CPU fan speed has started to vary

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My CPU fan has started to vary in speed.  At least I don't think it
did before.

It's no big deal to take off the fan again and clean it better, but
I'd appreciate your opinions on this anyhow.  

It's either 4821 or 4991rpm, no other values.  
I'm using ASUS Probe to measure the speed.  It shows a wobbly but
basically horizontal line in the graph it gives.

Is this bad?

Okay, after a few hours the speeds have changed to 4891, 4821, and
4787 rpm.

IIRC, it was a straight line, no visible wobbles, until yesterday,
when I removed the fan to get "soot" off of the heak sink prongs.
There was also soot on the fan blades, which I wiped off pretty well,
but maybe I didn't do it completely or evenly where the one blade is
overlapped by another.

It also seems to vibrate when my finger is on it, which I don't
remember happening before.  

Is this vibration and speed variation really caused by uneven soot?


(P.S.  A) Cleaning the prongs and the aluminum box around part of it
did lower the CPU temp by 15 to 20 degrees, so it's below spec now,
Now it's usually 138 F.  I have an 800 MHz cpu; I forget which brand,
on this self-assembled 98/XP computer.   I think the fan is 2.25
inches square.

        B) The soot came from a badly adjusted oil furnace this past
winter.  It also got on tv screens and some on the wall near the heat
output ducts.  It's also conceivable that it wasn't badly adjusted but
I closed too many first floor registers (3 or 4 out of 4) and this
interfered with the combustion, at least they tell me that but I don't
see how since the combustion air and the heated circulating air are
supposed to be separate.   Yet there is no denying the soot, which is
also supposed to be in the combustion air, not the rest of the house.)

Re: CPU fan speed has started to vary

mm wrote:

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You're concerned by what amounts to about a 3% reduction in measured
fan speed? I would guess that your PSU +12v varies by that much. Unless
your CPU is showing signs of over heating, I wouldn't fret about it.

Re: CPU fan speed has started to vary

On Sat, 29 May 2010 02:16:49 -0400, mm

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I suspect this is just a sampling rate issue, that when the
board samples the RPM it does not do it at the same
frequency as the fan RPM so sometimes it gets a count of
4821, other times 4991.  Either that or it could be as UCLAN
wrote, your 12V power rail is fluctuating a little bit based
on what the load is on it or the 5V rail.  For example a
change in video card or CPU power/processing state could
make the voltage change some.

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No that is fine... so long as the RPM doesn't drastically
lower which could be a sign of an impending fan bearing

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I imagine that by cleaning it you slightly altered its speed
a bit, but that it now fluctuates based on what I wrote
initially, that the sampling rate of the motherboard is just
a little less in sync now.  It is not a problem.

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In that case I would replace it.

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That could cause it, or it is spinning faster so the
vibration that was there before has grown.  You don't need
to wonder about this though, you can clean the fan more so
you know it isn't uneven soot.

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It's old enough I would go ahead and replace the fan,
especially if you have a spare fan lying around that is
decent quality.  If it is the original Intel heatsink with
the fan in a plastic shroud it becomes more difficult to
replace as a standard fan has no obvious way to mount on it,
but you could strap one on with nylon wire ties, or if you
are handy in a shop with machine tools you could tap a
threaded hole in the heatsink base and buy some long screws
from the hardware store to mount the fan... just be sure
they don't stick out of the bottom of the sink and keep it
from good contact with the CPU or that they short out traces
on the motherboard or damage it.

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If you have to you can get a fan wet when cleaning it.  A
small toothbrush with detergent solution on it might be in
order, but don't let it sit on the fan too long trying not
to get any in the bearing which would dissolve the grease
and flush it away.  Rinse with plain tapwater, shake out
excess, and allow to dry for a day or two before applying
power again (or several hours near a fan or vent, etc).

However, I'd just replace the fan... even if you get it
working smoothly again it is late in its expected lifespan
and fans are cheap.

Re: CPU fan speed has started to vary

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I would agree with Kony - the wobble in the line is probably just something
you didn't notice before. However, if blowing out the dust reduced your CPU
temperature by 15-20c, then it must have been very dirty, so perhaps the fan
has a little dirt inside and needs another clear out. Perhaps a little dust
has blown into the fan rotor and it will fail soon, but even it if does then
its not a big deal - fans are easy to replace. even if you can't get one the
correct size, you can mount a larger one with a plastic (or cardboard) duct.

Re: CPU fan speed has started to vary

Thanks to you, GT, and UCLAN and Kony.   I've been monitoring temps
and fan speeds closely and will continue to do so, and it's okay.
Plus the Asus Probe goes the front and beeps if any of the two
meausred temps, one measured fan speed, or 4 measured voltages gets
out of range.

Best of all, the variation in fan speed on the diminished after 2 or 3
days and tonight it diminished again and the line is flat at 4856 or
4787**, but there is no visible change to the line!

**I presume the variation doesn't really exist but it appears to for
the reason Kony gave.

Asus-Probe will also keep a history of these measurements, displayed
in graphical form, but I didn't start that right away and for what I
have the time is compressed so it probably would look flat no matter

But I may do the other things suggested anyhow, depending on what
happens with another partially assembled computer.  

thanks a lot.

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