In Reply to: High/varying idle, usu. way above 1000 rpm...?? posted by JamesM on November 14, 2000 at 14:27:57:
The problem you may also be experiencing could be a need to reset the CO and base Idle speed, with no other sensor/switch malfunctions and/or adjustments required. If an attempt is made to set the idle by simply adjusting the Idle speed screw, other poor performance symptoms may occur, such as causing the engine to die when you lift off the throttle. Specifically, adjusting the idle without going through the procedure could affect coast-off performance or idle performance during engine warm-up.
I on the Digest(around March 19, 1999) for setting the idle on the M5 & M6 S38B35 engines. This procedure was also written-up in the Roundel in 1996(?) for a S14 which has the same idle system as the S38B35.
The S38B35 engines have a manual idle speed set-point adjustment. I have found an occasional need(3 times over the last 7 years) to reset the CO adjustment on the Air Fuel Meter and open loop Idle speed in-order recover normal Idle. Fuel blend changes (winter/summer, MTBE, etc.) create a need to reset the CO and idle adjustment. The more sophisticated Bosch DME's with a hot wire MAF accomplishes this as a learning function in the CO & idle compensation circuit in the DME. In most cases when I have noticed a high idle condition the DME is actually functioning open loop. Therefore, if the S38B35 engines had an engine check light, it would have been lit!!
The three pins of the throttle switch connector can be shorted together as a substitute for a Throttle Switch Bridge. Myself, I prefer the simplicity and convenience of using the BMW Throttle Switch Bridge tool.
-----message I sent March 19, 1999 to BMW UUC Digest---------
The 88, M6 S38 has an Idle control valve and there is an air bypass port that is used to sets the median(closed loop) idle speed.
Attempts to set the idle without using a Throttle Switch Bridge will produce mediocre results at best (you'll just be guessing).
Below are the basics to successfully setting the Idle.
Throttle Switch Bridge, BMW p/n 88 88 613 4 010
Clamp on Tachometer, Idle setting is 850 +/- 50 rpm's, Fluke 88, recording/averaging meter is the best,
Volt meter, recording/averaging Fluke 87 or 88 Set meter to; 40volt range, record/average.
1)Check to make sure the throttle switch closes just as the throttle butterfly's close (the closer this switch is set, the better the engine will transition between idle and non-idle throttle positions.
2) Connect the voltmeter between pin 5 (DME oxygen sensor signal) and 19 (gnd) at the Diagnostic Connector.
3) Start the engine, while the engine is warming up observes the voltage reading at pin 5 on the Diagnostic Connector.
4) Once the engine is warm the oxygen sensor signal should read between 5-7 volts (6 volts +/- 3 volt spec.). If you don't have an averaging volt meter, your volt meter will cycle every second or so between 0 volts and 12 volts the cycling should be 50% duty cycle. The cycling is demonstrating that your fuel injection system is running closed loop. If the oxygen sensor signal is not cycling then you will need to diagnose and repair the DME open loop problem. The problem could be something as simple as the CO adjustment on the Air Flow Meter.
5) Shut-off the engine and install the Throttle Switch Bridge on the cable connector for the throttle switch. The Throttle Switch Bridge allows the idle circuit in the DME to operate in open loop mode and sets the idle control valve a nominal position.
6) Start the engine, check to see that the oxygen sensor signal is reading 5-7 volts (after reset the recording function) or that the voltage is cycling at a 50% duty with a standard volt meter. (the engine will not rev or operate normally with the Throttle Switch Bridge installed)
7) Adjust the base Idle speed screw to 850 rpm's (large brass slot screw recessed in the middle of the 3-4 throttle body) using the record/ averaging function will help accurately set the idle speed as it fluctuates. The minimum and maximum record measurements on the Fluke will help you understand the idle smoothness.
8) Shut-off engine, remove the Throttle Switch Bridge and re-connect the throttle switch.