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Old 08-20-2011, 08:52 PM   #1
dkrab
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RPM Fluctuation at 25mph

I've searched and searched, and haven't found an answer to this anywhere: When driving steady speed about 25 mph, the rpm will oscillate up and down about 200 or so rpm, and when it dips down, I hear a brief deep rumble. The oscillation frequency is about one cycle every 1.5 seconds or so. The RPM will oscillate between about 2200 and 2000 or so. If I move the auto shifter to sport mode, it downshifts, moving the RPM higher and eliminating the oscillation. If I tip into the throttle with the trans still in regular automatic mode, it accelerates smoothly, no fluctuation. Only when I am going a steady speed of 25 mph, with the auto trans in regular mode, does this happen. It's disconcerting, and I want to get it fixed. It's been doing it for a long while. I replaced the MAF, and both upstream O2 sensors. No codes. Any ideas?
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Old 08-23-2011, 11:15 AM   #2
dkrab
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I suppose no one has any suggestions. A bit more info on what's been replaced already:

New MAF
New O2 sensors (upstream)
Vanos kit installed
one new camshaft sensor (intake side)
new rubber intake boot
new CCV system
new valve cover gasket
new water pump and thermostat, new coolant jug (can't see how this would affect the issue at hand, though)
trans fluid changed every 60K and was done 30K ago, never had shifting problems, ZF automatic transmission
spark plugs changed about 60K ago, with correct NGK plugs. When old plugs were romoved at 90K they still looked pretty good.

Total miles on car is 150,000
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Old 08-23-2011, 11:41 AM   #3
FastHackem
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Mine does this too I've noticed. Almost like its changing gears back and forth or something.
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Old 08-23-2011, 12:07 PM   #4
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I'm not a BMW automatic transmission expert, but based on my previous experiences with other autos, this is pretty normal.

An auto has a torque converter that is not fully locked all the time. Assuming that is what is happening, this is fine. However, it is possible that your torque converter or transmission have started slipping ever so slightly. Similiar to when a manual transmission has a slipping clutch.

I wouldnt worry about it unless it starts slipping on acceleration, shifting strangely, or feels like excessive drag(tc not engaging) while accelerating.

You could go ahead and have it flushed just to see if that helps.
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Old 08-23-2011, 12:09 PM   #5
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Yeah I was thinking torque converter too. Doesnt bother me too much cause it only happens at 35mph and below really.
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Old 08-23-2011, 01:18 PM   #6
dkrab
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Mine does not feel like it's trying to change gears, it's just a rhythmic oscillation of RPM up and down slightly. If you pay attention you can feel it in mild pulses of acceleration, as if the driver is doing it with the accelerator pedal. I hope it's not the torque converter. Is there any way to check it?
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Old 08-23-2011, 01:35 PM   #7
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It sounds like you have excess slippage in your transmission. Is your car high mileage?
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Old 08-23-2011, 01:38 PM   #8
dkrab
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150K would qualify as high mileage. It only does it when I am going about 25mph, steady speed. Tip in or let off the gas, and it stops. I surely hope it's not transmission related. I have had the fluid changed every 60K, and it's only 30K through this change, but I could do an early change to see what happens. But that LTwhatever fluid for the ZF is expensive, and I am reluctant to change to a different fluid such as Redline because you can't get all the old stuff out and I don't want to mix them.
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Old 08-23-2011, 01:42 PM   #9
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One thing you can do is pull out your dipstick and check 2 things out about the fluid. One note the color, see if it is still somewhat red, or if it is turning brown. Also, check the smell of the fluid. Does it smell like burning?? If it smells like burning, its a good indication of slippage.
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Old 08-23-2011, 02:28 PM   #10
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Quote:
Originally Posted by DerRoteBaron View Post
One thing you can do is pull out your dipstick and check 2 things out about the fluid. One note the color, see if it is still somewhat red, or if it is turning brown. Also, check the smell of the fluid. Does it smell like burning?? If it smells like burning, its a good indication of slippage.
There is no dipstick for the transmission. Only your finger.
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Old 08-23-2011, 03:26 PM   #11
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http://forum.e46fanatics.com/showthr...740&highlight=

