you took out. Make sure the gasket between the throttle body and IAB still seems to be in good condition. If the gasket is okay, then reinstall the idle bypass using the bolts you removed earlier. Finally, reconnect the wiring harness. Hopefully, this will solve your idle problems.

Idle Reset And Adjustment

First you will need to disconnect the battery for 30 minutes or more. Doing this should erase the EEC-IV's memory. Now, disconnect the harness on the idle air bypass. Reconnect the battery, and start the engine. With the engine running, use a screwdriver to turn the stop or set screw on the throttle body. The picture below illustrates the location of the stop screw on my aftermaket throttle body. Turning the screw one way will raise idle speed, the other way should lower. Adjust the idle speed so the car is not struggling to run. Usually 5.0s will idle well around 800 RPMs stock. 5.0s with larger cams will probably need to be set around 900 RPMs. Once your idle is set the way you want it, shut off the engine. Reconnect the idle air bypass wiring connector. I reccomend checking, and setting the throttle position sensor voltage at this point. You can read the next section for instructions on setting the TPS voltage. Once the TPS voltage is taken care of, start the engine again. Let the car idle with no accessories on for a couple minutes. If you have an electric fan, watch your temperature and be sure it doesn't overheat. Now turn the car off for a few minutes. Restart the enigne and let it run for a couple minutes with every accessory turned on. If you have an automatic, make sure the car can go into gear without cutting out. Now shut the car off again. Letting the car idle with accessories off and on helps the EEC-IV learn the idle codes over again.

Checking And Setting TPS Voltage

First locate the throttle position sensor (TPS) on the top of the throttle body. Use the image below if neccessary. Now connect a digital volmeter to the + positive (green wire) and - negative (black wire) that come out of the TPS. Turn the ignition to key on, engine off. Now check the voltage reading between teh green and black wires with the voltmeter. The voltage should be around 0.96 to 0.98. If you don't get a reading from .96 ot .98v, you will need to adjust the sensor to achieve these numbers. To adjust the TPS voltage, loosen the two screws that hold the sensor in place. It should be lose enough that it will move, but not removed form the throttle body. Turn the TPS clockwise to increase and counterclockwise to decrease voltage. In some cases, you will need to make the screw holes larger in order to acheive the proper voltage. If neccessary, use a file or a small drill bit on the holes. Be very careful not to get too close to the edge or break any part of the sensor. Once you have the TPS voltage to the setting you desire, tighten the screws and recheck the voltage to be sure you didn't move anything while tightening the screws.