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      06-29-2015, 10:40 AM   #1
jgoens
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Serpentine belt changes speed at idle.

Just noticed today after cold start a sound like the fan is running fast then slow. I usually just go after starting the car in the morning but today I was idling for a couple minutes in the driveway. I opened the hood and noticed the belt would speed up and slow down. I think I have heard this a few times before after cold start when I was stopped at a nearby signal. Is that normal? The rpm does not change...in fact nothing seems off..it's just a sound.
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      06-29-2015, 11:33 AM   #2
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Quote:
Originally Posted by jgoens View Post
Just noticed today after cold start a sound like the fan is running fast then slow. I usually just go after starting the car in the morning but today I was idling for a couple minutes in the driveway. I opened the hood and noticed the belt would speed up and slow down. I think I have heard this a few times before after cold start when I was stopped at a nearby signal. Is that normal? The rpm does not change...in fact nothing seems off..it's just a sound.
That is physically impossible. The belt is driven off the crank. You may be hearing the A/C compressor cycle on and off which would cause a change in sound.

The fan, btw, is electric and will vary in speed depending on what the DME tells it to run at.
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      06-29-2015, 11:43 AM   #3
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Quote:
Originally Posted by jwzimm View Post
That is physically impossible. The belt is driven off the crank. You may be hearing the A/C compressor cycle on and off which would cause a change in sound.

The fan, btw, is electric and will vary in speed depending on what the DME tells it to run at.
The ac is off. I was kind,of thinking that the belt can only run faster if rpm change. Just from looking it is hard to tell. Maybe the dme is sending signal to the fan to speed up and slow down...probably not normal.
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      06-29-2015, 11:57 AM   #4
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Quote:
Originally Posted by jgoens View Post
The ac is off. I was kind,of thinking that the belt can only run faster if rpm change. Just from looking it is hard to tell. Maybe the dme is sending signal to the fan to speed up and slow down...probably not normal.
No, it sounds perfectly normal. It sounds like you are looking for a problem that does not exist. As I said, the belt is driven off the engine and only changes RPM based on engine speed. The fan will do its thing to control engine coolant temp.

Stop worrying.
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      06-29-2015, 12:04 PM   #5
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Quote:
Originally Posted by jwzimm View Post
No, it sounds perfectly normal. It sounds like you are looking for a problem that does not exist. As I said, the belt is driven off the engine and only changes RPM based on engine speed. The fan will do its thing to control engine coolant temp.

Stop worrying.
I'm looking for reassurance that I don't have a problem..you think it is fine so that is good. I just have not seen that in any other car I have owned.
thanks.
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      06-29-2015, 12:11 PM   #6
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Quote:
Originally Posted by jgoens View Post
I'm looking for reassurance that I don't have a problem..you think it is fine so that is good. I just have not seen that in any other car I have owned.
thanks.
No prob. Enjoy the car. They are a ton of fun!!
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      06-29-2015, 12:42 PM   #7
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If you suspect something, take it to the dealer.. i wouldn't take my chances with the serpentine belt...
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      06-29-2015, 02:41 PM   #8
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Originally Posted by E90 M335i View Post
If you suspect something, take it to the dealer.. i wouldn't take my chances with the serpentine belt...
Why, so they can charge him $175 diagnostic fee to say everything id fine? If he were still in warranty I would agree with you but I doubt he is.
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      06-29-2015, 02:42 PM   #9
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Quote:
Originally Posted by E90 M335i View Post
If you suspect something, take it to the dealer.. i wouldn't take my chances with the serpentine belt...
Why, so they can charge him $175 diagnostic fee to say everything is fine? What does it being the serpentine belt have to do with it? If it snaps he would lose the alternator, A/C compressor and power steering. All of which are not required to safely control the vehicle.

If he were still in warranty I would agree with you but I doubt he is.
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      06-29-2015, 03:11 PM   #10
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Quote:
Originally Posted by jwzimm View Post
Why, so they can charge him $175 diagnostic fee to say everything is fine? What does it being the serpentine belt have to do with it? If it snaps he would lose the alternator, A/C compressor and power steering. All of which are not required to safely control the vehicle.

If he were still in warranty I would agree with you but I doubt he is.
I do still have a little over a month of warranty and then it is cpo. I don't think it is with the actual belt and like you said it probably is not the car making the belt go faster..which I probably misdiagnosed...it is the fan going faster then slower and it only happens intermittently upon cold start. Might be hard going to the dealer since I cannot replicate it easily and by the time I drove there everything would look fine. My car has 26k miles on it..not even close to a belt change.
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      06-29-2015, 04:22 PM   #11
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If you're auxiliary fan is running faster than normal it's a good sign the waterpump is on it's way out.
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      06-29-2015, 07:43 PM   #12
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If you're auxiliary fan is running faster than normal it's a good sign the waterpump is on it's way out.
Even if it happens upon startup and not at operating temp?
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