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Alternator belt too tight? Strange sound.

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Alternator bushings replaced last month, now, if the car has been sitting for more than 5-6 days I'll get a low, non-metallic humming thump (almost a 'thwub') for the first minute or so of idling. If there's the slightest bit of heat in the car, no noise. The noise also seems to last longer in proportion the amount of cranking required to start the car. The alternator is quite new, the p/o just reused the bushings, so I don't think the alternator itself is crapping out. It doesn't sound like any belt noise I've ever heard, but I really don't think it's the engine itself, as it is coming from the very front of the car and is definitely not timing chain noise. 

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Think maybe water pump . Do you see any weeping?


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7 minutes ago, Dudeland said:

Think maybe water pump . Do you see any weeping?


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Haven't noticed weeping, could be happening given the age of the car, I've no record of the pump being replaced even during the p/o's engine rebuild. That said, the cooling system seems rock solid, good temps even in 90 degree heat here. You'd think it would happen more often though, and not just after 5+ days of sitting. 

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Another way to check the water pump.  Grab a fan blade (engine off, of course 😉 ) and see if you can wiggle it axially.  If you can rock it on its shaft, then the water pump bearings are going.  And then check the weep hole (at 6 o'clock)--if you see a whitish stain dribbling down from the weep hole, the seals are failing due to wobbling bearings.  

 

And BTW, new alternators have been known to be "not so good" right out of the box....

 

mike

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So what are the physics at play here? If it is the water pump, why is it only loud on a dead cold motor? 

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I had a old GTI and it would make this wobbly thump sound sound when cold.  The water pump was completely shot, I couldn't notice it was weeping because it is in a difficult location to see. 

 

My best guess is that the thing needed to warm up, pressurize the system to force a little coolant out, which has a lubricant in it, and it would lube it enough to the point it wouldn't bind so much and make the thumping sound.

 

What really helps with these sort of things is a video. 

 

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22 hours ago, Dudeland said:

My best guess is that the thing needed to warm up, pressurize the system to force a little coolant out, which has a lubricant in it, and it would lube it enough to the point it wouldn't bind so much and make the thumping sound.

Good point.  I had that happen to a center support bearing on my driveshaft and the idler pulley on my A/C compressor.  Both squealed like stuck pigs when cold, but shut up within a few blocks when what little grease remained in the bearings got warm.

 

mike

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You could always just take the belt off and see if it shuts up.

 

That's also a good way to tell how the water pump and alternator bearings feel.

 

t

 

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21 minutes ago, TobyB said:

You could always just take the belt off and see if it shuts up.

 

There ya go.  

 

I should relay a story, just in case it's valid here, or for anyone with a tii.

 

I had a terrible noise upfront on start up and decel until the engine was warm.  Timing chain, oil pump chain, small end bearing.  I was ready to dismantle the engine to find it.

 

Turns out it was the (tii) alternator bracket that was attached with the wrong length bolt.  M8X28 vs. M8X25.  And although the bolt was tight it was bottoming out and the bracket was loose, to scrape and bang against the timing cover.  Easy fix but hard to diagnose.

172191006_BracketRacket2.thumb.jpg.6680103b732ee07b87d345636e30c7a8.jpg

 

 

 

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