Read^
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DUMB YOUR MOM.
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Old 08-23-2011, 04:20 PM   #12
dkrab
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Nightmare, it looks like you're tracking down a driveline vibration. I have had my car up to 110 mph recently (last week) and it's smooth as butter. Would that eliminate the flexjoint/guibo, ujoint, etc. as a possible cause? The RPM only fluctuates at a steady speed of about 25mph, not under acceleration. I would like to find the highest probability cause first, and go from there. But I'm running out of high probability causes!
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Old 01-12-2013, 08:51 PM   #13
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Quote:
Originally Posted by dkrab View Post
I've searched and searched, and haven't found an answer to this anywhere: When driving steady speed about 25 mph, the rpm will oscillate up and down about 200 or so rpm, and when it dips down, I hear a brief deep rumble. The oscillation frequency is about one cycle every 1.5 seconds or so. The RPM will oscillate between about 2200 and 2000 or so. If I move the auto shifter to sport mode, it downshifts, moving the RPM higher and eliminating the oscillation. If I tip into the throttle with the trans still in regular automatic mode, it accelerates smoothly, no fluctuation. Only when I am going a steady speed of 25 mph, with the auto trans in regular mode, does this happen. It's disconcerting, and I want to get it fixed. It's been doing it for a long while. I replaced the MAF, and both upstream O2 sensors. No codes. Any ideas?
Read this->

http://www.bimmerfest.com/forums/sho...d.php?t=459899
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Old 01-12-2013, 08:51 PM   #14
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And then read about it's function -->

Click image for larger version

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Old 01-12-2013, 08:52 PM   #15
always-there
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Replace em'.
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Old 01-12-2013, 10:53 PM   #16
dkrab
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I get no Pcode. It's been doing this for a few years now, but it does get worse on long trips. Any way to check these before replacing them? Looks like both sensors will cost about 120 bucks. I will be doing a trans fluid change soon, so it would be a perfect time to change them, but if I could be more certain before dropping even more money into it, that would be great. I am not keen on throwing parts at it if I can help it.
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Old 01-12-2013, 11:09 PM   #17
always-there
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$120's a lot cheaper than several hundred replacing the burned clutches/stuck valves on valve body, or the torque converter.
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Old 01-13-2013, 09:52 AM   #18
dkrab
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No doubt about that. But, I am not sure these sensors are actually bad, especially considering it has thrown no codes. This would be an exercise in replacing parts to see what happens. Not that I haven't done that before, but I would like to nail it down better if I can. But if I can't, I'll replace them with the next fluid change (within a month).
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Old 02-18-2013, 05:25 PM   #19
ryanwalexander
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I have the exact same symptoms it sounds like. Started a few months ago but I've been just hoping it would go away.

RPM fluctuation like the torque converter seems to want to be locking up, then it drops the RPMs too low and then slips again. Happens probably every 3 seconds or so when I'm at steady speed. Under acceleration or under no power it doesn't occur. There is also a low pitch hum from the transmission that seems to be associated and occurs more when the transmission is loaded. (when it's highly loaded, I feel like it's probably still occurring, but the engine drowns out the sound.)

Happens around 45-70 km/h (30-45 mph.)

There aren't any symptoms until the car is at operating temperature (both no hum and no RPM fluctuation.)

Transmission has original fluid in it, so I've been wondering if maybe some of it has leaked out and it's running low even though I don't see leaks underneath.

There's also this weird loud buzzing sound that happens when the car starts up when it's less than 5C (40F) outside.

Any new info here? I'm hoping I don't need a new transmission since I just went back to school and don't have a lot of cash.

2002 325i. Car has 273k km on it (170k miles).

Last edited by ryanwalexander; 02-18-2013 at 05:31 PM. Reason: clarification
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Old 02-18-2013, 08:43 PM   #20
dkrab
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Well, mine would do it regardless even before it was fully warmed up. I heard no buzzing sound at very cold startup. I have changed the trans fluid every 60K miles. Oddly enough, it hasn't done it at all since I changed the big rearmost differential bushing, which was severely worn out. Not sure if that had anything to do with it, but it's been perfect ever since (about a month). By the way, my car has the ZF transmission.
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