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Old 06-13-2019, 11:24 PM   #1
ben325I
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Ongoing RPM dip issue SOLVED!!!!!!

So for the past few years, my 5spd 325I have been having issues where my RPMS would drop below 500 rpms whenever I press on the clutch before immediately picking back up. Turning the wheel ever so slightly to where you can here the pump would also cause the rpms to drop slightly. Blipping the throttle while in neutral would also cause the rpms to drop for a split second before picking back up. It seem to do it under any electrical load. I've learned to deal with it to the point that I've accepted it as being normal for a 17 year old car. The car was purchased new in Sept 2002 and now has 135k miles.

Things I've replaced or have done during the past 20k miles/3years not in any paticular order....

Coils/plugs
MAF
Fuel Pump/fuel filter
Cooling system refresh
complete CCV along w/ boots/vacuum lines
OFHG
Valve Cover gasket / Vanos lines
Belts/pulleys/hoses
New Battery (2 months old)

So yesterday morning the car stalled pulling out of my garage. I decided to take it to a shop rather than have to tear everything apart once again not knowing exactly where to start. I took it to my local indy shop in Fullerton, Ca and dropped it off. Told him to keep it for as long as he needs it. I get a call 2 hours later saying car is fixed and to pick it up.

This is what it says on the repair order...
"No DTC stored. Checked injectors and clean fuel injection system through idle circuit. Reset DME adaptation values and performed hard reset of electrical system."

Best $100.00 I ever spent on the car. Car revs freely and strongly. Throttle response is spot on. And best of all, no more dropping of engine speed below 500rpms. Rev it and let go, the needle holds steady at around 650-700rpm free of any erratic movements. No more anxiety over this issue.

Could it have been this easy all along? I'm glad I didn't throw parts at it and only replaced what needed to be replaced but I could see home mechanics replacing parts unnecessarily over this. My drive by cable 323i doesn't have this issue and now it makes sense. I'm enjoying driving my car again. Anyway, just thought I'd share this and hopefully help someone experiencing what I experienced.

Cheers
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Old 06-13-2019, 11:28 PM   #2
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The only thing in that list that sounds likely to fix the problem is cleaning the idle circuit, presumably the ICV (Idle Control Valve).
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Old 06-13-2019, 11:29 PM   #3
ben325I
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The only thing in that list that sounds likely to fix the problem is cleaning the idle circuit, presumably the ICV (Idle Control Valve).
But I've done that personally and didn't fix it.

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Old 06-14-2019, 11:50 AM   #4
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so what exactly is a "hard reset of the electrical system" - I'm not following. This is not a PC it's a car.
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Old 06-14-2019, 11:56 AM   #5
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Give it a few days and then report back. What are your fuel trims did you ever chrck them?
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Old 06-15-2019, 06:36 AM   #6
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so what exactly is a "hard reset of the electrical system" - I'm not following. This is not a PC it's a car.
Disconnect negative cable from battery and touch it to the positive cable to drain off all residual electrical charge. Makes sure all control modules get a hard reset.
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Old 06-15-2019, 06:40 AM   #7
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Ben, it's a good idea to update your DME software to the latest. See my thread for the details.
https://forum.e46fanatics.com/showthread.php?t=1081716
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Old 06-16-2019, 12:29 PM   #8
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I don't have the necessary tools to check fuel trims but I'm happy to report the car is driving like a new e46 just as I remembered driving it off the lot 17 years ago
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Old 06-16-2019, 01:24 PM   #9
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Disconnect negative cable from battery and touch it to the positive cable to drain off all residual electrical charge. Makes sure all control modules get a hard reset.
That's pointless, especially the part about touching the positive cable.

Most modules are diode protected against reverse voltage. You aren't going to discharge their capacitors by touching cables. Anyone that suggests touching cables is working from superstition, not knowledge.

Generally the processors on the modules save their important state to non-volatile memory (e.g. EEPROM) immediately when the car is turned off. They shutdown completely a little less than 20 minutes after turning off the car. On a typical module the only part that remains powered is a bus receiver that turns on main power when it sees data on the communication bus. When the modules wake up they reinitialize everything and load their state from non-volatile memory.

There are ways the system can screw up, especially body modules saving incorrect state when the battery slowly dies with repeated bus wake-ups (opening doors and trunk, etc). But those problems don't apply to the DME.
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Old 06-16-2019, 03:11 PM   #10
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That's pointless, especially the part about touching the positive cable.

Most modules are diode protected against reverse voltage. You aren't going to discharge their capacitors by touching cables. Anyone that suggests touching cables is working from superstition, not knowledge.

Generally the processors on the modules save their important state to non-volatile memory (e.g. EEPROM) immediately when the car is turned off. They shutdown completely a little less than 20 minutes after turning off the car. On a typical module the only part that remains powered is a bus receiver that turns on main power when it sees data on the communication bus. When the modules wake up they reinitialize everything and load their state from non-volatile memory.

There are ways the system can screw up, especially body modules saving incorrect state when the battery slowly dies with repeated bus wake-ups (opening doors and trunk, etc). But those problems don't apply to the DME.
Yeah, hard to say what OPís tech was doing and why. Agree that the ICV cleaning may have been the fix.

I have had multiple occasions where a DME or EGS lost its mind and no longer responded, needing a full system power down to clear it out. Shorting power to ground does that, and the easiest way is pulling the negative battery cable off and touching it to the positive cable.

If youíve done embedded systems design and firmware you know about the sweet release of a hard reset.
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Old 06-16-2019, 03:54 PM   #11
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Yeah, hard to say what OPís tech was doing and why. Agree that the ICV cleaning may have been the fix.

I have had multiple occasions where a DME or EGS lost its mind and no longer responded, needing a full system power down to clear it out. Shorting power to ground does that, and the easiest way is pulling the negative battery cable off and touching it to the positive cable.

If youíve done embedded systems design and firmware you know about the sweet release of a hard reset.
Again, shorting the battery cables *does nothing*.

You could make an argument that simply removing power might do something. For instance, latch-up is very difficult to confirm or disprove, but there isn't evidence to suggest that the DME has issues that can be solved by power cycling.

I've done my fair share of microcontroller based system design, both hardware and software. And more than my fair share of OS-level code on larger systems.
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Old Today, 08:45 PM   #12
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I don't have the necessary tools to check fuel trims but I'm happy to report the car is driving like a new e46 just as I remembered driving it off the lot 17 years ago


Fuel trims are like blood pressure: too high and thatís a sign of deeper problems.

Too low and your car just faints!
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