|
In Reply to: Whats the real idle speed ? posted by matt evans on October 18, 2001 at 16:17:13:
I have a lightweight flywheel.
I am getting the impression that the warm idle should be 950. Thus, i have a _very_ low warm idle.
My cold idle is around 1100.
Do you suppose its the flywheel doing this ? Or should I look to something else ?
Thanks,
Matt
1) Don't replace the ICV with a manual unit because you will loose the coast off.
2) Since the 88, M5/M6 is without an "Check Engine" light how will you know when the DME 02 circuit is operating open loop?
3) Since 88, M5/M6 S38 also has a closed loop Idle control circuit that performs functions such as "Coast-Off" and has an idle adjustment bypass port how can the proper open loop idle speed be set?
4) Is your idle rough?
5) Does your engine occasionally die when coasting down to idle?
Make no mistake, this is not the end-all, be-all for idle problems. The Bosch X5DC plugs should be replaced at every Inspection (15K miles), the distributer cap & rotor should be replaced at 60K miles and the you will want to check the vacuum at each throttle body to ensure the throttles are evenly set(use a check valve in series with your vacuum gauge). You could also have a leaky injector that will raise havoc with Idle performance.
The S38B35 engines have a manual idle speed set-point adjustment on the 3-4 throttle body and an manual CO adjustment on the AFM that set the DME closed-loop for the O2 sensor at idle. 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 performance.
Fuel blend changes (winter/summer, MTBE, etc.) create a need to reset the CO and idle adjustment. The more sophisticated Bosch DME's 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 "Check Engine" light, it would have been lit!! When I first purchased my M6 it would occasionally die. Since I correctly set-up the ICV that problem is gone.
Attempts to set the idle without using a Throttle Switch Bridge will produce mediocre results at best (you'll just be guessing). Adjusting the CO screw on the AFM without monitoring the O2 sensor will cause the DME To operate open loop at idle.
Below are the basics to successfully setting the CO and open-loop Idle speed.
Tools:
Throttle Switch Bridge, BMW p/n 88 88 613 4 010(shorts all three pins together)
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.
Procedure:
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 pin 19 (gnd) on a 20 pin 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 average (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 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 open-loop idle
smoothness.
8) Shut-off engine, remove the Throttle Switch Bridge and re-connect the throttle switch.
Enjoy!!
© 1997 -
2008
Jump Internet Inc. All rights reserved.
