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- Ever wondered where did that missing parts gone from a worn out belt??? :) (and eventually not just the missing ones goes there the whole material from the grooves) ---
- Oh that's easy: It's just falls off and goes away like small dust. No? :dunno:
- Yes mostly but not all. It seems that we have found a heaven like reserve where the belt parts can hide away. And that "shelter" is in your car!!! Wonderful, isn't it? ;)

Here is the full story (you can skip to the main point if you don't want to take the time):
It's a story which was fun all the way, so I decided to share with you… I own an 2001 530i with many-many miles in it. Since it's a very good car I own it for a long time and repaired everything on it when a problem occurred. I HAD one problem which seem to never disappear. During winter days the car has some squeaking noise - you know like if some belt pulley bearing were worn out. I replaced 3-4 years ago all the pulleys with bearing. For about a year everything seemed fine. Then the winter came again and the squeaking was back. I replaced the alternator belt and also the alternator bearing (well it was not really worn but…). Again… everything is ok. I was happy. So winter came again and the squeak was back… again, I replaced every bearing (well they were running "too fast" when I turned them by hand but they were not really noisy) BUT this time I did not replaced the belt itself. Squeaking noise stayed! Hooups… I was a little bit confused and started to see I had not solved the problem since years. I decided to take apart the car again and start to search for something which degrades my driving belts or something like that (despite I hate to remove that fan shroud and the fan you know fragile plastic parts, coolant spill, etc. or it's just because I've done it so many times ***61514;).
What have I found? For the first time nothing… but after the belt was removed and I stared a while into the engine bay I noticed that something is wrong with the water pump pulley. It was like some parts were missing from it or like this. But after a closer look turned out that not missing anything but there are more stuff in it than it should be… The pulley slots was full of the remaining bits of former old belts. Now I realized that last time when I removed this pulley (preventive water pump replacement) I did not noticed anything like that. The reason was that the built up inside the slots was so even that it wasn't noticeable - since back than I don't have a new pulley beside the old one to see if the slots are deep enough or not. But seems so that something has changed… maybe the reason was that this time I was a little bit "lazy" and did not started the squeak repair immediately as the winter began and I used the car with this fault for a while. During this (ab)use period some part of the built-up started to fell out from the slots. Now I cleaned the pulley and the problem went away. The engine revs up a little bit better from idle and it's like the whole engine were "better put together" - nice side effects anyway ***61514;…

Main thing is:
The water pump pulley slots has a built-up from the old driving belts causing slip after the new serpentine belts "brake in period". Pictures below showing the built-up, clean pulley and the comparison between a new and a used belt's profile. Below that you can read a partial DIY and tips that will make your life a little bit easier if you remove the belts, fan, etc.

The build-up inside the pulley slots (during the removal, when started nearly just one inches were missing in one or two slots) - at least the half of the depth of the slots were gone...:
Image 1-2

Pulley when clean:
Image 3

Old belt (but this belt is still in mint condition, it just has adapted the form of the pulley):
Image 4

New belt:
Image 5


Recommendations:
If you buy a used car (even with low mileage) you should check belts and see if the drive is okay and clean by every means. (Years back when I bought this car used I only replaced a badly worn belt and some pulleys).
If you own a car and you have to replace belts too often (let's say yearly or so) or just sometimes you hear a squeak you should check this also.
If you have the same problem like me you may don't even notice this problem if you live somewhere like California - but the car do not really happy to running like that (in my opinion). This kind of squeaking goes away when the weather was a little bit warmer but the problem does not.

DIY tips:
In this forum you can find many threads about L6 or V8 gasoline engines and cooling system overhauling or fan removal. I don't want to write this down again but some tips that may help:
- During assembly threading the fan to back its place can be a struggle if you don't know where the threads start. So I recommend that before you start the actual assembly process try to screw on the fan (without the shroud and other things). Mark the points where the threads begin on both sides (fan and pump) and it's easy to match them. (It's a real pain if you try to find this position with the shroud already in its place).
- When you start working with serpentine belts you can buy a new belt or belts for your car even if you think they not worn. They are relatively cheap and if the old ones are still strong you can keep these new ones in the trunk.
- Be aware that not worst belt wear when a belt has cracks "crossed" the grooves. These belts may run for many miles from this point (of course you should not risk this either) but cracks, wear, dilapidation across the length of the belt refer some additional problems (like pulley bearing troubles).
- If you are not sure about the belts condition replace it.
- If you replaced worn part in the belt drive replace the belt.
- When you remove the water expansion tank be aware that has a sensor with electrical connection. You should remove the plug and take care of that connector under the tank. It's very easy to break it down - particularly when you assembling the car and try to maneuver with the shroud, etc.
- The water tank has a small return pipe on the top. This hose connected to the radiator on the other side of the radiator's top. Better undo the hose on the tank's side because if you brake the (fragile, small diameter & plastic) connector on the radiator you will end up with a leaking radiator…
- If you don't plane any other water related work (you don't release the water) also be prepared that small return pipe: coolant will flow from this pipe if you move the water level up (by lifting the tank above the pipe)
- Fan shroud will became fragile by the time - be prepared you will break (chip) it on some points (just saying :) don't take it personally)…
- It's maybe evident but: You don't have to tight too much the fan to the water pump because it's a reverse thread - your fan can't just fly away but you have a struggle if you have to remove it again and it's too tight (actually you don't even have to tight it at all but of course use BMW specified forces at this part for extra safety)
- If your car has the power steering reservoir "around" the fan shroud (I mean the hose clamps access with tools blocked by the shroud and/or the intake parts) it's a GREAT time to check the clamps under the reservoir. By the time the rubber hose losing some of it's diameter and the clamps can came a little loose on it. It will cause small a small leak. You can see that always dirty M54, M52, etc. engine bays because of this (of course the hoses can leak badly after they get harder and crack ... its another story)
 

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