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Old 06-19-2017, 05:29 AM   #1
canosoto86
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Question about rpm issue

So my 330i while driving without cruise control on, the RPMs go down and then up at about 400rpm intervals, it get really annoying.
I was beginning to believe it was torque converter, but I noticed that whenever I use the cruise control the car drives perfect without any RPM fluctuation whatsoever.

Any idea what can be causing this? I have no codes, I already scan the whole thing with PA soft, INPA and nothing there either.

-Can this be caused by:
A-Throttle Body?
B-Transmission Input and Output speed sensors?
C-Anythibg else you may believe to cause this issue?

Thanks in advance.
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Old 06-19-2017, 12:34 PM   #2
pgib8
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can you turn off your DSC and see if it still does it, just out of curiosity?
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Old 06-19-2017, 12:52 PM   #3
jdstrickland
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If the car does not change the engine speed when the Cruise is on, then any change that comes when it is off has to be related to your foot. The 330 uses a fly-by-wire (electronic throttle control) where the gas pedal sends a signal to the throttle body to tell it to open or close depending on what your foot is doing. If you foot is doing nothing, then I'd guess that the sensor on the pedal or the sensor on the throttle body is to blame.

The pedal has a position sensor that the computer counts pulses to arrive at a position. The throttle body has a stepper motor that moves whatever number of pulses that the computer tells it to move. The stepper motor holds the position, 200 pulses for example, until the computer tells it to add or subtract 50 pulses from the current position. I don't know what the numbers are, I'm only trying to illustrate what's happening. The gas pedal parks at zero, for example. You push the pedal and number of pulses goes to the computer. The computer tells the throttle plate to open the amount that correlates to the number of pulses. All of this should feed through the OBD II diagnostic control, so if there is an issue of counting or performing pulses, the OBD II should be giving you an error code.

The Cruise goes to whatever position it needs and holds that position. I'm not sure how it controls the increase or decrease in throttle position that results from arriving at a hill, either up or down hill, to hold the set speed. The cruise accomplishes an increase or decrease in throttle position without regard to what your foot is telling the computer to do. This should narrow the diagnostic effort to the throttle pedal position sensor.

Another input is from the MAF. It tells the computer what the air density is. The air can be cold and dense or warm and thin, and this makes a difference in the fuel trim (air/fuel ratio), that if the trim is constantly adjusting then the engine speed can be changing too. The argument against this diagnosis is that it should affect the Cruise also. Since the cruise is not affected, then the MAF should not be a player. This brings me back to the gas pedal position sensor. The gas pedal position is used for driving without the Cruise, and should be over-ridden when the Cruise is being used.
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Old 06-20-2017, 07:27 AM   #4
markusmarkus
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Scan the engine computer (DME) and the tranny computer (EGS) for codes. The SES light does not have to be on for codes to be present. The SES light is on only when emissions related codes are present.
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Old 06-20-2017, 07:42 AM   #5
canosoto86
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pgib8: DCS does no difference, only when the cruise control in engaged that the rpm are fully stable and normal.

Markusmarkus: I have scanned using PAsoft, INPA and have no codes or errors related.

Jdstrickland: Thats my guess is something with the throttle, between the pedal and throttle body, was leaning more towards the foot sensor as the throttle was replaced, Also MAF is new OEM, I changed it when I replaced the engine 4 months ago.
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Old 06-20-2017, 08:11 AM   #6
Yewzer B Lewzer
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Speeds below when the Torque Converter locks you will see the fluctuation. Above you will not. The speed is roughly 60MPH but can vary based on your throttle inputs. Go on the highway at 65-70 and drive gently like the CC would and see if the TC locks. Your RPM should be more stable then.

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Last edited by Yewzer B Lewzer; 06-20-2017 at 08:12 AM.
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Old 06-20-2017, 08:32 AM   #7
canosoto86
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As mention earlier, with cruise control there is no RPM issue whatsoever, i can set cruise control at 25mph, 65 or 80mph which I have tried and there is no issue with rpm when using the cruise control. I drove the car for 45 minutes straight on Sunday from Mass to Ct, cruise set at 70mph and the whole drive was smooth, when I got to CT and cruise was off the issue started and I was driving at same speed as with cruise on.

Even driving car in manual mode has same issue, only cruise fixes it, thats why am leaning towards something throttle related rather than torque converter.

Last edited by canosoto86; 06-20-2017 at 08:33 AM.
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Old 06-21-2017, 07:14 AM   #8
markusmarkus
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Question about rpm issue

I'm wondering if this could be a DME software issue. I say this as there are no codes, and the only difference is the gas pedal. If the gas pedal was bad the DME would certainly have codes.

Last edited by markusmarkus; 06-21-2017 at 07:16 AM.
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Old 06-21-2017, 07:38 AM   #9
canosoto86
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The DME has been flashed to eu3 map, but before the flash still had the issue.
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