Setting Idle
09-13-2020, 09:58 AM
#1
Rennlist Member
Thread Starter
Setting Idle
1987 S4 automatic
I haven't done a lot of driving with my S4, just a trip to the gas station (2 mi round trip) and to the DMV for a title inspection (14 mi RT). During both trips, once the car warmed up, the idle was too low and she'd die at a stop. Drove smooth under regular acceleration. At that time, the cold idle was good. However, now the cold idle is low too. It searches a little bit, but for the most part is stable, just low. I've searched but haven't found a definitive way to set it. So any thoughts on how to get it to the proper level? Do I just need to drive it a bit more?
I haven't done a lot of driving with my S4, just a trip to the gas station (2 mi round trip) and to the DMV for a title inspection (14 mi RT). During both trips, once the car warmed up, the idle was too low and she'd die at a stop. Drove smooth under regular acceleration. At that time, the cold idle was good. However, now the cold idle is low too. It searches a little bit, but for the most part is stable, just low. I've searched but haven't found a definitive way to set it. So any thoughts on how to get it to the proper level? Do I just need to drive it a bit more?
09-13-2020, 12:13 PM
#3
Rennlist Member
Thread Starter
Switching the AC on does not make a difference, but the AC isn't working - the compressor doesn't kick on. I haven't gotten to that part of the restoration but I imagine the freon is long gone.
09-13-2020, 12:22 PM
#4
Rennlist Member
Idle speed is controlled by the LH computer and the ISV (idle stabilizer valve) under the intake-- a variable air valve. LH measures RPM and regulates the idle speed by opening or closing the ISV as needed. So if the idle speed is not right, then something else is wrong-- typically a lean fuel mixture, or possibly the ISV or idle-switch as Fred mentioned. .
Mixture is also regulated by the LH, which takes an airflow measurement from the MAF and the engine RPM and computes how much fuel is needed to go with the measured airflow. The LH also monitors the mixture in the tailpipe with an O2-sensor which measures the air/fuel ratio, which allows the LH to make fuel adjustments as the engine runs.
Both the MAF and the O2 sensor will tend to cause a lean mixture as they age, and both are good for 60-80K miles. So depending on the history of the car, either (or both) would be the primary suspect. O2-sensors are relatively cheap, it is a Bosch 13048 Oxygen Sensor and screws into the top of the cross-pipe just forward of the cats. Unless it has been changed already I would replace that first, then if the problem persists get the MAF checked or replaced.
If you want to approach this scientifically, grab your multimeter and find the disconnect for the O2-sensor in the CE-panel area-- it will be a rubber 3-pin connector. The two white wires on the sensor side are the heater, the black wire is the signal. Warm the engine up, measure the voltage on the black connection-- you can stick the meter probe into the back of the connector and make contact, and connect the black to chassis ground somewhere. The sensor signal is in the 0-1 volt range, a lean mixture will be around 0.2v, and a rich mixture will be around 0.7v. The actual voltage doesn't matter, but if everything is happy then you should see it oscillate between the low (lean) and high (rich) reading once or twice a second as the LH "hunts" for the proper mixture. If it is stuck at the low voltage and the idle is weak, then it is likely MAF. If it tracks up and down normally but the idle is weak then likely the O2 sensor.
After replacing either MAF or O2 sensor, disconnect the battery for a moment to reset the LH adaptation so that it can "learn" the new sensor.
Mixture is also regulated by the LH, which takes an airflow measurement from the MAF and the engine RPM and computes how much fuel is needed to go with the measured airflow. The LH also monitors the mixture in the tailpipe with an O2-sensor which measures the air/fuel ratio, which allows the LH to make fuel adjustments as the engine runs.
Both the MAF and the O2 sensor will tend to cause a lean mixture as they age, and both are good for 60-80K miles. So depending on the history of the car, either (or both) would be the primary suspect. O2-sensors are relatively cheap, it is a Bosch 13048 Oxygen Sensor and screws into the top of the cross-pipe just forward of the cats. Unless it has been changed already I would replace that first, then if the problem persists get the MAF checked or replaced.
If you want to approach this scientifically, grab your multimeter and find the disconnect for the O2-sensor in the CE-panel area-- it will be a rubber 3-pin connector. The two white wires on the sensor side are the heater, the black wire is the signal. Warm the engine up, measure the voltage on the black connection-- you can stick the meter probe into the back of the connector and make contact, and connect the black to chassis ground somewhere. The sensor signal is in the 0-1 volt range, a lean mixture will be around 0.2v, and a rich mixture will be around 0.7v. The actual voltage doesn't matter, but if everything is happy then you should see it oscillate between the low (lean) and high (rich) reading once or twice a second as the LH "hunts" for the proper mixture. If it is stuck at the low voltage and the idle is weak, then it is likely MAF. If it tracks up and down normally but the idle is weak then likely the O2 sensor.
After replacing either MAF or O2 sensor, disconnect the battery for a moment to reset the LH adaptation so that it can "learn" the new sensor.