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In Reply to: Jekyll and Hyde driveabilty - or am I nuts? posted by Todd Kenyon on November 03, 2001 at 20:30:18:
But wait, there's more. On any given drive, I notice a fairly subtle but clear to me difference in the way it drives. Some trips, it feels heavy, engine sounds louder, rougher, under load. Throttle response seems sluggish. Other times, it drives beautifully. But this is the kind of thing where if you don't drive the car all the time, you may not notice it.
SO I am convinced these problems are related somehow. I am also convinced I am not imagining the warm driveabilty changes. I have not tested fuel pressure, but reg and filter are nearly new. I have also not pulled injectors, but sniffer gave good HC readings, meaning probably not a leaky injector. But maybe a bad spray?
There's even more. The voltage required to make a spark is high, like 30kV versus normal 8-10kV, yet I am getting a good burn (HC reading). But maybe this points to a clogged injector? Cap, rotor, and wires are good, plugs new.
One mechanic (both guys honest, experienced) thinks its probably not injectors, other thinks could well be. Does any of this ring any bells for anyone? WHy would it drive fine sometimes, bad other times (subtle) and almost always be crappy if completely cold....
If anyone actually gives me a lead that works, the car is yours. Well, ok maybe not, but would happily ship beer "care package" your way...
I suggest checking the O2 sensor function by way of the Diagnostic connector pin 7(described below). I suspect your engine isn't always operating closed loop and since the M5 doesn't have an engine check light this is the best way to quickly check the DME/O2 closed loop function. It is also possible your AFM is malfunctioning(that is the bucking) or one of your injectors is leaking from time to time. Have you checked the fly wheel/crank sensors to make sure you have a good signal?
Basic idle and O2 setup.
The S38B35 engines have a manual idle speed set-point adjustment(brass screw) 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 (brass screw) 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.
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